# No, poco 240vac is single phase.



## TimPa (Aug 15, 2010)

concrete_joe said:


> no, poco 240vac is single phase.


???
I didn't say that it was not single phase. 

Joe, in residential power distribution, single phase is applied to the primary of a transformer which has a secondary that is center tapped. the output of this transormer is two 120v hots, 180 degree out of phase from each other (but _still considered single phase_). 120 v + 120 v = 240 vac hot-hot.
the neutral conductor is connected to the center tap which provides the 120 volt potential to either hot line. so the three lines from the pole are hot-hot-neutral. ground is established at the dwelling.


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## concrete_joe (Oct 6, 2014)

TimPa said:


> ???
> I didn't say that it was not single phase.
> 
> Joe, in residential power distribution, single phase is applied to the primary of a transformer which has a secondary that is center tapped. the output of this transormer is two 120v hots, 180 degree out of phase from each other (but _still considered single phase_). 120 v + 120 v = 240 vac hot-hot.
> the neutral conductor is connected to the center tap which provides the 120 volt potential to either hot line. so the three lines from the pole are hot-hot-neutral. ground is established at the dwelling.


nope, the two 120's are the same phase, not 180 apart. its a center tap transformer, single phase.


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## curiousB (Jan 16, 2012)

concrete_joe said:


> nope, the two 120's are the same phase, not 180 apart. its a center tap transformer, single phase.


Joe you are just wrong here. Let it go.

The two feeds are exactly inverted from each other. They are called split phase. That by mathematical definition means they are 180 degrees or Pi radians out of phase of each other. Yes they are derived from a single phase of the 3 phase HV feeder network but we're not talking about that.


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## Desertdrifter (Dec 10, 2009)

*240V Safety Disconnect Switch*



concrete_joe said:


> nope, the two 120's are the same phase, not 180 apart. its a center tap transformer, single phase.


Take it to the technical forums. For the interest of DIYers here the definition works. Safety is what counts. You just add confusion

The voltage is relative to the center tap. 

Out in the field they are called phases all the time. Regardless


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## concrete_joe (Oct 6, 2014)

curiousB said:


> Joe you are just wrong here. Let it go.
> 
> The two feeds are exactly inverted from each other. They are called split phase. That by mathematical definition means they are 180 degrees or Pi radians out of phase of each other. Yes they are derived from a single phase of the 3 phase HV feeder network but we're not talking about that.


no they are not, its ONE FEED, its a center tap secondary winding, ONE WINDING. where exactly does a second magnetic field come in on this single secondary winding ?? please stop calling it dual phase, 180 shift, whatever, its ONE PHASE !!!!!

if a single 240 service is two phase, then do you have six phases when you get 3-phase service ??? residential service is 240V CENTER TAP SINGLE PHASE.


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## Desertdrifter (Dec 10, 2009)

Just start your own thread on the topic already. Here's another thread balled up over this.


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## beenthere (Oct 11, 2008)

Made this its own thread for you Joe.

Please refrain from discussing phasing in the other thread, thank you.


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## concrete_joe (Oct 6, 2014)

beenthere said:


> Made this its own thread for you Joe.


awesome, great. 

so to make this thread short, its ONE PHASE. 
anyone else care to show me thats its more than one phase?


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## beenthere (Oct 11, 2008)

The 2 lines are indeed 180 degrees out of phase with each other. And it is single phase. 

If they were the same phase, you wouldn't get 240 volts when you checked them with your meter. 


http://www.assocpower.com/index.php?Single-Phase-Power


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## concrete_joe (Oct 6, 2014)

wow, i have to say, that link has some bad info. there is not "two 120v sources". its a single secondary winding providing 240vac single phase.

and its not possible to get the ends of a single coil of wire to be out of phase, its the same wire, the magnetic field hits the whole coil at the same time. you want more phases? then you need more coils.

slice & dice it anyway you like, one coil, one phase. line1 and line2 (a single phase) flip 60x/sec.


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## beenthere (Oct 11, 2008)

concrete_joe said:


> wow, i have to say, that link has some bad info. there is not "two 120v sources". its a single secondary winding providing 240vac single phase.



Maybe you should get your oscilloscope out and find out you are not correct.


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## beenthere (Oct 11, 2008)

http://www.nojolt.com/three_phase_and_single_phase_electric.shtml


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## Desertdrifter (Dec 10, 2009)

The point is the 240v is a single phase. The center tap the makes it a 1/2 phase. So measuring from center you get 120v each way. If relative to the center tap they were "in phase" you'd get ZERO when measuring across the hot legs. However relative to the center tap they are out of phase so you get 240v. 

They are called legs, hots, phases and who knows what else in the field. No it's not a phase of the 3 phase nature off a single poco pot. 

However on a 2 or 3 pot bank with a lighter pot, you do have phase A and Phase B next to the center/neutral tap. Still 120/240 feeding single phase lighting load off of one pot in the bank. But they are part of a 3 phase set up 

Don't get hung up on the letter of the word.


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## Desertdrifter (Dec 10, 2009)

concrete_joe said:


> slice & dice it anyway you like, one coil, one phase. line1 and line2 (a single phase) flip 60x/sec.


There's not 2 sources and you left out the center tap in your commentary. With out it there's no 120v. Thus "split phase".


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## concrete_joe (Oct 6, 2014)

current flows through the coil in one direction only per half cycle of the generator.

it doesnt matter if you mark one side +240 and other side zero, or one side +120 and other side -120, its a single phase and the two channel of oscope will overlay each exactly the same when one channel(neg) and other channel(pos) is connected to the center tap (aka "neutral"). however, if you put both channel(neg) on neutral the oscope will display what looks like two 120v sources 180 apart, but they are not, you just flipped the one probe around.

and to note, the heads of these arrows are always more positive than the tail end as electrons move from neg to pos.


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## Desertdrifter (Dec 10, 2009)

concrete_joe said:


> anyone else care to show me thats its more than one phase?


I believe the original reply to you was it's a split phase by Jump Start.


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## ddawg16 (Aug 15, 2011)

Here, this will confuse you even more.










It's single phase....and if you measure one side (referenced to neutral) to the other side of a typical 240Vac home electrical, yes, the peaks are 180 deg out of phase to each other....but it's still single phase.


Here is another one....


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## concrete_joe (Oct 6, 2014)

so, in this pic, the two motors that share the neutral, if electrons flow from neg to pos than the secondary lines have potential as i have indicated. now explain to me how you get that from a center tap transformer? the only way is to have primary mag field opposite across what is depicted as two secondary coils, which isnt possible with SINGLE PHASE


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## ddawg16 (Aug 15, 2011)

It's just a reference.

Are you familiar with a Variac? Or Rheostat? Basically, it's a coil of wire with a sliding tap.

Take your center tap transformer....if you could 'move' that center tap, then the voltage reference would move with it.

Look at it this way...

Lets assume the bottom of the transformer is ground (forget neut and ground rods). In other words, it's our 'reference'.

Lets also assume the transformer at the top gives us 100v (ignore ac, dc, or anything else...lets keep it simple)

So now, bottom is 0V...top is 100V.

If you measure in the middle...it's 50V. Measure up 2/3's...66.6v

Get the idea? 

So now, remove the ground from the bottom. Now, your voltages are going to be from what ever point one lead is on to the other point.

On a center tap transformer, all we are doing is putting the 'reference' in the middle.

Going back to our 100v transformer, if we put our meter lead at 50 and then measure to each end...we get 50v

Does this confuse you more now?

Back to your center tap transformer. If you put a 240vac device across the 2 hots....no current flow goes through the neutral.

Conversely, if you put two devices from each hot to neutral...and they have 'exactly' the same load, then, no current flows through neut.

BTW...ignore the A numbers in the above pic....back when I did it I intentionally as a test to someone....I no longer have Visio on my puter so I haven't changed it.

If you have ever done any Spice calculations...you would understand.


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## curiousB (Jan 16, 2012)

oy vey. This is simple math.


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## Daniel Holzman (Mar 10, 2009)

The fastest way to get an argument amongst electricians going is to discuss single phase, split phase, or two phase, like this thread. I worked at Duke Energy for 8 months, with electrical engineers who worked three phase power all day, and even among them you could get a roudy argument going about just this topic.

I had just about given up understanding this, when one of them explained the situation to me, approximately as follows. The term "two phase" by tradition refers to two sources derived from separate windings, that are 90 degrees out of phase. This type of service is almost unknown in the United States now, I believe there is a small part of Baltimore that still uses it, but it was common around 1900, in fact I believe that Niagara Falls was initially wired with two phase generators. Three phase replaced two phase because it is more efficient, and has other superior electrical properties.

Nevertheless, two phase service lives on in many single phase electric motors, which need a second phase to start. The second phase is derived from the main single phase service in a variety of ways, one way is by using a capacitor to cause the secondary phase to be out somewhere on the order of 30 degrees from the primary phase. You can also do this with an inductor, but capacitor start motors are more common. In any case, a single phase motor will not start without a second winding out of phase with the first winding, for full review of this interesting topic you can check out capacitor start motor, or shaded pole motor, which should point out the many ways to derive a second phase from a single phase source.

Anyway, here is the nut. A true two phase service, where each phase was 180 degrees out of phase with the other, would look exactly the same as the typical split phase, single phase service at the average American house. The only difference would be the method of derivation of the second phase. In a "true" two phase service, the generator would need to have two out of phase windings, each winding 180 degrees out of phase with the other. Much like a three phase machine has three windings, each 120 degrees out of phase.

However, by tradition I guess, the split phase service we get at our house, which has exactly the same electrical properties as a "true" two phase service would have, is derived very differently. As has been noted on this thread, the U.S. 240 volt service is derived by center tapping the secondary of a single phase transformer. This has the effect of causing the second leg to have reversed polarity from the first leg, which simply means that at any given time, if leg one is reading +60 volts, leg 2 is reading - 60 volts. Using a root mean square device like a typical voltmeter, the average difference between the two legs is 240 volts, which creates an average difference of 120 volts between either leg and the neutral, which is maintained at 0 volts potential.

Curiously, you would get exactly the same result if you derived the two legs from two separate windings which were 180 degrees out of phase, but since the legs were derived from a single winding, not two windings, it is not typically called two phase, but split phase, since a single phase was split (center tapped). As to why the two processes (split phase versus two separately derived phases) look the same on an oscilloscope, the answer is that they are mathematically identical, which occurs because they are exactly 180 degrees out of phase, hence a negative polarity (created by the split phase) is mathematically identical to a two phase system with each phase out 180 degrees. Ciao.


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## dmxtothemax (Oct 26, 2010)

Daniel Holzman said:


> The term "two phase" by tradition refers to two sources derived from separate windings,


.

So are you saying that to be a true multi phase, 
then the windings must be completely separate ?

I was not aware of this ?

Could explain the confusion ?


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## brric (Mar 5, 2010)

http://www.nojolt.com/three_phase_and_single_phase_electric.shtml


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## Stubbie (Jan 7, 2007)

There is no such thing as 'split phase' in the sense of making 2 phases out of one. Terminology can make things confusing. 
The term 'split' simply means that we split off multi voltages from a single phase. 
The secondary winding of the transformer is a single coil. Everything is in series. From that winding we *split* off 120 volts by tapping the center of the coil. This has the effect of creating 2 voltages at each end of the winding (L1 and L2) that can live together when current is flowing on a *shared* neutral. The only way this can happen is if one voltage is peaking in the positive direction and one voltage is peaking in the negative direction at exactly the same time ..ie.. +120 volts and -120 volts. If this is not the case then the current would add on a shared neutral instead of cancelling out and only the difference remains. Some people like to use the term '180 shift' in the single phase waveform to accomplish the correct image of the two derived 120 voltages. If you check with an oscope L1 will peak exactly the same time as L2 but the peaks are 180 degrees opposite each other. If they both overlayed the same then there would be no 240 volts and all current from 120 volt loads would add on a shared neutral.


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## concrete_joe (Oct 6, 2014)

dmxtothemax said:


> .
> 
> So are you saying that to be a true multi phase,
> then the windings must be completely separate ?
> ...


its fundamental physics. one output coil from the generator is one phase. you need multiple coils to get multiple phases. "rotate" the coils in the generator and you get phase shift between the outputs.

as for the the cap starting motors, this doesnt shift the electrical phase, but it does shift the current phase, which is what's needed to induce a starting current. dont confuse the two......


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## TimPa (Aug 15, 2010)

concrete_joe said:


> current flows through the coil in one direction only per half cycle of the generator.
> 
> it doesnt matter if you mark one side +240 and other side zero, or one side +120 and other side -120, its a single phase and the two channel of oscope will overlay each exactly the same when one channel(neg) and other channel(pos) is connected to the center tap (aka "neutral"). however, if you put both channel(neg) on neutral the oscope will display what looks like two 120v sources 180 apart, but they are not, you just flipped the one probe around.
> 
> and to note, the heads of these arrows are always more positive than the tail end as electrons move from neg to pos.


This is a perfect pic you posted to explain the phasing... if you look at the red current arrows that you drew in the secondary, you'll see they are both going the same direction. in the bottom drawing, that would make the bottom of each coil half (-) negative, and the top of the coil half (+) positive. with respect to each other, that makes the two coils opposite polarity, or 180 degrees out of phase. faraday laws.


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## TimPa (Aug 15, 2010)

concrete_joe said:


> so, in this pic, the two motors that share the neutral, if electrons flow from neg to pos than the secondary lines have potential as i have indicated. now explain to me how you get that from a center tap transformer? the only way is to have primary mag field opposite across what is depicted as two secondary coils, which isnt possible with SINGLE PHASE


your red polarity marks are incorrect in this drawing. at any given instant, the polarity of the secondary coil halves will be - +. - + (top to bottom), or + -, + -. This would be the polarity that you would apply to each end of the coil halves. so the juction, or center tap, would be the positive end of one coil half, and the negative end of the other coil half, because of the current direction between the whole secondary.


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## Bob Sanders (Nov 10, 2013)

TimPa said:


> This is a perfect pic you posted to explain the phasing... if you look at the red current arrows that you drew in the secondary, you'll see they are both going the same direction. in the bottom drawing, that would make the bottom of each coil half (-) negative, and the top of the coil half (+) positive. with respect to each other, that makes the two coils opposite polarity, or 180 degrees out of phase. faraday laws.


No.
I see what Joe is saying in that regard and he is absolutely correct. I think the confusion here is people are looking at the center tap as part of the equation in a 240 volt circuit, and that's incorrect. There is no center tap involved in a 240 volt circuit so don't even figure it in. Put a piece of tape over it and cover it up. If you do that then you can see what you have... one big 240 volt transormer which produces one big 240 sine wave (not two 120 volt sine waves 180 degrees out of phase with each other.

In other words this is a 240 volt transformer... NOT two 120 volt transformers working opposite to each other.


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## TimPa (Aug 15, 2010)

i agree that when 240 volts is considered, the center tap is not in use, and the secondary power is in phase with the supply power. 

however, HIS PIC shows a center tap with 120 volt circuitry, so my info applies.


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## Bob Sanders (Nov 10, 2013)

TimPa said:


> however, HIS PIC shows a center tap with 120 volt circuitry, so my info applies.


Well, regardless of who's pictures are who's, the point is that you have to look at it in the same way a device connected to the transformer is looking at it (stove... hot water heater... etc). The center tap (or neutral) is not connected and therefore not involved at all. The device in question is therefore seeing one big 240 volt sine wave (as opposed to this idea of two 120's, 180 degrees out of phase)


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## zappa (Nov 25, 2011)

What's going on? Cletis said that there was a ruckus brewing over here.

*ddawg's* MIDDLE image and *curiousB's* image below, especially the blue and red lines, are the ones you guys should be paying attention to. They are by far the easiest to understand and they are in real time. I look at these waveforms almost every day and no one is making this stuff up. Imagine a (real time) vertical line intersecting the peaks and you will see how 240 volts adds up. It has to have another reference to push against, if that makes any sense.

Carry on...


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## curiousB (Jan 16, 2012)

concrete_joe said:


> its fundamental physics. one output coil from the generator is one phase. you need multiple coils to get multiple phases. "rotate" the coils in the generator and you get phase shift between the outputs. as for the the cap starting motors, this doesnt shift the electrical phase, but it does shift the current phase, which is what's needed to induce a starting current. dont confuse the two......


Wrong yet again.

A phase shift can be any phase angle. Yes 3 phase power has three phases 120 degrees apart.

A phase shift can be 90 or 180 degrees as well.

In split phase, yes it is a thing..... The phases are 180 degrees apart. 

http://en.m.wikipedia.org/wiki/Single-phase_electric_power


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## ddawg16 (Aug 15, 2011)

One important point...

A transformer can have several output windings 2 Is the most common (typical residential transformer). They just happen to tie two of the legs in serial.

You could have 4 windings. Regardless of how you wire them, the outputs all have the same phase. If you see one 180 deg out of phase with the other one, it's only because your 'reference' is on the other side of the winding.

In other words....as the voltage goes up on the primary, the secondaries follow suit at the same phase.


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## concrete_joe (Oct 6, 2014)

Bob Sanders said:


> Well, regardless of who's pictures are who's, the point is that you have to look at it in the same way a device connected to the transformer is looking at it (stove... hot water heater... etc). The center tap (or neutral) is not connected and therefore not involved at all. The device in question is therefore seeing one big 240 volt sine wave (as opposed to this idea of two 120's, 180 degrees out of phase)


exactly, and the reason why is, theres only one coil producing that sine wave voltage. and the physics backs this 100%. these pics with two 120's is just because the scope probe is set backwards (but logically correct because we call the far leg "hot"). 

can "you" (anyone) post some pics of what your oscope shows when you connect the probes like this? makes you wonder, huh....?








































curiousB said:


> Wrong yet again.
> 
> A phase shift can be any phase angle. Yes 3 phase power has three phases 120 degrees apart.
> 
> ...


what "phases", we already established there is just a SINGLE PHASE, so why do you keep saying "phases" ??


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## ddawg16 (Aug 15, 2011)

We can use this one...










1. Both Blue and Red
2. Blue or red...take your pick...but you had better be using 2 different scopes fully isolated because connecting the probe grounds like this is going to let some smoke escape.
3. Blue and Green
4. Green and Red


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## curiousB (Jan 16, 2012)

*Oy vey^2*

I showed in the graph the mathematic equations for each line of the split phase power service.

They are:

(SP1) Vp Sin (wt) (which can be rewritten as Vp Sin (wt + 0*π)) and

(SP2) Vp Sin (wt + π)

The wt part is simply 2π*f*t Where f is 60Hz, and t is time. So this component is locked to the frequency of the sin wave which is precisely 60.0000000000000Hz.

The second part in the Sin argument is called the phase shift component. In the first one it is zero or zero π. The second one is π (which is 180 degrees if you prefer degrees).

So by mathematic definition these two outputs have 180 differing phases. It doesn’t matter that they were derived from a single phase of a 3 phase (polyphase) feeder network. The 180 degree phase shift is due to the center tapping of the transformer and outputting the opposite ends to the windings.


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## concrete_joe (Oct 6, 2014)

curiousB said:


> *Oy vey^2*
> 
> I showed in the graph the mathematic equations for each line of the split phase power service.
> 
> ...


i know the equations. so where exactly do you think pie comes from, some magical component in a single coil winding? it comes from physical placement within the generator, two or more coils rotated from one another.

care to do the oscope probe exercise i listed as pics??


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Well, regardless of who's pictures are who's, the point is that you have to look at it in the same way a device connected to the transformer is looking at it (stove... hot water heater... etc). The center tap (or neutral) is not connected and therefore not involved at all. The device in question is therefore seeing one big 240 volt sine wave (as opposed to this idea of two 120's, 180 degrees out of phase)


It boils down to what point you are using as a reference. In your example, say L1 and L2 are starting out at zero volts. A short period later (at the exact moment in time) there will be say a 50 volt potential, 25 L1 and 25 L2. Eventually a 240 volt potential, 120 L1 and 120 L2. Looking at this on a scope, it will seem like 2 inverted waveforms which is a true picture of what is happening electrically.

If you decide to ground one of the hot legs, and call that your zero reference, then only the other leg will gain in voltage. This will produce one 240 volt waveform, similar to looking at one hot leg and neutral.

To better see what is happening electrically in the second scenario, monitor the ground (neutral) with a current probe and that inverted waveform will return. One showing voltage, the other showing current.


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## TimPa (Aug 15, 2010)

concrete_joe said:


> can "you" (anyone) post some pics of what your oscope shows when you connect the probes like this? makes you wonder, huh....?


the scope image that ddawg16 has provided (blue and red) is what you will see, except they will be at +/- 167 volts peak, not 120v. unless there are rms o-scopes out there. i haven't been exposed to one yet. i hope peak vs rms is not another discussion, or ruckus i think i read!


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## curiousB (Jan 16, 2012)

TimPa said:


> except they will be at +/- 167 volts peak, not 120v.


Agreed. I omitted that intentionally as I didn't want to stir the pot further by introducing peak voltage vs rms. I was worried the Internet might explode! :wink:


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## ddawg16 (Aug 15, 2011)

curiousB said:


> Agreed. I omitted that intentionally as I didn't want to stir the pot further by introducing peak voltage vs rms. I was worried the Internet might explode 😃😀😀


I assumed that as well. Some people have an issue understanding RMS (Root Mean Square)

One of the reasons one of my posts used a nice even number of 100


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## concrete_joe (Oct 6, 2014)

TimPa said:


> the scope image that ddawg16 has provided (blue and red) is what you will see, except they will be at +/- 167 volts peak, not 120v. unless there are rms o-scopes out there. i haven't been exposed to one yet. i hope peak vs rms is not another discussion, or ruckus i think i read!


right, the 240 and 120 is just the "rms" amplitude of the sine wave. peak should be around 169.73 for 120v(rms), all depends on where the poco sets their rms delivery and acceptable variance.


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## Bob Sanders (Nov 10, 2013)

ddawg16 said:


> One important point...
> 
> A transformer can have several output windings 2 Is the most common (typical residential transformer). They just happen to tie two of the legs in serial.
> 
> You could have 4 windings. Regardless of how you wire them, the outputs all have the same phase. If you see one 180 deg out of phase with the other one, it's only because your 'reference' is on the other side of the winding


Precisely.
A center tap transformer is a pretty versatile device and has in essence several different windings and therefore can be seen in several different ways... all ending up as a same (total) end result.

However, technically there is only really ONE proper way to look at it, and that's in the way the load is looking at it. In the case of 240 volts, it's a 240 volt transformer with one big 240 volt sine wave and the center tap has absolutely nothing to do with it.


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## concrete_joe (Oct 6, 2014)

[B said:


> ddawg16[/B]]
> _One important point...
> 
> A transformer can have several output windings 2 Is the most common (typical residential transformer). They just happen to tie two of the legs in serial.
> ...


does each winding have same direction, or are the windings opposite of each other?

like this (note the dots)


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## curiousB (Jan 16, 2012)

concrete_joe said:


> can "you" (anyone) post some pics of what your oscope shows when you connect the probes like this? makes you wonder, huh....?


sure...


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## ddawg16 (Aug 15, 2011)

Direction does not matter.....except in how the ends are terminated.

In your case above, you would actually only have 120Vac. Basically, you have two parallel windings. And, yes, that is done at times with control transformers. It's a way to up the available current


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## concrete_joe (Oct 6, 2014)

ddawg16 said:


> Direction does not matter.....


yikes? i do believe the direction of windings does matter. 

take a single long coil, then stretch it out right in the middle some, say about 12", while holding that center 12" section stretch one of the coils just a tad more and then flip it over. use the 12" section as the c-tap. the mag field from primary winding will induce voltage in the "two" secondary windings opposite from each other..... confusing isnt it...


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## ddawg16 (Aug 15, 2011)

concrete_joe said:


> yikes? i do believe the direction of windings does matter.
> 
> take a single long coil, then stretch it out right in the middle some, say about 12", while holding that center 12" section stretch one of the coils just a tad more and then flip it over. use the 12" section as the c-tap. the mag field from primary winding will induce voltage in the "two" secondary windings opposite from each other..... confusing isnt it...


read what I wrote. It doesn't mater if you wind it clock wise or counter clockwise. It's how you terminate the ends....THAT is what matters.


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## Desertdrifter (Dec 10, 2009)

In a 4 bushing secondary ab cd you can parallel the windings to 120v. Alley Cat Bad Dog. 
So.... ?


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## concrete_joe (Oct 6, 2014)

clw1963 said:


> In a 4 bushing secondary ab cd you can parallel the windings to 120v. Alley Cat Bad Dog.
> So.... ?


you mean like

a/c and b/d parallel? i am certain one leg of each coil is "dotted", right?

what do you get if you a/d and b/c? do you expect this to be the same??


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## Desertdrifter (Dec 10, 2009)

All I'm saying is coil one is a/b coil 2 is c/d. Connected in series with b/c together you get 120/240. Parallel with a/c tied and b/d tied its in phase. This is for a single phase primary 4 bushing secondary distribution xfmr. We externally tie the bushings together for 120/208 banks.

Edit-paralleled its straight 120v


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## concrete_joe (Oct 6, 2014)

clw1963 said:


> All I'm saying is coil one is a/b coil 2 is c/d. Connected in series with b/c together you get 120/240. Parallel with a/c tied and b/d tied its in phase. This is for a single phase primary 4 bushing secondary distribution xfmr. We externally tie the bushings together for 120/208 banks.


in series? what do you get if you series the coils as b-ad-c? the coils are still in series, right? is this any different than a-bc-d ?


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## Desertdrifter (Dec 10, 2009)

Schematically it's the same. FWIW, I'm not arguing either side right now. Just stating a fact of how we hook up transformers. Leads me to believe 120v current flows from L1 to N and from N to L2. When we have a MWBC the current on the neutral is cancelled out by opposing current flow. Or it's just seen as 240v load.

That's just a thought in my head. I may be totally wrong. Just not afraid to toss it out there. 

Physically on the pot you wouldn't do that. I'll take a picture of the pot and nameplate later if I have a chance.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Precisely.
> A center tap transformer is a pretty versatile device and has in essence several different windings and therefore can be seen in several different ways... all ending up as a same (total) end result.
> 
> However, technically there is only really ONE proper way to look at it, and that's in the way the load is looking at it. In the case of 240 volts, it's a 240 volt transformer with one big 240 volt sine wave and the center tap has absolutely nothing to do with it.


Are you saying that a 240 volt water heater is seeing one big 240 volt sine wave on its input? Can you tell me where L1 and L2 is located (position wise) in the sine wave.


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## ddawg16 (Aug 15, 2011)

One point that needs clarifying....

Think of a transformer as a multiplier/divider of voltage

The output of a transformer is a function of the ratio of the number of turns on the primary vs the secondary.

(I'm using simple numbers for simplicity...in reality, the # of turns in in the hundreds to thousands)

Assuming 100Vac going in....and we will ignore losses.....for simplicity

Input Output Voltage
turn turns out
100 100 100
100 50 50
100 75 75


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## Jump-start (Sep 26, 2012)

Sorry Im joining so late, but the terminology is trivial. Even on professional forums it goes on for pages whether split phase is single phase, or '2 phase'. The transformer primary may be fed from a single phase source, however the 180 degree opposite from the neutral and current cancelation properties are enough for some to call it 2 phase. 

There are phase shifting transformers that can actually spit out wave forms that lag or lead the input. Heck ronk makes static phase converters where capacitors and inductors turn single phase into 3 phase. Perhaps a center tap isn't that divine, but you still get 180 degrees out of phase relative to the neutral.


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## concrete_joe (Oct 6, 2014)

Jump-start said:


> There are phase shifting transformers that can actually spit out wave forms that lag or lead the input. Heck ronk makes static phase converters where capacitors and inductors turn single phase into 3 phase. Perhaps a center tap isn't that divine, but you still get 180 degrees out of phase relative to the neutral.
> 
> There are phase shifting transformers that can actually spit out wave forms that lag or lead the input. Heck ronk makes static phase converters where capacitors and inductors turn single phase into 3 phase. Perhaps a center tap isn't that divine, but you still get 180 degrees out of phase relative to the neutral.


really? using single input (single shaft with single primary induction)? i suspect not.

what you can do though is, build the secondaries so that the secondary outputs are separated from each other by some angular phase # (20 degress, 46 degrees, whatever).

caps and inductors phase shift the voltage-to-current picture.


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## Jump-start (Sep 26, 2012)

clw1963 said:


> Schematically it's the same. FWIW, I'm not arguing either side right now. Just stating a fact of how we hook up transformers. Leads me to believe 120v current flows from L1 to N and from N to L2. When we have a MWBC the current on the neutral is cancelled out by opposing current flow. Or it's just seen as 240v load.
> 
> That's just a thought in my head. I may be totally wrong. Just not afraid to toss it out there.
> 
> Physically on the pot you wouldn't do that. I'll take a picture of the pot and nameplate later if I have a chance.


You are correct. The neutral does indeed take the difference. 120 volts flow to the neutral, however, where opposite (180 degree out of phase) neutral current meets up such as at the panel buss bar current cancelation takes place. What doesn't cancel (such as 10 amps on black leg 12 amps on red) the 2 amps return to the transformer. The 2 amps then head to the winding the red leg is originating from and go there since this is where the imbalance is being drawn from . If the load is perfectly balanced there is no current flow on the neutral and it can even be removed. Voltage between the break and load will be theoretically be zero.


A 240 volt heater does see one giant 240 volt wave, however the coils to ground see it differently. If one was to measure with a DMV, one lead to ground the other to the center of the exact center of the heater coil it would be zero volts. As you move the meter lead toward either direction away from the exact center the voltage steadily goes up until you hit 120 volts at the far end where the coil starts. 

Not to confuse but 3 phase is much the same way. Connect and oscillograph to any phase to ground or any phase to another phase and its a perfect sine wave. However, all sine waves will be out of phase with each other. One will reach its peak while another is descending. 


The nameplate would be neat. If I understand its 2 windings 120 120 that may be in series or parallel. Just to add food for thought the windings in parallel must be correct in that if they are out of phase it will be a short circuit. But if both are in phase its just parallel 120 volts with current equally shared between windings.


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## Jump-start (Sep 26, 2012)

concrete_joe said:


> really? using single input (single shaft with single primary induction)? i suspect not.
> 
> what you can do though is, build the secondaries so that the secondary outputs are separated from each other by some angular phase # (20 degress, 46 degrees, whatever).


And what feeds a static phase converter? How does a shaded pole motor work?


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## beenthere (Oct 11, 2008)

Jump-start said:


> Sorry Im joining so late, but the terminology is trivial. Even on professional forums it goes on for pages whether split phase is single phase, or '2 phase'. The transformer primary may be fed from a single phase source, however the 180 degree opposite from the neutral and current cancelation properties are enough for some to call it 2 phase.
> 
> There are phase shifting transformers that can actually spit out wave forms that lag or lead the input. Heck ronk makes static phase converters where capacitors and inductors turn single phase into 3 phase. Perhaps a center tap isn't that divine, but you still get 180 degrees out of phase relative to the neutral.


2 phase, still used(or was 20 years ago) a little in philly. Is only 90° out.


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## Jump-start (Sep 26, 2012)

beenthere said:


> 2 phase, still used(or was 20 years ago) a little in philly. Is only 90° out.


I know, but my point is in theory anything that's out of phase can be called 2 phase since the definition fits. 

I could have 2 wires 10, 30, 68, 90, ect degrees apart relative to ground, but it still meets the definition of 2 phase. Perhaps no practical use exists and you have no 'true' neutral (same as in real 90* 2 phase) but that offset in waves is there. 


And fwiw, there is no such thing as a single phase motor.


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## concrete_joe (Oct 6, 2014)

Jump-start said:


> And what feeds a static phase converter? How does a shaded pole motor work?


oh, those items add components to achieve inductance or reactance which can affect when you see voltage or current (yes, an apparent "phase" shift). straight-up coil on coil, no.... add more stuff to the mix, ok.


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## curiousB (Jan 16, 2012)

Jump-start said:


> And fwiw, there is no such thing as a single phase motor.


Yes there is. I had one on my table saw. The starter cap was fried rendering the start windings the same as the main windings. I had to spin the motor then switch it on. Worked fine but was a pain.:laughing::wink:


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## concrete_joe (Oct 6, 2014)

curiousB said:


> Yes there is. I had one on my table saw. The starter cap was fried rendering the start windings the same as the main windings. I had to spin the motor then switch it on. Worked fine but was a pain.:laughing::wink:


but the phase mentioned is the phase shift between V and I, a very different topic.


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## Jump-start (Sep 26, 2012)

concrete_joe said:


> but the phase mentioned is the phase shift between V and I, a very different topic.


Ok Ill take it. But what about a 3 to 6 phase conversion? How is it done?


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## curiousB (Jan 16, 2012)

concrete_joe said:


> but the phase mentioned is the phase shift between V and I, a very different topic.


CJ,

I am so disappointed I prepared all the scope views in post #46 in response to your ominous makes "_*you wonder comment*_". Yet complete silence from you.......

Originally Posted by *concrete_joe*  
_can "you" (anyone) post some pics of what your oscope shows when you connect the probes like this? *makes you wonder, huh....?*_
​


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## Jump-start (Sep 26, 2012)

curiousB said:


> Yes there is. I had one on my table saw. The starter cap was fried rendering the start windings the same as the main windings. I had to spin the motor then switch it on. Worked fine but was a pain.:laughing::wink:


 :laughing::thumbup:

Good catch! Got me on this one.  

But, for motors not spun by hand have to decide rotation on another 'phase'. Either a capacitor or metal shadings in the stator to cause the first magnetic field to be displaced to establish rotation.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Are you saying that a 240 volt water heater is seeing one big 240 volt sine wave on its input?


Of course. It's a 240 volt transformer. What else would would the heater see??


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## concrete_joe (Oct 6, 2014)

curiousB said:


> CJ,
> 
> I am so disappointed I prepared all the scope views in post #46 in response to your ominous makes "_*you wonder comment*_". Yet complete silence from you.......
> Originally Posted by *concrete_joe*
> ...


silent? dunno. thanks for clarifying using your pic.


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## bernie963 (Dec 18, 2010)

just to add to the confusion: most utilities run high voltage primary with a neutral through a transformer to get 240 center tapped secondary. the old commonwealth electric (now part of Nstar) on Cape Cod and south coastal Massachusetts ran two high voltage primaries and no neutral through a transformer with a pole ground to get 240 center tapped secondary. Is that a two phase system??

bernie


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## Jump-start (Sep 26, 2012)

bernie963 said:


> just to add to the confusion: most utilities run high voltage primary with a neutral through a transformer to get 240 center tapped secondary. the old commonwealth electric (now part of Nstar) on Cape Cod and south coastal Massachusetts ran two high voltage primaries and no neutral through a transformer with a pole ground to get 240 center tapped secondary. Is that a two phase system??
> 
> bernie


 
Sounds like a pole pig connected phase to phase instead of phase to neutral.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Of course. It's a 240 volt transformer. What else would would the heater see??


Well, it's the whole purpose of this thread. Can you tell me where L1 and L2 is located (position wise) in the *one* sine wave. This is a very important question and they must be in a vertical plane, meaning time wise.


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## Desertdrifter (Dec 10, 2009)

FWIW here's and underground 4 bushing transformer and its nameplate.


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## concrete_joe (Oct 6, 2014)

zappa said:


> Well, it's the whole purpose of this thread. Can you tell me where L1 and L2 is located (position wise) in the *one* sine wave. This is a very important question and they must be in a vertical plane, meaning time wise.


its sees L1 at 240v (rms @ 60Hz) higher than L2 and vice versa, otherwise there would be no 240v on the heater. when in time is dependent on when you look.


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## zappa (Nov 25, 2011)

concrete_joe said:


> its sees L1 at 240v (rms @ 60Hz) higher than L2 and vice versa, otherwise there would be no 240v on the heater. when in time is dependent on when you look.


How do you accomplish that with one big 240 volt sine wave. When L1 is at the peak L2 has no potential in a vertical plane. With two sine waves and one inverted, then L1 and L2 are at the peaks during the same time period.

A residential 240 volt transformer as we are talking about here runs balanced with 240 volt loads. Just like balanced audio and digital signals, two signals with one inverted.


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## Daniel Holzman (Mar 10, 2009)

I did a little more research on the history of two phase machines. Two phase was common in the late 1800's, with the phases 90 degrees apart. This was known as quadrature, and is still used for certain specialty applications today, and apparently in some section of Philadelphia, as has been mentioned by others.

The reason the phases are 90 degrees apart is that a two phase motor with each phase 90 degrees apart will self start, just like a three phase motor, because the vector sum of two windings 90 degrees apart creates a rotating magnetic field. The two phase system lost out to the three phase system because the three phase system uses less copper per delivered kilowatt of power, and three phase motors run smoother than two phase motors.

The key point here is that a two phase system with phases 180 degrees out of phase will not self start a motor, because there is no rotating magnetic field. Since electric motors constitute a large percentage of electric use, a two phase system 180 degrees apart was not very useful, and was never built. Hence the term two phase is generally used to describe two phases 90 degrees apart.

The term two phase in general simply means there are two distinct phases that are x degrees out of phase with each other, where x cannot be zero. So a split phase system such as has been discussed in this thread can certainly be called two phase, REGARDLESS of how the two phases were derived. However, by convention, two phase generally refers to systems 90 degrees out, so I suppose the term split phase was invented to distinguish the 180 degree phase shift from the "traditional" 90 degree phase shift.

As to how a single phase transformer can generate two phases that are 180 degrees apart, the center tap essentially reverses the polarity on one phase, which is mathematically identical to delivering two distinct phases 180 degrees apart. There are all sorts of ways to shift phases, and some really odd devices like Scott transformers that can convert two phase to three phase (they not only shift phase, they add a phase). Too many ways to shift phase to mention them all, but my conclusion is that the U.S. split phase system could be called two phase, but is not for historical reasons. Enjoy the debate.


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## curiousB (Jan 16, 2012)

Daniel Holzman said:


> ... The two phase system lost out to the three phase system because the three phase system uses less copper per delivered kilowatt of power, and three phase motors run smoother than two phase motors...


I would also add with a polyphase power system when N>=3 the power delivered (on a balanced load) is a constant and not a sinusoid. This is good for the generators feeding the network and, as you state, results in smoother motors.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> How do you accomplish that with one big 240 volt sine wave. When L1 is at the peak L2 has no potential in a vertical plane. With two sine waves and one inverted, then L1 and L2 are at the peaks during the same time period.
> 
> A residential 240 volt transformer as we are talking about here runs balanced with 240 volt loads. Just like balanced audio and digital signals, two signals with one inverted.


Nope.
How do you get two inverted sine waves when the center tap is not even in the picture?

You're thinking about the center tap again. Remove it from the equation because it is totally moot in a 240vac example. What you have is a simple 240 volt transformer no different from the concept of a 12 volt transformer or a 24 volt one


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## ddawg16 (Aug 15, 2011)

Here is a good analogy for the performance issue of 2-ph.....

It's about par to a 3 cyl engine running on only 2 cylinders....


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## zappa (Nov 25, 2011)

Daniel Holzman said:


> I did a little more research


Nice writeup but I don't agree with your last paragraph. The neutral center tap has nothing to do with the inverted phase. A straight 240 volt transformer will do the same thing. The secondary is simply following the primary with each leg leg going off in a different direction. Peak, null, peak, null over and over again occurring at the same time.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Nope.
> How do you get two inverted sine waves when the center tap is not even in the picture?
> 
> You're thinking about the center tap again. Remove it from the equation because it is totally moot in a 240vac example. What you have is a simple 240 volt transformer no different from the concept of a 12 volt transformer or a 24 volt one


I have never mentioned the center tap. It has nothing to do with this discussion.


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## joecaption (Nov 30, 2011)

Is there really any reason this whole thing has not been deleted?


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## Bob Sanders (Nov 10, 2013)

zappa said:


> I have never mentioned the center tap. It has nothing to do with this discussion.


Once per cycle in a SINGLE 240 volt sine wave the top portion will peak at 120 and the bottom portion will peak at -120. That difference in potential totals 240 volts. This happens (here) 60 times per second. That's AC current.


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## concrete_joe (Oct 6, 2014)

Daniel Holzman said:


> As to how a single phase transformer can generate two phases that are 180 degrees apart, the center tap essentially reverses the polarity on one phase, which is mathematically identical to delivering two distinct phases 180 degrees apart. There are all sorts of ways to shift phases, and some really odd devices like Scott transformers that can convert two phase to three phase (they not only shift phase, they add a phase). Too many ways to shift phase to mention them all, but my conclusion is that the U.S. split phase system could be called two phase, but is not for historical reasons. Enjoy the debate.


so, simply by adding a wire to the center of the coil you get the two halves to be opposite of each other? to be opposite, doesnt this mean the magnetic field would have to be opposite on each half? not sure how that can happen if the primary winding creates a single magnetic field that flips 120x/sec. i am a tad confused, can you explain this part for me.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Once per cycle in a SINGLE 240 volt sine wave the top portion will peak at 120 and the bottom portion will peak at -120. That difference in potential totals 240 volts. This happens (here) 60 times per second. That's AC current.


The key information here is that they don't occur at the same time, hence no potential, no power.


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## concrete_joe (Oct 6, 2014)

hmmm, so at any point in time where do you get a diff of 240 if top peaks at 120 and bottom peaks at -120 some time later? wouldnt the amplitude of this sine wave need to ±peak at 240(rms) ??


Bob Sanders said:


> Once per cycle in a SINGLE 240 volt sine wave the top portion will peak at 120 and the bottom portion will peak at -120. That difference in potential totals 240 volts. This happens (here) 60 times per second. That's AC current.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Once per cycle in a SINGLE 240 volt sine wave the top portion will peak at 120 and the bottom portion will peak at -120. That difference in potential totals 240 volts. This happens (here) 60 times per second. That's AC current.


This magical inverted extra phase that we are talking about is only apparent to the load attached to it and an oscilloscope. It just happens, but you need to view it on a scope with 2 probes to understand it. There is nothing difficult here.


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## zappa (Nov 25, 2011)

concrete_joe said:


> hmmm, so at any point in time where do you get a diff of 240 if top peaks at 120 and bottom peaks at -120 some time later? wouldnt the amplitude of this sine wave need to ±peak at 240(rms) ??


Even if it peaked at 1000 volts there would still be no power produced. It has to peak high and low at the same time.


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> hmmm, so at any point in time where do you get a diff of 240 if top peaks at 120 and bottom peaks at -120 some time later? wouldnt the amplitude of this sine wave need to ±peak at 240(rms) ??


You *never* get opposing peaks to hit the plus peak in minus peak at the same time. That holds true for 24 volts ac 120ac 1200ac... etc.

The ENTIRE sine wave from one peak to the other is considered the total voltage. The two halves don't have to peak at the same time.... and don't.

The picture I noted above has shaft rotation of a generator beside it. One sine wave from 0 to +peak to 0 to -peak and back to 0 is one shaft rotation.

(It should be noted we're talking rms here)

In a 120 volt sine wave once per cycle the top half peaks at 60 volts and the bottom half peaks at -60 1/2 a cycle later. That's ac current


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## zappa (Nov 25, 2011)

Bob Sanders said:


> You *never* get opposing peaks to hit the plus peak in minus peak at the same time. That holds true for 24 volts ac 120ac 1200ac... etc.
> 
> The ENTIRE sine wave from one peak to the other is considered the total voltage. The two halves don't have to peak at the same time.... and don't.
> 
> ...


Bob, are you familiar with balanced audio, telephone, data?


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## ddawg16 (Aug 15, 2011)

I think a lot of people are overlooking something......

Reference.

How do you turn a +24 Vdc supply into a -24Vdc supply? Reverse the leads.

How do you make a 24Vdc supply and all you have is 24Vdc supplies? Put them in series. The - of the second supply connects to the + of the 1st one.....now you have 48Vdc.

If you are down in a trench......everything is above you. If you're on the hill, everything is below you.

It's all relative....to something


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Bob, are you familiar with balanced audio, telephone, data?


I'm not a pro in the area but I know the basic concepts. You can't have a balance if the center tap is not involved... which it isn't in a 240volt load


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## zappa (Nov 25, 2011)

zappa said:


> It boils down to what point you are using as a *reference*. In your example, say L1 and L2 are starting out at zero volts. A short period later (at the exact moment in time) there will be say a 50 volt potential, 25 L1 and 25 L2. Eventually a 240 volt potential, 120 L1 and 120 L2. Looking at this on a scope, it will seem like 2 inverted waveforms which is a true picture of what is happening electrically.
> 
> If you decide to ground one of the hot legs, and call that your zero reference, then only the other leg will gain in voltage. This will produce one 240 volt waveform, similar to looking at one hot leg and neutral.
> 
> To better see what is happening electrically in the second scenario, monitor the ground (neutral) with a current probe and that inverted waveform will return. One showing voltage, the other showing current.





ddawg16 said:


> I think a lot of people are overlooking something......
> 
> Reference.
> 
> ...


Like this? Way back on post 39. :whistling2:


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## Desertdrifter (Dec 10, 2009)

Getting back to the phase question. The time of the sinewave is constant as well as the oscillation or frequency. So the 120v waves will peak above and below at the same points. Just the amplitude will be half of the 240v sine wave. I suspect a sinewave from L1 to N and one from N to L2 will be fairly the same and except for amplitude would match the 240v. And all would sync up. Otherwise I couldn't parallel the 2 secondary coils on the transformer when creating a 120/208 bank. 
Just my thinking.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> I'm not a pro in the area but I know the basic concepts. You can't have a balance if the center tap is not involved... which it isn't in a 240volt load


Ok, I though it would be easier for you to grasp if you knew balanced circuits well. The transformer is operating balanced for 240 volt loads. Again, nothing to do with the center tap. It's just there for unbalanced 120 volt loads.


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## curiousB (Jan 16, 2012)

The answer was a long time ago.


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## ddawg16 (Aug 15, 2011)

zappa said:


> Like this? Way back on post 39. :whistling2:





curiousB said:


> The answer was a long time ago.


I was trying the Common Core method.....

Explain it in a weird way and maybe it will be clear.


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## Desertdrifter (Dec 10, 2009)

curiousB said:


> The answer was a long time ago.


If that's truly the answer on a single phase transformer, how can I parallel the secondary coils to be a single 120v output? 

It's kinda like a car battery, I can parallel them for more power or series them for more voltage.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Ok, I though it would be easier for you to grasp if you knew balanced circuits well. The transformer is operating balanced for 240 volt loads. Again, nothing to do with the center tap. It's just there for unbalanced 120 volt loads.


For me to grasp?
LOL!

Look... in order to create a balance (two opposing peaks at the same time) what you basically need are two windings firing 180 degrees from each other. You then need to take the top half of one wave and add it to the bottom half of the other.

That is not what is happening at the 240 volt level. The load sees the transformer as a SINGLE winding, therefore a SINGLE 240 volt wave.


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## zappa (Nov 25, 2011)

zappa said:


> *ddawg's* MIDDLE image and *curiousB's* image below, especially the blue and red lines, are the ones you guys should be paying attention to.


Post 32...


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## zappa (Nov 25, 2011)

I have run out of ways to explain it. Anyone want to order a pizza? I like any meats and extra cheese.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> I have run out of ways to explain it. Anyone want to order a pizza? I like any meats and extra cheese.


No no.

You said these are balanced... so prove your point. If I'm wrong I will most surely offer my apologies... but I'm not


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## Jump-start (Sep 26, 2012)

Daniel Holzman said:


> I did a little more research on the history of two phase machines. Two phase was common in the late 1800's, with the phases 90 degrees apart. This was known as quadrature, and is still used for certain specialty applications today, and apparently in some section of Philadelphia, as has been mentioned by others.
> 
> The reason the phases are 90 degrees apart is that a two phase motor with each phase 90 degrees apart will self start, just like a three phase motor, because the vector sum of two windings 90 degrees apart creates a rotating magnetic field. The two phase system lost out to the three phase system because the three phase system uses less copper per delivered kilowatt of power, and three phase motors run smoother than two phase motors.
> 
> ...


 
:thumbup:

Perfect explanation:thumbup: 

And, to make things even more interesting: 3 center tapped transformers can turn 3 phase into 6 phase!


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## Jump-start (Sep 26, 2012)

And a question to concrete joe: If a center tap transformer is single phase, then why isn't a 6 phase secondary called triple split phase? 

http://www.vias.org/matsch_capmag/matsch_caps_magnetics_chap6_12_07.html


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## zappa (Nov 25, 2011)

Well gee Bob....I'm at the bottom of my bag-O-tricks and there is nothing left. Ok..OK...one more try. You have a simple POCO transformer, or even a small signal transformer. FORGET the grounded neutral for a moment. The POCO side is unbalanced because the neutral is grounded. To make that balanced, pass it through a transformer and don't ground L1 and L2. Doesn't that automatically make the signal balanced?


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## zappa (Nov 25, 2011)

Jump-start said:


> And a question to concrete joe: If a center tap transformer is single phase, then why isn't a 6 phase secondary called triple split phase?
> 
> http://www.vias.org/matsch_capmag/matsch_caps_magnetics_chap6_12_07.html


:sleeping: :sleeping: :sleeping:


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## Desertdrifter (Dec 10, 2009)

The explanation for single phase transformers reversing one coil doesn't really work. A center tap doesn't change the windings of the second half of the coil. The coil is a whole. I doubt you could explain how the mutual inductance magically imposes and opposite field on the second half of the same coil.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> To make that balanced, pass it through a transformer and don't ground L1 and L2. Doesn't that automatically make the signal balanced?


No. It makes it your average transformer.

With a 240 volt center tapped transformer I can create a balanced 120 volt curcuit. I use one side of the center tap to push and the other side of the tap to pull (or in other words have two 60 volt peaks, one plus and one minus occur at the same time). 

But for a balanced 240 volt transformer I would need *TWO 240 volt windings* (a 480 volt center tapped transformer). take the top 1/2 of one 240 volt wave (which is +120 volts) and the bottom half of the wave on the other winding (-120 volts). The addition of the two wave is 240 volts balanced.


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## ddawg16 (Aug 15, 2011)

Actually, most of the transformers I've seen actually have two secondary windings. 

Depending on how I strap/wire it I can get

1. 240Vac only
2. 120Vac at double the current
3. 120/240Vac
4. Short it and let lots of smoke out


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## curiousB (Jan 16, 2012)

ddawg16 said:


> Actually, most of the transformers I've seen actually have two secondary windings. Depending on how I strap/wire it I can get 1. 240Vac only 2. 120Vac at double the current 3. 120/240Vac 4. Short it and let lots of smoke out


This isn't that complicated. Imagine a center tapped secondary where the center tap comes out in plain view. Now snip the wire to break the center tap. Now you have too identical windings on the secondary unconnected. Reconnect it and you have a 120/240 split phase output (most common), put then in parallel (ensuring the snipped windings do not get connected back to each other) and you get 120vac only but at double the current output.


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## Jump-start (Sep 26, 2012)

zappa said:


> :sleeping: :sleeping: :sleeping:


:laughing: Typical response when ones proven wrong. I doubt he will answer it.


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## Desertdrifter (Dec 10, 2009)

Schematic of a 4 bushing (2 secondary coil) transformer. 

This is an honest question. Given the real life example. If the 2 coils as shown are out of phase when connected for 240. How can I parallel them at the leads, while physical relationship of the coils does not change? 

The explanation of the center tap reversing the direction of the second coil or second half of one is like trying to connect 2 batteries in series tying them at a common terminal.


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## micromind (Mar 9, 2008)

A transformer is constructed of two or more windings and an iron core around which they are wound. 

There can be only one phase on a core, not more than one, no matter how many windings or taps. 

In a 3Ø transformer, all 3 cores can be connected together, but there are 3 cores and 3 sets of windings and it's still one phase per core. Even an open delta transformer, which has only two cores and two sets of windings still has only one phase per core. 

Therefore, if the transformer has only one core, it's single phase no matter how you look at it.


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## ddawg16 (Aug 15, 2011)

micromind said:


> A transformer is constructed of two or more windings and an iron core around which they are wound.
> 
> There can be only one phase on a core, not more than one, no matter how many windings or taps.
> 
> ...


Yep....

Like this....


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## bernie963 (Dec 18, 2010)

ddawg, you mentioned the"common core" method of logic to solve this problem. First, that will set us back to the beginning of time and second, we might as well make a movie about "transformers"!!!

This forum is an education all to itself, I never stop learning, and laughing.

bernie


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## bernie963 (Dec 18, 2010)

Jump-start, What is a "polepig" ? Is it just a pole mounted transformer with phase to phase primary? On Cape Cod, they do run 3 phase , and use two for this set up. Advantages and disadvantages compared to phase to neutral. Maybe this should be on a new thread?

bernie


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## TimPa (Aug 15, 2010)

clw1963 said:


> Schematic of a 4 bushing (2 secondary coil) transformer.
> 
> This is an honest question. Given the real life example. If the 2 coils as shown are out of phase when connected for 240. How can I parallel them at the leads, while physical relationship of the coils does not change?


 X4 + X2 = L1, X3 + X1 = L2. L1 TO L2 = 120VAC w/double current of single coil. Parallel connections of equally sized coils must be in-phase ( - to -, + to +), serial connections of any sized coils are typically out of phase (- to +, - to +).


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## Desertdrifter (Dec 10, 2009)

TimPa said:


> X4 + X2 = L1, X3 + X1 = L2. L1 TO L2 = 120VAC w/double current of single coil. Parallel connections of equally sized coils must be in-phase ( - to -, + to +), serial connections of any sized coils are typically out of phase (- to +, - to +).


I wasn't asking advice on how to hook up the transformer. I do it. The question was about split phase and the rest of the discussion taking place. I've found myself on both sides of the fence at this point. 

If the physical relationship of the secondary coils remains constant and if in series they are out of phase, by 180 degrees, why does paralleling work just by changing the lead connections? Wouldn't the coils want to buck against each other if that's truly the case? 

Isn't the fact they can be paralleled evidence that each 120 leg is just a smaller version of the whole and the reason you get 240 has nothing to do with the 120 legs being out of phase with each other or anything else but merely because its the "whole" of the two? 










This earlier picture represents why I can successfully parallel coils. IMO. 
Current flows the same direction, in sync and that's why the coils can be paralleled. This is what leads me to believe the two sides of the center are just smaller version of the whole.


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## TimPa (Aug 15, 2010)

clw1963 said:


> I wasn't asking advice on how to hook up the transformer. I do it. sorry, i took your question literally. The question was about split phase and the rest of the discussion taking place. I've found myself on both sides of the fence at this point.
> 
> If the physical relationship of the secondary coils remains constant and if in series they are out of phase, by 180 degrees, why does paralleling work just by changing the lead connections? isn't changing the connections how we change any circuit configuration.Wouldn't the coils want to buck against each other if that's truly the case? they have to the same exactly - # of turns, wire gauge, etc.
> 
> ...


 ......


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## concrete_joe (Oct 6, 2014)

so question. if L1 and L2 are 180 shifted (+120 and -120 at their peaks at the same time), then please explain how in this pic (previously posted) how the current from each leg is flowing in same direction on the N wire ??? electrons flow from "-" to "+", if the voltages are 180 apart (but phase sync'd, because afterall it is only a single phase) then the currents must also be phase shiftfed too. to have current flow like this L1A and L1B would both need to be -120V.


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## zappa (Nov 25, 2011)

If anyone is still interested I took some pictures. Like I mentioned in post #88, I'm not saying that this is an extra phase being produced, but more like a balanced circuit with one signal inverted. This is what a 240 volt single phase load would see.

This is a small power transformer set up just like a single phase residential. The white alligator clip is the neutral grounded back to the input. The scope probes are monitoring L1 and L2. Notice the relationship of the 2 waveforms and how they peak and null together but with a 180 degree phase shift. These waveforms can be added together and produces voltage.




















As a comparison I inverted half of the secondary winding. Notice how the waveforms are occurring at the same time with no shift and generates basically no voltage. (6 millivolts)


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## concrete_joe (Oct 6, 2014)

zappa said:


> These waveforms can be added together and produces voltage.


but (-120) + (+120) = 0 




zappa said:


> As a comparison I inverted half of the secondary winding.


you physically flipped one coil around? how so? the windings are physically the same direction no matter how you connect their ends, etc. to get reversed induction you need to physically flip the coil.

lets not forget the basics
http://www.youtube.com/watch?v=r7kp8MJRmw0


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## zappa (Nov 25, 2011)

concrete_joe said:


> but (-120) + (+120) = 0
> 
> 
> 
> ...


It's a potential between the two voltages. Not physically flipped. Look at how the black alligator clip has been moved.


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## Desertdrifter (Dec 10, 2009)

So this is what the coils look like before and if parallel with the probes connected the same orientation L1 to N and N to L2? 

I think the issue is trying to justify voltage in a sinewave relationship between two halves of the coil when that's not the case. It's based on how much of the coil you are reading. That's why a floating neutral gives whacked readings but always adding up to full coil voltage. If you read at 1/4 or 3/4 point you'd get 180/60 or 60/180. All about the amount of coil being read. 
If there was truly an inverted phase relationship, you'd have issues paralleling coils. Schematically and in practice a center tap coil and 2 coils in series are the same. In fact 3 bushing (center tap) pots are routinely internally changed so you can make a 120/208 overhead bank.


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## concrete_joe (Oct 6, 2014)

ah, makes more sense now...... adding these two yields the whole coil, (+120)+(+120)=+240, and (-120)+(-120)=-240. makes perfect sense on how it really works.


clw1963 said:


> So this is what the coils look like before and if parallel with the probes connected the same orientation L1 to N and N to L2?
> 
> I think the issue is trying to justify voltage in a sinewave relationship between two halves of the coil when that's not the case. It's based on how much of the coil you are reading. That's why a floating neutral gives whacked readings but always adding up to full coil voltage. If you read at 1/4 or 3/4 point you'd get 180/60 or 60/180. All about the amount of coil being read.
> If there was truly an inverted phase relationship, you'd have issues paralleling coils. Schematically and in practice a center tap coil and 2 coils in series are the same. In fact 3 bushing (center tap) pots are routinely internally changed so you can make a 120/208 overhead bank.


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## zappa (Nov 25, 2011)

clw1963 said:


> So this is what the coils look like before and if parallel with the probes connected the same orientation L1 to N and N to L2?
> 
> I think the issue is trying to justify voltage in a sinewave relationship between two halves of the coil when that's not the case. It's based on how much of the coil you are reading. That's why a floating neutral gives whacked readings but always adding up to full coil voltage. If you read at 1/4 or 3/4 point you'd get 180/60 or 60/180. All about the amount of coil being read.
> If there was truly an inverted phase relationship, you'd have issues paralleling coils. Schematically and in practice a center tap coil and 2 coils in series are the same. In fact 3 bushing (center tap) pots are routinely internally changed so you can make a 120/208 overhead bank.


Transformers confuse me. I just switched the winding to show the difference in phase relationship.


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## concrete_joe (Oct 6, 2014)

zappa said:


> Transformers confuse me. I just switched the winding to show the difference in phase relationship.


thats just it, you didnt change the phase at all, all you did was flip the voltage over, an apparent phase shift, but its not. they are phase-sync'd and both start at zero, cross zero, and end at zero all at the same time.....

all you really get is increased voltage as you walk across the coil, thats it.


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> so question. if L1 and L2 are 180 shifted (+120 and -120 at their peaks at the same time),


It's not. What Zappa doesn't seem to understand is the you need two windings to accomplish the task of having peaks oppose at the same time.... and although there are two 120volt windings in a 240 volt center tapped transformer, There is *ONLY ONE* 240 volt winding.


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> so question. if L1 and L2 are 180 shifted (+120 and -120 at their peaks at the same time), then please explain how in this pic (previously posted) how the current from each leg is flowing in same direction on the N wire ??? electrons flow from "-" to "+", if the voltages are 180 apart (but phase sync'd, because afterall it is only a single phase) then the currents must also be phase shiftfed too. to have current flow like this L1A and L1B would both need to be -120V.


EXACTLY. :thumbup:
Look at the picture. One big 240 (or 220 in this case) sine wave. You can't balance as Zappa is suggesting without 2 windings. There is only ONE 240 volt winding.


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## curiousB (Jan 16, 2012)

concrete_joe said:


> thats just it, you didnt change the phase at all, all you did was flip the voltage over, an apparent phase shift, but its not. they are phase-sync'd and both start at zero, cross zero, and end at zero all at the same time..... all you really get is increased voltage as you walk across the coil, thats it.


Wrong again

A 180 degree phase shift is an inversion. The same as multiplying by -1 or swapping the coil wires. You keep wanting to referring this back to the 3 phase 120 degree phase shifts which it has nothing to do with.


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## concrete_joe (Oct 6, 2014)

curiousB said:


> Wrong again
> 
> A 180 degree phase shift is an inversion. The same as multiplying by -1 or swapping the coil wires. You keep wanting to referring this back to the 3 phase 120 degree phase shifts which it has nothing to do with.


the physics tells the story. its a single mag field which will induce a single induced voltage on the secondary coil, one sine wave for our 60Hz sine wave power generation. hook up the probes any way you like to get funny pictures, its one sine wave. you cant have phase shift between voltages unless you have more than one phase. i am ready to be shown this is wrong.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> It's not. What Zappa doesn't seem to understand is the you need two windings to accomplish the task of having peaks oppose at the same time.... and although there are two 120volt windings in a 240 volt center tapped transformer, There is *ONLY ONE* 240 volt winding.


You are trolling....right???? I switched to the special smoke and mirror time warp probes, just for you Bob. :laughing:


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## Bob Sanders (Nov 10, 2013)

zappa said:


> You are trolling....right???? I switched to the special smoke and mirror time warp probes, just for you Bob. :laughing:


And I also noticed you're using TWO windings. :thumbsup:

Once again... a 240 volt transformer only has ONE 240 volt winding (I should say the standard 240 volt center tapped transformers as noted above... because they do make them with multiple 240 volt windings)

Are suggesting this picture is lying??


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## concrete_joe (Oct 6, 2014)

curiousB,

please indicate the current flows for these connected loads, just need directional arrows, not the magnitude, etc.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> And I also noticed you're using TWO windings. :thumbsup:
> 
> Once again... a 240 volt transformer only has ONE 240 volt winding (I should say the standard 240 volt center tapped transformers as noted above... because they do make them with multiple 240 volt windings)


Is this a Canadian thing? Common core maybe? Are you gonna ask me to order a real POCO transformer next? :shifty:


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## ddawg16 (Aug 15, 2011)

Bob Sanders said:


> And I also noticed you're using TWO windings. :thumbsup:
> 
> Once again... a 240 volt transformer only has ONE 240 volt winding (I should say the standard 240 volt center tapped transformers as noted above... because they do make them with multiple 240 volt windings)
> 
> Are suggesting this picture is lying??


If they are being energized from the same AC source, then it will behave as a single 240Vac transformer.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Is this a Canadian thing? Common core maybe? Are you gonna ask me to order a real POCO transformer next? :shifty:


No. I expect you to come up with proof that the above picture is lying... not some high school thing with a couple of small sine waves on a scope. If what you're saying is true then there should be plenty of literature on it. :thumbsup:

Speaking of that... you have shown two sine waves... which anybody can do, but you haven't added them together... and this is where you fall apart.

A 240 volt center tapped transformer has 2 120 volt windings but what you are in essence doing in your flawed logic is adding one entire 120 volt sine wave to the top half of a 240 sine wave and the other entire 120 volt to the bottom half. You CAN'T do that! (you could but it would require some accompanied electronics which a transformer does not have). You can only add top halves with top halves and bottom halves with bottom halves and in the case of a 120 volt sine wave the top half is +60 (not + 120) and the bottom half is -60 (not -120)


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## ddawg16 (Aug 15, 2011)

Simpson 260 VOM....one of the best meters made...but your's are in better shape than mine.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Is this a Canadian thing? Common core maybe? Are you gonna ask me to order a real POCO transformer next? :shifty:


What you are suggesting is this:










And it can certainly be done... but not without two windings of the same voltage.
I can do this at 120 volts with a 240 volt center tapped transformer because there are two 120 volt winding firing 180 degrees from each other, but I can't do it at 240 volts because there is only ONE 240 volt winding.

Now... back to this picture. Have a look at that 240 volt single phase wave and tell me if it looks anything like the wave blue/yellow wave. If you're saying it's lying... then prove it.


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## zappa (Nov 25, 2011)

Hear ya go Bob....I swear that I didn't break the case open and shove another winding in there.


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## Bob Sanders (Nov 10, 2013)

And I can do the EXACT same thing with my scope (use two separate leads and flip one 180 degrees... but you're sidetracking now (I have a feeling it's because you have recognized you're wrong :thumbsup: ) 

I told you... I don't want a glittering sideshow. I want proof that the 240 volt single phase wave depicted in the shot joe brought... is lying. If you can't do that then there is little more to discuss.


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## zappa (Nov 25, 2011)

ddawg16 said:


> Simpson 260 VOM....one of the best meters made...but your's are in better shape than mine.


They are little meters that I have wired up for dedicated tasks. The one with the selector switch is for DC milliamps, the other for AC milliamps.


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## Desertdrifter (Dec 10, 2009)

That's originally DDawgs print and it is at least partially wrong. It shows the neutral amps as being additive. Dawg mentioned it was for a test. So maybe some false readings provided for student to correct. 
The 120 legs should not be opposed. IMO.


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## ddawg16 (Aug 15, 2011)

clw1963 said:


> That's originally DDawgs print and it is at least partially wrong. It shows the neutral amps as being additive. Dawg mentioned it was for a test. So maybe some false readings provided for student to correct.
> The 120 legs should not be opposed. IMO.


The wrong values on the neut were intentional...but I really should fix it.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> And I can do the EXACT same thing with my scope (use two separate leads and flip one 180 degrees... but you're sidetracking now (I have a feeling it's because you have recognized you're wrong :thumbsup: )
> 
> I told you... I don't want a glittering sideshow. I want proof that the 240 volt single phase wave depicted in the shot joe brought... is lying. If you can't do that then there is little more to discuss.


It's impossible for a single sine wave to output power without another reference Bob. What's the current/voltage going to push against? It's gotta have something to push against. Ground, another sine wave, but not air.


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## curiousB (Jan 16, 2012)

concrete_joe said:


> curiousB, please indicate the current flows for these connected loads, just need directional arrows, not the magnitude, etc.


Well I could but it's just the prior voltage charts I posted. The one difference is with the vertical axis divided by 1,000,000. Otherwise identical.


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## curiousB (Jan 16, 2012)

Bob Sanders said:


> What you are suggesting is this: And it can certainly be done... but not without two windings of the same voltage. I can do this at 120 volts with a 240 volt center tapped transformer because there are two 120 volt winding firing 180 degrees from each other, but I can't do it at 240 volts because there is only ONE 240 volt winding. Now... back to this picture. Have a look at that 240 volt single phase wave and tell me if it looks anything like the wave blue/yellow wave. If you're saying it's lying... then prove it.


The lower diagram in wrong. The neutral current from the two motors isn't additive. Only the net difference flows in the neutral which would be 3 A. This is the whole concept around MWBC and, dare I say, split phase residential power systems.


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## Bob Sanders (Nov 10, 2013)

curiousB said:


> The lower diagram in wrong. The neutral current from the two motors isn't additive. Only the net difference flows in the neutral which would be 3 A. This is the whole concept around MWBC and, dare I say, split phase residential power systems.


The lower diagram has nothing to do with the argument at hand. It's the upper diagram in the shot Joe put up which I am focusing on.

Zappa seems to be under the impression a single phase 240 volt wave coming off a 240volt center tapped transformer looks like this (opposing peaks):










I keep trying to tell him it doesn't. it simply looks like any other single phase sine wave like this:


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## Bob Sanders (Nov 10, 2013)

zappa said:


> It's impossible for a single sine wave to output power without another reference Bob. What's the current/voltage going to push against? It's gotta have something to push against. Ground, another sine wave, but not air.


Your logic is completely and totally flawed and you need to go back to your basic electrical books.

The voltage of a sine wave is measured by the difference in potential from peak to peak in one cycle. a 120 volt sine wave looks EXACTLY the same as a 240 volt sine wave, only half its size. You must consider the entire cycle when talking about "pushing off" things and you're only considering 1/2 the cycle which is why it is not making sense in your head.


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## curiousB (Jan 16, 2012)

Bob Sanders said:


> The lower diagram has nothing to do with the argument at hand. It's the upper diagram in the shot Joe put up which I am focusing on. Zappa seems to be under the impression a single phase 240 volt wave coming off a 240volt center tapped transformer looks like this (opposing peaks): I keep trying to tell him it doesn't. it simply looks like any other single phase sine wave like this:


 Zappa is right if your put the scope probes ground leads on neutral and then one probe on one end of the winding and the other at the other end. Then both traces will will move around +/- 120V. One will be phase shifted from the other by 180 degrees. In fact since the tap is neutral and that is bonded at the panel to ground you don't even need to connect the ground clips. They already are referenced to ground. 

You are right if you put the ground lead at one end of the transformer and the probe at the other end. (Although yours won't work because attaching ground to one phase will be a short to ground). If you put scope on an isolation transformer then yours would work. 

So you are both right. You're just talking about a different referee point for the scope. 

It's like Einstein said, it's all relative. The scope measures relative to the ground lead.


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## Bob Sanders (Nov 10, 2013)

curiousB said:


> You are right if you put the ground lead at one end of the transformer and the probe at the other end.


And this is what I keep trying to say! There is no neutral considered in a 240 volt load. So on a 240 volt center tapped transformer the load connects to L1 and L2 which is one big 240 volt sine wave.


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## concrete_joe (Oct 6, 2014)

so if connecting gnd of oscope probe to either L1 or L2 of xformer is a short, than how can you read 240v using one probe if the probe gnd is not isolated?


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> so if connecting gnd of oscope probe to either L1 or L2 of xformer is a short, than how can you read 240v using one probe if the probe gnd is not isolated?


It must be an isolated ground on my scope because I have no issues connecting my probe between L1 and L2... done it many times.


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## curiousB (Jan 16, 2012)

Bob Sanders said:


> It must be an isolated ground on my scope because I have no issues connecting my probe between L1 and L2... done it many times.


Could be. My scope is almost an antique. A lot of newer ones are battery powered so can be easily isolated from earth ground.


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## Bob Sanders (Nov 10, 2013)

My scope connected to a 240 volt welder outlet:










One big 240 volt sine wave.

Now you do have to be careful when using 2 probes because the grounds are not isolated from each other so you can short through them.
(probe is on L1 and ground is on L2)


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## ddawg16 (Aug 15, 2011)

curiousB said:


> Could be. My scope is almost an antique. A lot of newer ones are battery powered so can be easily isolated from earth ground.


Antique? Let me show you an antique










This is not my actual scope...mine is up in the attic


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## Bob Sanders (Nov 10, 2013)

ddawg16 said:


> Antique? Let me show you an antique


Well, the hitachi you see in my pic has got to be at least 30 years old (it's a crt). Haven't used it in a long time but decided to drag it out just to settle this silliness!


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## concrete_joe (Oct 6, 2014)

back in the day that oscope was probably thousands... my owon, a cordless / digital memory recording / 2ch / color tft / 25MHz was something like $216. wow, they have come a long way.


Bob Sanders said:


> My scope connected to a 240 volt welder outlet:
> 
> 
> 
> ...


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> back in the day that oscope was probably thousands... my owon, a cordless / digital memory recording / 2ch / color tft / 25MHz was something like $216. wow, they have come a long way.


Lol!
Yup. I bought that thing new and if I remember correctly it cost around $1600


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## ddawg16 (Aug 15, 2011)

Bob Sanders said:


> Well, the hitachi you see in my pic has got to be at least 30 years old (it's a crt). Haven't used it in a long time but decided to drag it out just to settle this silliness!


My HP was made in the 60's. That makes it about 50 years old.

Single channel...100Khz...and weighs about 50 lbs

Takes 5 min for the CRT to warm up enough to see the trace.

It still works.


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## Desertdrifter (Dec 10, 2009)

zappa said:


> It's impossible for a single sine wave to output power without another reference Bob. What's the current/voltage going to push against? It's gotta have something to push against. Ground, another sine wave, but not air.


You're not serious?


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## zappa (Nov 25, 2011)

Bob Sanders said:


> Your logic is completely and totally flawed and you need to go back to your basic electrical books.
> 
> The voltage of a sine wave is measured by the difference in potential from peak to peak in one cycle. a 120 volt sine wave looks EXACTLY the same as a 240 volt sine wave, only half its size. You must consider the entire cycle when talking about "pushing off" things and you're only considering 1/2 the cycle which is why it is not making sense in your head.





Bob Sanders said:


> And this is what I keep trying to say! There is no neutral considered in a 240 volt load. So on a 240 volt center tapped transformer the load connects to L1 and L2 which is one big 240 volt sine wave.


I didn't use probe grounds for any of these measurements. This is the part that is confusing you. With your logic, you are saying if you hook up *L1 only* to both sides of a 240 volt load it will work. Measure voltage or a waveform on *L1 only* by just using L1 and no other reference. You can't do either one which I'm sure you know. The reason for this is there is no reference or "push point" for the current to do its work.

With one waveform inverted, the energy is able to push off of each other and produce work. If both of them are going up and down in unison there is no potential or energy between the two. You saw that example on my scope when both waveforms were following each other. They produced no voltage, or only 6 millivolts, because there was a slight phase difference.


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## zappa (Nov 25, 2011)

clw1963 said:


> You're not serious?


Grab either L1 or L2 but not both and see what you can make it do.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> I didn't use probe grounds for any of these measurements. This is the part that is confusing you. With your logic, you are saying if you hook up *L1 only* to both sides of a 240 volt load it will work. Measure voltage or a waveform on *L1 only* by just using L1 and no other reference. You can't do either one which I'm sure you know. The reason for this is there is no reference or "push point" for the current to do its work.


I didn't hook up L1 only. I connected the scope across L1 and L2 just as you would any other 240 volt load. The sine wave you see in the scope is EXACTLY what any 240 volt load is seeing... and using. Anything beyond that is coming from your imagination and your flawed logic.


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## Bob Sanders (Nov 10, 2013)

clw1963 said:


> You're not serious?


That's basically what I have been saying to him for the last couple of pages. Maybe you try for a while because it's clear I'm not getting through to him.

I think the problem here is that he doesn't know how to use a scope.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> I didn't hook up L1 only. I connected the scope across L1 and L2 just as you would any other 240 volt load. The sine wave you see in the scope is EXACTLY what any 240 volt load is seeing... and using. Anything beyond that is coming from your imagination and your flawed logic.


You are only seeing half of the picture (L1 or L2 only) by hooking your scope up that way.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> That's basically what I have been saying to him for the last couple of pages. Maybe you try for a while because it's clear I'm not getting through to him.
> 
> I think the problem here is that he doesn't know how to use a scope.


Heeheehee....keep digging that hole deeper Bob. :whistling2:


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## Desertdrifter (Dec 10, 2009)

zappa said:


> Grab either L1 or L2 but not both and see what you can make it do.


You're not understanding what's being observed. 

That sine wave is a visual representation of what a voltmeter reads the AC RMS value is between L1 and L2.


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## Desertdrifter (Dec 10, 2009)

I want to throw out there that as far as the transformer goes the secondary bushings are generally labeled X1 X2 X3. No "N.". 

As I noted before, even on a 3 bushing transformer the internal leads can be changed to parallel 2 halves of the coil.

To get like (apples to apples) readings for the secondary sine wave before and after X2 you would have to measure X1 to X2 then move the lead from X2 to X3 and from X1 to X2. Current/voltage will be flowing the same direction at any point in time. Because hey, they're in series. My guess is IF you over laid all 3 sinewaves, the 2 120v would match near exact and the 240v would peak and zero at the same time as the 120v just with higher amplitude.


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## beenthere (Oct 11, 2008)

Continued personal attacks, will get the thread cleaned up. And some of the post that get removed. May have what you consider important/relevant info.


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## zappa (Nov 25, 2011)

clw1963 said:


> You're not understanding what's being observed.
> 
> That sine wave is a visual representation of what a voltmeter reads the AC RMS value is between L1 and L2.


Why are there 2 separate and distinct waveforms on a scope? One for L1 and the other for L2. Without them being paired together 180* out of phase they are worthless. No potential, no reference, no workie.

Stand on an insulated metal platform with 13.8kv attached to it. Without another phase or ground nearby there is nothing going on. Yet, there is a big sine wave on that cable.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Why are there 2 separate and distinct waveforms on a scope? One for L1 and the other for L2. Without them being paired together 180* out of phase they are worthless. No potential, no reference, no workie.


You're not interpreting (or using) the scope correctly.
If you were to use two probes (1 on L1 and one on L2) what would be seeing is *THE SAME* wave only the other way. In essence you are seeing the same wave from two different angles.


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## Bob Sanders (Nov 10, 2013)

These people are measuring an ac source. Note how they are only using ONE probe.

https://www.youtube.com/watch?v=g0tBJlOEz00

https://www.youtube.com/watch?v=7ywcmziZdd8


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## Desertdrifter (Dec 10, 2009)

zappa said:


> Why are there 2 separate and distinct waveforms on a scope? One for L1 and the other for L2. Without them being paired together 180* out of phase they are worthless. No potential, no reference, no workie.
> 
> Stand on an insulated metal platform with 13.8kv attached to it. Without another phase or ground nearby there is nothing going on. Yet, there is a big sine wave on that cable.


You're a little lost on the subject. Your conclusions are incorrect.


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## zappa (Nov 25, 2011)

Bob Sanders said:


> These people are measuring an ac source. Note how they are only using ONE probe.


Exactly why I asked if you were familiar with balanced circuits. Both of these examples are measuring single ended, unbalanced, referenced to ground signals.

This would be the same as measuring either L1 or L2 to neutral but not L1 to L2.

The picture of you measuring 240 volts can be done also if your probe ground is isolated from system ground. But you are only seeing the voltage potential between L1 and L2 and not the two signals that make up L1 and L2. You are seeing the *green trace* but not the blue or red in curiousB's image.


----------



## busman (Nov 7, 2008)

Bob Sanders said:


> These people are measuring an ac source. Note how they are only using ONE probe.
> 
> https://www.youtube.com/watch?v=g0tBJlOEz00
> 
> https://www.youtube.com/watch?v=7ywcmziZdd8


Those are coaxial inputs. What neither video showed is that there are two probes on the other end of that cable.

http://en.wikipedia.org/wiki/Test_probe

Mark


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## Bob Sanders (Nov 10, 2013)

zappa said:


> But you are only seeing the voltage potential between L1 and L2 and not the two signals that make up L1 and L2. You are seeing the *green trace* but not the blue or red in curiousB's image.


In other words, I'm seeing EXACTLY what a 240 volt load would see if I connected it to L1 and L2. :thumbsup:

You are not using your scope correctly. You do not use 2 probes on a single source to measure voltage and if you do you are simply seeing the *SAME* wave from the other direction.

If I take a volt meter and place the red lead on L1 and the black and on L2 I get a reading of 240 volts. Now if I reverse the leads I get the same reading because I am measuring the *SAME* voltage. The ONLY difference between a meter and a scope is that a scope can pick up on, and draw a picture of the source measured with leads reversed. If you use two probes you are merely seeing the EXACT SAME wave at different times in its cycle. (a volt meter can only read voltage, while a scope can read voltage *AND* *TIME*) What you see in a true balanced system (balanced microphone) are two completely different waves from two different sources used in a "push/pull" senario. A 240 volt center tapped transofrmer only has ONE 240 volt winding and therefore can only generate one 240 volt source. It is not the same thing at all.

A dual trace scope is much like having two voltmeters in the same box so you can measure two *DIFFERENT* sources at the same time.

As for "no worki" if you don't have opposing peaks, sorry man, you are so far off base I don't even know where to begin.


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## Bob Sanders (Nov 10, 2013)

busman said:


> Those are coaxial inputs.
> Mark


I give up.


----------



## busman (Nov 7, 2008)

Bob Sanders said:


> I give up.


It's best to give up when you have no clue what you are talking about. Voltage is a relative measurement that is a difference in potential between two points, period. Even if you were to have a single wire probe, it would still be measuring between that probe and the other side of the oscilloscope circuit, whether anything is connected to it or not. Regardless, please tell me how a coaxial jack has a single conductor.

Mark


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## Desertdrifter (Dec 10, 2009)

Zappa, the two waves you refer to are derived from DIVIDING a 240v sine wave in half not by adding them together to make 240v. For your statement to be true, every straight 240v appliance would need a neutral to work. 

The voltage wave form only demonstrates the voltage characteristic of the line measured. The platform example you give is only to show you need to complete the circuit for CURRENT to flow. (Bird on a wire) In other words the platform has POTENTIAL but needs to complete the circuit to do any meaningful work. It does not need another waveform to complete the circuit. Just a place for current to flow. 

Mark, I think that's what Bob has been pointing out. If you read the latest part of the thread. Apparently according to Z, can't have 240v without having 2 120v legs making it up.


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## Bob Sanders (Nov 10, 2013)

clw1963 said:


> Mark, I think that's what Bob has been pointing out. If you read the latest part of the thread. Apparently according to Z, can't have 240v without having 2 120v legs making it up.


:thumbup::thumbup::thumbup:

In a manner of speaking anyway. I believe what Z is saying in a round about way is that the upper 1/2 of the sine wave is pushing while at the same time the lower 1/2 is pulling... kind of like one person on the front pulling a car with a rope while at the same time someone is on the back pushing..... and that indeed is how things work on a balanced microphone... but you are using two different and opposing sources for that. Let's say I'm going to connect a transformer to a speaker to create a push/pull balance (i'd blow the speaker, but I'm just using this as an example) If I take a 240 volt center tapped transformer and I connect L1 and N to a coil and glue it to one side of the speaker cone, then I take L2 and N and connect to a coil and glue that coil to the opposite side of the cone. I now have a balanced cone operating at 120 volts. One coil will push while the other coil pulls at the same time. What he is not seeing is that I am using two different sources to do that (L1,N) and L2,N)

He's coming to this conclusion because he's using 2 probes (with no ground clip involved according to him) and measuring L1 and L2 and coming up with 2 waves. Of course... if you use two probes on a dual trace scope, you're going to get 2 waves.
What he is not understanding is that he is simply seeing the same wave at a different time in the cycle (180 degrees). He's taking that as 2 different waves, and in his mind laying them on top of each other to produce opposing peaks (or a push/pull in the above speaker example)


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## beenthere (Oct 11, 2008)

clw1963 said:


> Zappa, the two waves you refer to are derived from DIVIDING a 240v sine wave in half not by adding them together to make 240v. For your statement to be true, every straight 240v appliance would need a neutral to work.
> 
> The voltage wave form only demonstrates the voltage characteristic of the line measured. The platform example you give is only to show you need to complete the circuit for CURRENT to flow. (Bird on a wire) In other words the platform has POTENTIAL but needs to complete the circuit to do any meaningful work. It does not need another waveform to complete the circuit. Just a place for current to flow.
> 
> Mark, I think that's what Bob has been pointing out. If you read the latest part of the thread. Apparently according to Z, can't have 240v without having 2 120v legs making it up.


Turn off your main breaker to your house. Disconnect your neutral from the panel box. Now read one leg feeding the switched off main breaker to ground. Do you read 240, or 120 volts.


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## Desertdrifter (Dec 10, 2009)

beenthere said:


> Turn off your main breaker to your house. Disconnect your neutral from the panel box. Now read one leg feeding the switched off main breaker to ground. Do you read 240, or 120 volts.


What's your point?


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## beenthere (Oct 11, 2008)

clw1963 said:


> What's your point?


The legs are each 120 volts. So in reality, you do need two 120 volt legs to make 240 volts.

Its not like we're talking about a 277 volt circuit, which is 1 hot, 1 neutral.


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## Bob Sanders (Nov 10, 2013)

beenthere said:


> The legs are each 120 volts. So in reality, you do need two 120 volt legs to make 240 volts.


Only because you have split a 240 volt transformer in half and measuring one side, and that's why they use a center tapped transformer.... you can do that... but do you NEED a center tapped transformer to generate 240 volts? No. This will do the exact same job in a 240 volt application:


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## Desertdrifter (Dec 10, 2009)

You're mistaken. If that's reality then the open wye open delta 240v bank I just put up with no ground or neutral reference, no center tap connected to anything hasn't been working because there's not 2 referenced center taps to create 4 120v legs to create the 3 240v reference points.

Now in your example, throw a breaker on apply some single leg load. You read 140/100. How does that work? Apply some more load from the same leg, , now your 150/90. Will your 240 load still work?


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## Desertdrifter (Dec 10, 2009)

beenthere said:


> The legs are each 120 volts. So in reality, you do need two 120 volt legs to make 240 volts.
> 
> Its not like we're talking about a 277 volt circuit, which is 1 hot, 1 neutral.


Yeah you are. In reality. Just a 277 2 bushing pot is generally tied on one side together with other pots. But in transformer reality it's the same theory and design characteristics of any other wound transformer. A ratio of coils, mutual inductance and yada yada yada. You could place a center tap on that, ground it and have a 277/138.5 pot. Couldn't you?

Edit- I realize you said circuit, but we have single phase 2 bushing 277 transformers(pots as I've been referring to them), which we can build a pole mounted 480 bank when needed. So since the discussion is about utility provided power I used the 277 transformer as my reference.

Edit 2- How does a residential meter keep track of what power you use with no neutral reference? L1 and L2 stabs only?


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## zappa (Nov 25, 2011)

A little MS paint will fix anything. :yes:

Correct











*Missing reference*. Could someone *PLEASE* tell me how this is going to produce any voltage or current.


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## Desertdrifter (Dec 10, 2009)

See my last edit and explain how the metering works. You're just wrong.


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## zappa (Nov 25, 2011)

clw1963 said:


> See my last edit and explain how the metering works. You're just wrong.


Tell me why I'm wrong.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> *Missing reference*. Could someone *PLEASE* tell me how this is going to produce any voltage or current.


You drew over top of it. It's called the zero mark. A 240 volt sine wave produces +120 volts relative to 0 and then -120 volts relative to 0 a half cycle later for a total potential of 240 volts.

I'll try one more time. I'll make this as simple as I can.
We'll do this in slow motion with a 120 volt example...

If I take a generator with one coil in it and I start turning it from the 0 mark I will begin to produce the upper part of the sine wave. The light connected to the generator starts to light. It gets brighter and brighter until the magnet is perfectly aligned with the coil (1/4 turn of the generator and at this point we are pumping +60 volts relative to 0). As we continue to turn the generator the magnet starts going away from the coil and now we start the downside of the sine wave and the light starts to grow dim. At 1/2 turn of the genny we are back down to 0 volts and the light is off as the pull between the south and north pole of the magnet are now at the same distance from the coil.

Now... as the opposite side of the magnet becomes closer we start generating the same 1/2 wave but in the opposite direction which produces a peak of -60, and the light does the same thing (gets brighter then dimmer and off again) volts relative to 0 (this is a half cycle later). when the generator completes one turn we are back to o volts.

In order to do what you believe is happening we would have to place *TWO* coils in that genny so that one coil is being acted upon by the north pole at the same time as the other coil is being acted upon by the south pole.

I'm sure I've left something out here... but I'm too tired to go back and look!


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## Stubbie (Jan 7, 2007)

I'm a little lost at what the argument is ... 

Zappa's last post is correct in my opinion. The 1st diagram is the diagram you see in every trade book discussing the subject. Two 120 volt legs offset 180 degrees due to the center tap reference or neutral point. Are we saying that the two legs would overlay the same as in the next diagram he shows?


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## zappa (Nov 25, 2011)

Bob Sanders said:


> You drew over top of it. It's called the zero mark.


Zero in reference to what? At this point you are holding one of your probes in thin air expecting to measure something.

Lets make this even simpler and forget generators and transformers for a moment. You are stating that a 240 volt load will see just one big sine wave like my bottom image. Explain how that works so I (we) can understand it.


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## Bob Sanders (Nov 10, 2013)

Stubbie said:


> Two legs offset 180 degrees due to the center tap reference or neutral point.


OMG!
There is no neutral in a 240 application! Take the center tap right out of the equation. It's totally moot! In a 240v application
These two diagrams produce the exact same thing:


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## zappa (Nov 25, 2011)

Stubbie said:


> Are we saying that the two legs would overlay the same as in the next diagram he shows?


Worst than that. The second leg is completely missing according to his theory.


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## Desertdrifter (Dec 10, 2009)

zappa said:


> Tell me why I'm wrong.


If you can't have 240 with out 2 x 120, how can a residential revenue meter, track usage without the neutral or center reference? It's only plugged into L1 and L2.


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## Stubbie (Jan 7, 2007)

Calm down Bob I'm not saying your wrong. I just don't understand. What transformer configuration are we discussing ? You keep saying 240 volt transformer I thought the discussion was about a 120/240 volt utility transformer and the characteristics of the two 120 volt legs. And yes Bob there isn't a neutral for 240 volts in the USA.
So is the argument about the wave characteristics for 240 volts ?

Bob this discussion is very hard to follow for us novel electricians and most have dropped out long ago I'm trying to understand. This discussion interests me.


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## zappa (Nov 25, 2011)

clw1963 said:


> If you can't have 240 with out 2 x 120....


What post? I never said that.


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## Desertdrifter (Dec 10, 2009)

Stubbie said:


> I'm a little lost at what the argument is ...


Yet you thank for a post? 

Should we start over or do you go back and read a bit? 

Need a new thread on transformer theory I guess. 

No disrespect to anyone. It's a frustrating, yet learning (for me anyway) lively discussion.


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## zappa (Nov 25, 2011)

Stubbie said:


> So is the argument about the wave characteristics for 240 volts ?


Yes...


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## Stubbie (Jan 7, 2007)

clw1963 said:


> Yet you thank for a post?
> 
> Should we start over or do you go back and read a bit?
> 
> ...


 
No I think I just drop out like the rest have a good day.


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## Desertdrifter (Dec 10, 2009)

zappa said:


> What post? I never said that.





zappa said:


> It's impossible for a single sine wave to output power without another reference Bob. What's the current/voltage going to push against? It's gotta have something to push against. Ground, another sine wave, but not air.


You're saying there needs to be 2 sine waves to make 240v aren't you?


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## Desertdrifter (Dec 10, 2009)

Stubbie, I meant no offense.


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## zappa (Nov 25, 2011)

clw1963 said:


> You're saying there needs to be 2 sine waves to make 240v aren't you?


240 volts with no ground reference....like what we are talking about.

2 sine waves with a phase difference. 180 degrees in this case, 120 degrees if you are talking 3 phase.


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## Desertdrifter (Dec 10, 2009)

zappa said:


> 240 volts with no ground reference....like what we are talking about.
> 
> 2 sine waves with a phase difference. 180 degrees in this case, 120 degrees if you are talking 3 phase.


So, how does a revenue meter work?

Also, I'll revisit this later. Need a break.


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## zappa (Nov 25, 2011)

clw1963 said:


> So, how does a revenue meter work?
> 
> Also, I'll revisit this later. Need a break.


I'm not sure why you think a meter has anything to do with this discussion. Whether it be 240 loads or 120 loads, all current has to flow through L1, L2, or both. The math, if any, I would have think about more or try to look it up.


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## beenthere (Oct 11, 2008)

Bob Sanders said:


> Only because you have split a 240 volt transformer in half and measuring one side, and that's why they use a center tapped transformer.... you can do that... but do you NEED a center tapped transformer to generate 240 volts? No. This will do the exact same job in a 240 volt application:


Yes it will, And L1 and L2 can't both be positive at the same time.


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## beenthere (Oct 11, 2008)

clw1963 said:


> So, how does a revenue meter work?
> 
> Also, I'll revisit this later. Need a break.


Doesn't matter if your using 120, or 240 volts. The meter is only interested in watts.


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## curiousB (Jan 16, 2012)

Up to page 15 but the answer has always been at the start.


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## zappa (Nov 25, 2011)

beenthere said:


> http://www.nojolt.com/three_phase_and_single_phase_electric.shtml


The answer was here also. I totally blew by it without clicking on the link.


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## Desertdrifter (Dec 10, 2009)

curiousB said:


> Up to page 15 but the answer has always been at the start.


I believe that answer is correct relationship wise, but I also wonder if it's not an accurate representation of what's actually happening.

I believe it's possible for the different views to be correct. What it comes down to the the method and standard of how we test.

If I place probe B on the centertap of the transformer and probe A on L1 you'll get split phase 1, then move probe A to L2 you'll get split phase 2, correct? I believe that's how it's being represented and that may be the standard to observe the relationship.

If I place probe A on the center tap and probe B on L2, would that generate a sine wave similar to split phase 1?


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## zappa (Nov 25, 2011)

clw1963 said:


> I believe that answer is correct relationship wise, but I also wonder if it's not an accurate representation of what's actually happening.
> 
> I believe it's possible for the different views to be correct. What it comes down to the the method and standard of how we test.
> 
> ...


Is the center tap that you are speaking of the grounded neutral? Big difference in the way you measure it grounded vs. ungrounded. If it's grounded you only use one probe and get one waveform.


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## Desertdrifter (Dec 10, 2009)

CuriousB, what are the equations on the picture here?


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## curiousB (Jan 16, 2012)

Well I am almost afraid to comment for fear of setting this into another 15 page rant.:laughing:

The equations are shown. They are the phase to neutral voltage (120VAC) of each half of the split phase power found in most homes.

Vp is the peak voltage (I said it was 120 volts but it is really closer to 160 but I didn't want to take us off tangent with an RMS vs peak voltage debate).

So Vp Sin (wt) is the equation for that voltage at any point in time where t is time in seconds. W is the greek letter omega but isn't a valid character here so I just used w. w= 2 * Pi * f where f is frequency in cycles/second. So you could rewrite the equation to V=Vp* Sin (2*Pi *60 * t) where t is in seconds.

The second equation is the other phase relative to neutral and since it is phase shifted 180 degrees there is the + Pi component inside the Sin ( )

The last equation is just subtracting the first equation from the second. This is the phase to phase voltage (240 VAC).


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## dmxtothemax (Oct 26, 2010)

Flogging a dead horse !


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## Desertdrifter (Dec 10, 2009)

Don't worry your little head off DMX. I'm not flaming anyone or demanding I'm right. Just trying to vet out some things.

If you read my posts on this subject, I began thinking split phase. Then some of the posts gave me pause to consider my trade and how my transformer hook ups are done by habit. We have a transformer tech that TTRs the pots and says they're good, then I just hook them up because I know how from doing them all these years. I started considering some things and sought some answers to recall what I had forgotten and to learn some things new. Right now I just want to see how this works out. My only intention is to learn. Hopefully others will have an open mind as well. We are all passionate about our trades. I see the pros trading info about code to learn from one another. Who knows what will happen here. 

CuriousB, so, is the only reason we see that 180 degree shift (pi), because of how the test probes are hooked up relative to S1?


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## sparky90 (Aug 1, 2014)

Hey guys, this has gone too far and too many people have brought in too many obscure relationships that most people don't understand. The POCO transformer is a single phase 240v center tapped transformer. It is not a 2 phase transformer it is single phase center tapped. 2 120v potentials are derived from L1 to CT and the other 120v from L2 to CT then across l1 l2 you get 240v. I think over the years, electricians especially use the term "phases" to represent L1 and L2. Phases is not the right term but serves it's purpose only to identify L1 from L2 to keep things straight. It would be better if electricians used Leg A and Leg B instead of phase A and phase B because too many young upcoming electricians think they re really phases and they are not. They are Legs of the 240 volt transformer in relation to the CT only. Not 2 phase, not split phase, single phase 240v center tapped. Period end of story. Now here is a question that many electricians get wrong, who have been electricians for many years and they get this question wrong. If I have a 14-3 wire on a shared neutral circuit and I put a 100 watt light bulb on the black wire circuit and a 100 watt light bulb on the red wire circuit. What will happen to the light bulbs if I cut the neutral wire at the panel? DO not give me explanations. Just give me an answer. Let's see what answer we get. Here are your choices. If I cut the neutreal wire the following will happen.
A. Both bulbs get very bright and the bulbs burn out
B. Both bulbs get very dim
C. Both bulbs go out
d. One bulb goes out
E one bulb gets bright and the other dim
F. Nothing happens

Your answer please


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## beenthere (Oct 11, 2008)

Nothing happens.


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## zappa (Nov 25, 2011)

Don't know why you're asking this question in this thread.

F. Nothing happens


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## sparky90 (Aug 1, 2014)

zappa said:


> Don't know why you're asking this question in this thread.
> 
> F. Nothing happens


Because surprisingly as many people who do not understand the original discussion don't get this right either


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## ddawg16 (Aug 15, 2011)

You left out a few details.

Do both bulbs have exactly the same resistance?

Are the wires exactly the same distance?

Do you have good connections?

Has the load center been defluxed and degaused?

Is the flux capacitor properly installed?



Yea....nothing happens.:wink:


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## TimPa (Aug 15, 2010)

assuming they are balanced circuits, they will both remain illuminated. good question.


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## Daniel Holzman (Mar 10, 2009)

If you lose the neutral in a MWBC you will get 240 volts across the two bulbs, which will cause them to turn very bright or burn out.


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## Desertdrifter (Dec 10, 2009)

Thanks for your input sparky90.

I find nothing wrong in your statement. 

But for the sake of GOOD discussion I wanted to pursue the answers to the questions in a civil way. 

If someone wants to post a graphic. Lets understand what it truly represents and learn.


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## zappa (Nov 25, 2011)

Daniel Holzman said:


> If you lose the neutral in a MWBC you will get 240 volts across the two bulbs, which will cause them to turn very bright or burn out.


240 volts in series with two 120 volt bulbs so nothing different would happen.


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## rjniles (Feb 5, 2007)

Daniel Holzman said:


> If you lose the neutral in a MWBC you will get 240 volts across the two bulbs, which will cause them to turn very bright or burn out.


Nope. Since both bulbs are the same wattage / resstance, they continue to drop 120 volts each and stay lighted at the same brightness.

Nothing happens. If they were different wattages, the game changes.


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## curiousB (Jan 16, 2012)

Yes. This us why when you have a suspect main neutral connection in a panel the voltage on each leg of the split phase can vary. Moreover they vary in concert with each other. If one goes down 6 volts the other goes up the same amount. This is due to the difference in the load on each leg, without a good neutral the neutral line floats.


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## sparky90 (Aug 1, 2014)

Daniel Holzman said:


> If you lose the neutral in a MWBC you will get 240 volts across the two bulbs, which will cause them to turn very bright or burn out.


Sorry Dan, in the question presented you are wrong.


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## sparky90 (Aug 1, 2014)

clw1963 said:


> Thanks for your input sparky90.
> 
> I find nothing wrong in your statement.
> 
> ...


Is DC 2 phase? This can be better understood if we look at 2 car batteries in series. Take 2 car batteries. put them in series, where the 2 tie together that is the Center tap, just like the POCO transformer. You read 12 volts from either terminal to CT on the ends, you get 24 volts. Take the center tap away and you have a single 24 volt battery. Take the center tap away from the POCO transformer and you have a single 240v transformer.


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## Desertdrifter (Dec 10, 2009)

sparky90 said:


> Is DC 2 phase? This can be better understood if we look at 2 car batteries in series. Take 2 car batteries. put them in series, where the 2 tie together that is the Center tap, just like the POCO transformer. You read 12 volts from either terminal to CT on the ends, you get 24 volts. Take the center tap away and you have a single 24 volt battery. Take the center tap away from the POCO transformer and you have a single 240v transformer.


Funny you should use that example. I drew it out last week and discussed it with someone here. As I said I find nothing wrong with what you posted. If fact, here's one of my drawings illustrating the graphic I was referring too. 








IMO the last example is best example of what the graphic is showing is.


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## Jump-start (Sep 26, 2012)

I know this is slightly off topic, but it has to do with 120/240 and open neutrals. Both videos explain it really well imo:



http://www.youtube.com/watch?v=O-Q0M32puGY




http://www.youtube.com/watch?v=HsSlQnGCvdg


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## concrete_joe (Oct 6, 2014)

sparky90 said:


> Is DC 2 phase? This can be better understood if we look at 2 car batteries in series. Take 2 car batteries. put them in series, where the 2 tie together that is the Center tap, just like the POCO transformer. You read 12 volts from either terminal to CT on the ends, you get 24 volts. Take the center tap away and you have a single 24 volt battery. Take the center tap away from the POCO transformer and you have a single 240v transformer.


but if you put the "neg" probe of meter on the center you will read +12 on one side and -12 on the other. if you match the probe leads per bat terminal you will get +12 and +12, etc. flip the bats over (keep probes as-is) and you get -12 and -12.

lets ask another Q. if you lift poco neutral between transformer and service panel, will you get a jolt if you hang onto the poco neutral and service panel gnd ???


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## curiousB (Jan 16, 2012)

concrete_joe said:


> lets ask another Q. if you lift poco neutral between transformer and service panel, will you get a jolt if you hang onto the poco neutral and service panel gnd ???


I wouldn't suggest trying it but if the loads in the house at that moment are balanced on each feed (I am avoiding using the polarizing term phase) then the neutral voltage will be zero and there should be not differential to the transformer neutral. That would require balance which is not very likely. 

Even if they weren't balanced loads a loose neutral might only cause neutral to drift a few volts one way. So the difference from the transformer neutral might only be say, 8 volts. That might not give you a jolt either.

As an example. Put 1500Watts of load on one leg and 2300W on the other leg. Lets say resistive load to keep it simple (avoid reactive issues of capacitive or inductive loads). Voltage on each leg will be 120VAC with neutral working. The currents of each leg will be different due to different loads. The neutral will carry the difference of the two leg loads.

Remove the transformer neutral connection and the house neutral will float such that current in each leg is the same (because you have no neutral for difference current to flow through). The heavier loaded side @2300W will now have 95VAC across it the and the 1500W side will now have 145VAC across it. This is how electronics products get damaged with a bad neutral. They can get stressed by excess voltage.


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## Jump-start (Sep 26, 2012)

Here are a few:

http://www.mikeholt.com/documents/calculations/formulas/neutral3wire.xls

http://www.mikeholt.com/mojonewsarc.../Open-Service-Neutral-Protection~20050725.php


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## Jump-start (Sep 26, 2012)

Off the net, just for giggles 2nd link in post 16:

https://soundforums.net/threads/4812-Why-An-Open-Neutral-Kills-120V-Devices?


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## sparky90 (Aug 1, 2014)

I worked for one electrical contractor who never used shared neutral circuits for fear if the neutral ever got compromised and there was expensive electronic equipment on the one side that the voltage would rise on, the equipment would be destroyed. Using a single neutral per feed eliminates the danger of blown up appliances should the neutral be compromised. On the other hand I worked for 2 other contractors that used shared neutral circuits all the time and the panels are loaded with them. Of course we have no control of the incoming neutral from the service if that goes open, then the entire electrical system is compromised in the whole house. When doing wiring in my own home I stick to a single neutral per run with no sharing but if the boss says run a shared neutral I do it. I also tend to spin the red and black wires from a shared neutral circuit right up to the breakers hoping that the next guy in there will realize to keep those wires on different legs. I am actually in favor of the code change requiring DP breakers on those circuits.


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## rjniles (Feb 5, 2007)

Current code calls for handle ties ( or 2 pole breaker).


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## concrete_joe (Oct 6, 2014)

shared neutral?? does it really matter in the feeds? its all shared neutral at the service panel to poco. home runs to each endpoint makes no diff if poco neutral (and ground) lift off.


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## beenthere (Oct 11, 2008)

Daniel Holzman said:


> If you lose the neutral in a MWBC you will get 240 volts across the two bulbs, which will cause them to turn very bright or burn out.


Since they are in series. The load resistance has doubled. So they won't get any brighter or hotter then they were before the neutral was disconnected/cut.


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## zappa (Nov 25, 2011)

concrete_joe said:


> shared neutral?? does it really matter in the feeds? its all shared neutral at the service panel to poco. home runs to each endpoint makes no diff if poco neutral (and ground) lift off.


Which post are you commenting on?


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## rjniles (Feb 5, 2007)

If the POCO neutral goes open but the panel neural remains bonded to ground, will that prevent all the 120 branches from seeing the full 240?


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## zappa (Nov 25, 2011)

rjniles said:


> If the POCO neutral goes open but the panel neural remains bonded to ground, will that prevent all the 120 branches from seeing the full 240?


It very much depends on if you are connected to neighbors by water pipe or something conductive. If you are on well water and by your self then the voltage would swing more easily.


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## Desertdrifter (Dec 10, 2009)

rjniles said:


> If the POCO neutral goes open but the panel neural remains bonded to ground, will that prevent all the 120 branches from seeing the full 240?


I've never seen a swing +/- 35 volts for an open neutral to a residence.


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## concrete_joe (Oct 6, 2014)

rjniles said:


> If the POCO neutral goes open but the panel neural remains bonded to ground, will that prevent all the 120 branches from seeing the full 240?


arent the neutral and gnd blocks isolated from each other...... the center tap from poco should have a good path to gnd though, thereby now you have earth as neutral....??


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## beenthere (Oct 11, 2008)

concrete_joe said:


> arent the neutral and gnd blocks isolated from each other...... the center tap from poco should have a good path to gnd though, thereby now you have earth as neutral....??



Earth is not a good conductor.


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## rjniles (Feb 5, 2007)

concrete_joe said:


> arent the neutral and gnd blocks isolated from each other...... the center tap from poco should have a good path to gnd though, thereby now you have earth as neutral....??


The neutral and ground are bonded in the main panel. (Or should be.)


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## Jump-start (Sep 26, 2012)

If the piping under the street is metal and your neighbors have the same water bond as you (they should per NEC) an open neutral may go un-noticed since current will travel onto the metal water mains, to your neighbor's water bonds and return to the transformer using their neutral.


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## Jump-start (Sep 26, 2012)

concrete_joe said:


> arent the neutral and gnd blocks isolated from each other...... the center tap from poco should have a good path to gnd though, thereby now you have earth as neutral....??


If the utility uses a multi grounded neutral system the return path will be excellent.


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## concrete_joe (Oct 6, 2014)

rjniles said:


> The neutral and ground are bonded in the main panel. (Or should be.)


then whats the issue with sharing the terminal blocks with white neutrals and bare coppers?? doesnt NEC say they should be grouped together by type?



beenthere said:


> Earth is not a good conductor.


"good", maybe not as low ohmed as #2 AL from service panel to transformer, but it must carry a good portion of current, no?


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## Desertdrifter (Dec 10, 2009)

concrete_joe said:


> then whats the issue with sharing the terminal blocks with white neutrals and bare coppers?? doesnt NEC say they should be grouped together by type?
> 
> 
> "good", maybe not as low ohmed as #2 AL from service panel to transformer, but it must carry a good portion of current, no?


At the main they are generally permitted together. At sub panels neutrals and grounds are separated so as to only provide one path back to the main.

One of the reasons earth seems to conduct so well in respect to neutral and ground current is because of grounding policies. Never to far to a ground rod or bond and up to a neutral.


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## rjniles (Feb 5, 2007)

concrete_joe said:


> then whats the issue with sharing the terminal blocks with white neutrals and bare coppers?? doesnt NEC say they should be grouped together by type?



There is no issue with sharing the terminal blocks with white neutrals and bare coppers (at the first disconnect location - main panel). It is only after the first disconnect (sub panels) that the neutrals and grounds must be electrically isolated.


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## beenthere (Oct 11, 2008)

concrete_joe said:


> "good", maybe not as low ohmed as #2 AL from service panel to transformer, but it must carry a good portion of current, no?


Actually, it doesn't.


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## Jump-start (Sep 26, 2012)

concrete_joe said:


> "good", maybe not as low ohmed as #2 AL from service panel to transformer, *but it must carry a good portion of current, no*?


Rarely, if ever. Ground rods are usually over 25ohms, sometimes into the hundreds of thousands. Even at 25ohms the current will be minimal to the point little difference will be made. Take 120 volts to a 25ohm rod, 4.8amps will flow. Chances are, unless you have a metal water main and interconnected neighbors, an open neutral will be felt.


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## Jump-start (Sep 26, 2012)

bernie963 said:


> Jump-start, What is a "polepig" ? Is it just a pole mounted transformer with phase to phase primary? On Cape Cod, they do run 3 phase , and use two for this set up. Advantages and disadvantages compared to phase to neutral. Maybe this should be on a new thread?
> 
> bernie


Sorry for the late reply but I just am now looking back through this thread which is a good read.

A pole pig is just slang for a pole mounted transformer. They come in single and double bushing. A single bushing unit is connected only to a grounded wye system that has a multi grounded neutral. Basically a wire that acts a ground and a neutral. One of the leads is grounded to the tank and the tank is then bonded to the grounded neutral. A double bushing unit has both leads connected to an insulated bushing on top . These can either be connected phase to phase or phase to neutral. The neutral may be insulated or multi grounded. 

If interested I can go into the advantages vs disadvantages but that is a HUGE engineering subject.


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## analogmusicman (Feb 4, 2008)

yes,I agree "wholeheartedly" that the 2 120V legs are 180 deg. out of phase (otherwise certainly your electric oven wouldn't work or any other 240V devices) but I want somebody (maybe an engineer in this forum) to explain how and why those legs get that way, it's certainly not straightforward and I was asleep that day in "tech. school" 

tnx,


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## MTN REMODEL LLC (Sep 11, 2010)

I think it's kind of a phase , like when a lady turns 50 or so.... basically not understandable to a layman GC.

(Actually, wish I could of followed the discussion... it was above my head... lateral service or not...:laughing:


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## nap (Dec 4, 2007)

ok, here is something to screw with everybody.


if you connect an oscilliscope to L1 and L2 you will get a single sine wave showing a 240 volt swing from peak to peak

now, if you connect to L1 and center tap OR L2 and center tap you will get a single sine wave with a a 120 volt swing from peak to peak


Now here is the magic:

if you connect 3 leads with the center tap as the reference point, you will get a dual sine wave, each with a swing of 120 volts but exactly opposite of each other so you have a 240 volt difference at each peak point where when you see the single wave the total between the high and low is offset. 



now what you actually have powering anything at 240 volts is the first scenario. 

I believe the reason everybody gets so confused is the fact is you do not have 120 plus 120 = 240. You have 240 volts and you are splitting it in half to get the 120. Why you get what looks to be opposing phases isn't really opposing phases (when you think of it in terms of phases of a transformer) but opposing phases as in time shift. Think about it for a minute and it might sink in. The sine shows both voltage and time. when you use the center tap as reference you get what appears to be opposing phases but what the really are is simply a representation of the wave form as referenced to the center tap at any given time point. It will be negative to the center from L1 at the same time it is positive from the center to L2.


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## nap (Dec 4, 2007)

analogmusicman said:


> yes,I agree "wholeheartedly" that the 2 120V legs are 180 deg. out of phase (otherwise certainly your electric oven wouldn't work or any other 240V devices) but I want somebody (maybe an engineer in this forum) to explain how and why those legs get that way, it's certainly not straightforward and I was asleep that day in "tech. school"
> 
> tnx,



actually they are not 180º out of phase at all because you do not have 2 phases unless you want to count it being split creates 2 phases (which is does in time but not in terms of a phase when one speaks of a transformer). You have a 240 volt transformer and that is why you get 240 volts when you connect L1 and L2 to a load.


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## nap (Dec 4, 2007)

rjniles said:


> If the POCO neutral goes open but the panel neural remains bonded to ground, will that prevent all the 120 branches from seeing the full 240?


nope. You do not need a ground at all. You don't need the "neutrals" of the primary and secondary connected either (and the reason they are is because they are both grounded). In fact, it has nothing to do with the POCO neutral. In fact, often times they do not even bring a neutral (delta wound transformer has no neutral)


you have 120/240 because the ratio of windings from primary to secondary create the 240 volts and when you tap the center of the transformer you get 120 volts each way just like when I am connecting a transformer and adjust the tap setting to compensate for a low or high voltage from the POCO so I have the right voltage for my customer.


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## analogmusicman (Feb 4, 2008)

gee "NAP",you make it sound like the 180 deg. phase shift between the 2 hot legs of the transformer secondary is a mere "illusion". I have to disagree! if this phase shift were not an actual,physical thing, your oven just wouldn't work neither would your DVM read 240V when placed across the two legs. if I'm wrong about the "illusion" then my apologies.

tnx,


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## nap (Dec 4, 2007)

analogmusicman said:


> gee "NAP",you make it sound like the 180 deg. phase shift between the 2 hot legs of the transformer secondary is a mere "illusion". I have to disagree! if this phase shift were not an actual,physical thing, your oven just wouldn't work neither would your DVM read 240V when placed across the two legs. if I'm wrong about the "illusion" then my apologies.
> 
> tnx,


disagree all you want because there is no phase shift. It is simply that when using the center tap as a reference point, at any moment in time the waves are inverse from each other. 

you apparently missed the point that the reason there is 240 volts is simply because from L1 to L2 of the one, single winding it is the proper ratio compared to the primary windings to provide 240 volts.


so yes, the out of phase issue is only an illusion because you do not use the center tap when tapping the transformer for 240 volts. When you view the wave form of the 240 volt connection, there is a single sine wave peaking at at 240 volts. . The only time you see the opposing waves is when you toss in the center tap and then because it is in the center, it reads + voltage to one leg while it reads - voltage to the other. It is an illusion because it is to that center tap only which has nothing to do with providing 240 volts.


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## nap (Dec 4, 2007)

ok, check this:

the larger wave form is what you have when you connect to L1 and L2 with your leads from the scope. The red wave is what you get when you connect from the center tap to either L1 or L2. The blue wave form is what you get when you connect from the other L to the center tap.

all of this is going on in the time frame exactly as you see it there. There is no additive to come up with 240 volts and the only reason the 2 120 volt wave forms appear to be out of phase is you are using the center tap as the reference point. From center to one end will be + at the exact same time from the center to the other end will be -. That inclusion of the center tap is what is confusing people.


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## concrete_joe (Oct 6, 2014)

nap said:


>


nap, yes, its one winding of the generator, which is a single phase of many (depends on the generator). but sadly this graph is not accurate because the voltage ratings (and meter readings) are RMS voltages, not peak voltages.

and, here's where things get odd. in the pic, the appearance of the 180degree out of phase in fact are the two poles with center tap reference (common), and note their diff, it = 240v, yep, put your meter probes into the hot side of two outlets that are on different poles and you'll get 240v. you can power a 240v device simply by using two 120v outlets. 

bottom line is, US residential is single phase power.
http://www.diychatroom.com//www.pinterest.com/pin/create/extension/


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## nap (Dec 4, 2007)

Joe, a scope reads true voltage, not rms. now the actual voltage is incorrect because it gives the rms voltage but I suspect the creator did this to remove confusion. If the used true voltage it would start a discussion about that.


The reason it appears out of phase is because the the two 120 volt waves are using opposites leads of the scope. There is 1 lead being used as reference for both the waves but it is on the opposite end of the windings being measured. That makes it the + lead for one side of the transformer while it is simultaneously the - lead for the reading on the other half of the transformer


If I use 4 leads and hook them up in the same respective direction the waves will overlap.


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## concrete_joe (Oct 6, 2014)

nap said:


> Joe, a scope reads true voltage, not rms.


yes. and? this fact doesnt change the bad graph (axis is bad, that's all).


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## nap (Dec 4, 2007)

Not a thing wrong with the graph other than it used nominal voltage in rather that true voltage.


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## concrete_joe (Oct 6, 2014)

nap said:


> Not a thing wrong with the graph other than it used nominal voltage in rather that true voltage.




a graph with bad #'s or labels on the axis is in fact a bad graph. 

rip off the #'s from axis and use the lines as an illustration to discuss what the voltage would look like on a scope, then i am good with that. :thumbsup:



> *graph 1*
> 
> (grăf)_n._*1. * A diagram that exhibits a relationship, often functional, between two sets of numbers as a set of points having coordinates determined by the relationship. Also called _ plot_.


http://www.diychatroom.com//www.pinterest.com/pin/create/extension/


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## nap (Dec 4, 2007)

THe numbers are merely reference to the "240 volt" wave form and the two "120 volt" wave forms. It allows one to identify and compare the three wave forms. You can put whatever numbers you want on there if it makes you feel better. Call them A B and C if you like. It makes no difference and it doesn't invalidate what is shown on the graph. 


Ya know, since I'm among the very few arguing your side of the discussion, it doesn't make a lot of sense you would argue irrelevant minutia with me.


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## Desertdrifter (Dec 10, 2009)

Both 120 legs have the same wave form just at half the amplitude.


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## nap (Dec 4, 2007)

Btw; what residentisl 120/240 power is accurately defined as Is a single phase power with that phase being "split" from which the 120 volt is derived. 


Another point; 

There are two different definitions of the word "phase" in play here. thsre is a power phase which is referring to power derived from a single winding or multiple windings being acted upon in identical time which actuslly brings in the second definition of phase being a reference to the wave forms in the same point in their cycle at the same time. 

So for the purposes of the discussion 240 volt power is a single phase power as there is only one wave form produced when displaying it on a scope


Ya'all are getting confused by trying to factor in the spitting of the transformer and resulting split phase display of 2 120 volt wave forms that appear to be 180deg out of (time) phase. The problem with that is you are using a single reference point which is in the middle of the two second reference points. It will always show an opposite polarity at the same instant just the same as if you took to batteries and connects the + of one to the - of the other and used that point as your reference point from which to read the voltage to the two remaining terminals at the same moment. One voltage will be displayed as - while the other will be displayed as + but the problem with that is - or + is only meaningful when comparing two voltage sources using the same polarity within the leads to the meter/scope. 


It's like trying to read pressure between three containerd one having 1 psi, the second having 2 psi and the third having 3 psi. 

Using 3 sensors with a common one in the 2 psi container you will read 1 psi + pressure to the one with 1 psi and you will read 1 psi - to the container with 3 psi but that is only relevant when measuring to all 3 containers. When you measure to the 2 containers (1 psi and 3 psi containers) you get a reading of 2 psi. Depending which lead you consider the reference or base, you will get either a + or a - number. It is simply a means to compare one to the other and what either reads to the third container is irrelevant.


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## concrete_joe (Oct 6, 2014)

clw1963 said:


> Both 120 legs have the same wave form just at half the amplitude.


they are in fact the same waveform, it comes from a single magnetic field across a single coil that is center-tapped. as Nap says, this apparent phase change (no phase change at all) comes from the fact that the center tap is the GND reference point. thus when A side looks like its on the + side to GND, the B side looks like its on the - side to GND, but all at the same time, thus an apparent phase shift.

its single (1) phase no matter how you slice and dice it.


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## analogmusicman (Feb 4, 2008)

this has to be my last post since this topic is a "can of worms" with all sorts of confusing misinformation.
yes,a center tapped transformer (center tapped on the secondary side) makes the transformer act in a special way when that center tap is grounded. (ie. one of the hot legs has it's waveform inverted)
I won't go into detail about your "broiler element" turning red but just trust me that no current is going to flow through that element unless the two hot legs that the element is connected to are out of phase with each other. (180 deg.)
don't believe me? think it's an "illusion"? just get a piece of "graph paper" and draw the voltage sine waves of the 2 hot legs, both with the waveforms "in phase" and "out of phase by 180 deg. you decide which set of waveforms is going to produce current in your "broiler element". hint: use your basic electricity knowledge.
sheesh,all I wanted to know is why one leg gets inverted! 

tnx,


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## Desertdrifter (Dec 10, 2009)

analogmusicman said:


> this has to be my last post since this topic is a "can of worms" with all sorts of confusing misinformation.
> yes,a center tapped transformer (center tapped on the secondary side) makes the transformer act in a special way when that center tap is grounded. (ie. one of the hot legs has it's waveform inverted)
> I won't go into detail about your "broiler element" turning red but just trust me that no current is going to flow through that element unless the two hot legs that the element is connected to are out of phase with each other. (180 deg.)
> don't believe me? think it's an "illusion"? just get a piece of "graph paper" and draw the voltage sine waves of the 2 hot legs, both with the waveforms "in phase" and "out of phase by 180 deg. you decide which set of waveforms is going to produce current in your "broiler element". hint: use your basic electricity knowledge.
> ...


You're wrong. 
Think of it as one horse is 120, two working together is 240. When yoked together they keep pace and are twice as strong as one alone. They're not walking out of phase. They are in phase (pace).


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## Daniel Holzman (Mar 10, 2009)

There is a very nice discussion about transformers and phasing here http://jacobs-online.biz/understanding_transformers.htm

I am not going to repeat the discussion, it is somewhat lengthy but very informative. I will quote the section that explains how it is possible to get two legs that are 180 degrees out of phase from a single secondary winding on a transformer.

"The phase between one end of a coil and the other are also 180° out of phase. When one end is at the positive peak, the other end will be on the opposite peak. Since there must be a difference in voltage between two points for current to flow, the two ends of the winding must be opposite voltage at any point in time."

This basically explains how a center tapped transformer produces two legs that are 180 degrees out of phase. This happens because one end of the winding is 180 degrees out of phase with the other end of the winding, and the center tap is effectively at zero volts (it is grounded), so by connecting to the two ends of the secondary transformer winding, we get two legs that are 180 degrees out of phase. We would call this two phase power, except the term two phase historically meant the two phase were 90 degrees apart, so to distinguish this is called split phase, because the two phases are generated by splitting a single winding. Ciao.


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## nap (Dec 4, 2007)

> analogmusicman;1912417]this has to be my last post since this topic is a "can of worms" with all sorts of confusing misinformation.
> yes,a center tapped transformer (center tapped on the secondary side) makes the transformer act in a special way when that center tap is grounded. (ie. one of the hot legs has it's waveform inverted)


let's just stop right there. Grounding has nothing to do with any of this.




> I won't go into detail about your "broiler element" turning red but just trust me that no current is going to flow through that element unless the two hot legs that the element is connected to are out of phase with each other. (180 deg.)


and again, still wrong. Did you miss the fact this is a waveform for a 240 volt supply at a residential service:










do you see any inverted waveforms? Do you see any 120 volt waveforms? of course not because there aren't any. 





> don't believe me? think it's an "illusion"? just get a piece of "graph paper" and draw the voltage sine waves of the 2 hot legs, both with the waveforms "in phase" and "out of phase by 180 deg. you decide which set of waveforms is going to produce current in your "broiler element". hint: use your basic electricity knowledge.


um, look above. That IS the waveform you will get when you connect a scope to L1 and L2 of a residential service.




> sheesh,all I wanted to know is why one leg gets inverted!


it is an illusion. each leg is reading from center to the end. That means it is viewed as positive in relation to one end while it is viewed as negative in relation to the other end at the same moment in time. If I were to use 4 leads and connected the same (marked) lead to the left connection (presuming the transformer is laid out L1- center tap-L2) (where the "a" lead of the first set would be connected to L1 and the "a" lead would be connected to the center tap with the "b" lead of each set connected to the opposite end of the intended connection) the waveforms would overlay each other exactly.

the inverse you see is only because you are using a reference point that is in the middle of the transformer. Voltage being negative or positive is simply a statement of relationship between the two points being measured and since you are reversing the polarity of the connections when using three leads to show both 120 volt waveforms, you will see them as inverse of each other.


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## nap (Dec 4, 2007)

daniel, riddle me this:


let's use your post for conditions.


that means the relationship between L2 and C is one is neg and one is positive, right?

Ok, let's arbitrarily stop time for an instant and make L2 positive. That makes C negative, right?


great, now catch this:

if C is negative, the L1 MUST be positive, right? 

I'm sure you agree so great.

Now, since we have just proven L1 AND L2 are both positive, please explain where there would be any inverse wave form when comparing those 2 sources.


and now, let's expand this. Since we also tap 240 volts from this very same transformer, at this moment in time we have just determined both L1 and L2 are both positive.....

hmm, didn't your link just say we can't have that and still have current flow?

I think I have just tore a hole in the space time continuum.


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## concrete_joe (Oct 6, 2014)

folks, nap has this right.

this boils down to the basics of magnetic flux across a coil. the mag field hits the secondary coil (all of it, the coil) all at the same time (for the xfrmer secondary that feeds your home). the xfrmer i have in front of my home has 6 coils, 3 primary and 3 secondary (3-phases of power) and each phase powers different sets of homes (basically 3 xfrmers in a single box). to have phase shift the mag field must hit coils at different times, as they do in a spinning generator that has coils that are physically separated by some # of degrees (aka "phase shift").

have to say it again, the 120/120 you have in your home is from a single phase of power, no phase shift at all. what you see on meter or scope is happening all at the same time, unlike what you would see if you scoped 3 phases of power from the poco, etc.


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## nap (Dec 4, 2007)

until dan or analogmusicman can invalidate my argument about both ends of the transformer being the same polarity, I don't think they are going to say much. 

the problem with all of this is:

the 2 120 volt sources really have nothing to do with the 1 240 volt source. They are trying to compare apples to kumquats. The phase relationships between the 2 120 volt sources has nothing to do with the phase position of the 240 volt source, at least in a functional matter. Yes, because the neutral is where it is, the 2 120 volt sources oppose (rather than the claimed add) each other. That is how you can have a mwbc with equal current on each of the ungrounded conductors and absolutely no current on the neutral conductor. 

but it is a matter of perspective regarding the relationship between the two 120 volt sources and that has nothing to do with the 240 volt source.


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## TimPa (Aug 15, 2010)

nap said:


> Did you miss the fact this is a waveform for a 240 volt supply at a residential service:


Actually your voltage scale is incorrect. the peaks will be at 339.5volts positive and 339.5 volts negative. The 240 vac reference is at rms (root mean square) = 0.707 x peak.


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## Bob Sanders (Nov 10, 2013)

nap said:


> ok, here is something to screw with everybody.
> 
> 
> if you connect an oscilliscope to L1 and L2 you will get a single sine wave showing a 240 volt swing from peak to peak
> ...


Thank you.
I spent a considerable amount of time arguing this point earlier in this thread and finally gave up. Even dug out my scope and took a pic to try and prove it.

If you connect a load across L1 and L2 you get a SINGLE 240 volt sine wave. The two 120 volt legs have NOTHING to do with the equation because the center tap is not part of the circuit. For all intents and purposes the load is coming from SINGLE 240 volt winding off the transformer.... NOT two 120 volt windings.


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## Bob Sanders (Nov 10, 2013)

analogmusicman said:


> this has to be my last post since this topic is a "can of worms" with all sorts of confusing misinformation.
> yes,a center tapped transformer (center tapped on the secondary side) makes the transformer act in a special way when that center tap is grounded. (ie. one of the hot legs has it's waveform inverted)
> I won't go into detail about your "broiler element" turning red but just trust me that no current is going to flow through that element unless the two hot legs that the element is connected to are out of phase with each other. (180 deg.)
> don't believe me? think it's an "illusion"? just get a piece of "graph paper" and draw the voltage sine waves of the 2 hot legs, both with the waveforms "in phase" and "out of phase by 180 deg. you decide which set of waveforms is going to produce current in your "broiler element". hint: use your basic electricity knowledge.
> ...


This is wrong.
You only get two 120 volt waves if you involve the center tap. Connecting a load across L1 and L2 COMPLETELY omits the center tap.

To get an accurate picture of what is going on, you must only look at what the load is seeing.


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## nap (Dec 4, 2007)

TimPa said:


> Actually your voltage scale is incorrect. the peaks will be at 339.5volts positive and 339.5 volts negative. The 240 vac reference is at rms (root mean square) = 0.707 x peak.


Ya Ya ya. Already been through this point. It doesn't change anything regarding the topic at hand.


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## nap (Dec 4, 2007)

Bob Sanders said:


> This is wrong.
> You only get two 120 volt waves if you involve the center tap. Connecting a load across L1 and L2 COMPLETELY omits the center tap.
> 
> To get an accurate picture of what is going on, you must only look at what the load is seeing.


Finally somebody sees the light. 


But what do we do about the tear in the space time comtinuum As a result of the fact we have current flow while both ends of the transformer are of the same polarity?


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## Desertdrifter (Dec 10, 2009)

TimPa said:


> Actually your voltage scale is incorrect. the peaks will be at 339.5volts positive and 339.5 volts negative. The 240 vac reference is at rms (root mean square) = 0.707 x peak.


For the purposes of the discussion the scale is fine.


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## Daniel Holzman (Mar 10, 2009)

Nap, I don't know is you are serious about the space time continuum, but I guess you either did not understand, or did not believe, the reference from Jacobs. At a given time, L1 is negative, the center tap (neutral) is 0 voltage, and L2 is positive. Since the voltage a device sees is the difference between the voltages on the two legs, the voltage from L1 to C is 120 - 0 = 120V, the voltage from L2 to C is -120 - 0 = -120, and the voltage between L1 and L2 is +120 - (-120) = 240V.

As to why the opposite ends of a single secondary transformer coil are at +120 and -120V at the same time, you have to read the Jacobs article for further information. And it has nothing to do with the space time continuum.


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## TimPa (Aug 15, 2010)

clw1963 said:


> For the purposes of the discussion the scale is fine.


not for someone looking in and thinking that waveform is correct! if you are going to say "_this is how something is_" , it should be accurate.


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## nap (Dec 4, 2007)

Dan, I used your and their statements to prove both ends of the transformer is the same polarity. 


My point is; everybody is trying to compare 2 things that simply cannot be compared in the way they are trying. Polarity, just like voltage, is relative. 

On top of that, the waveform for a 240 vac supply is as I posted. There is no 120 volt anything and there are no inverse waveforms.


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## Bob Sanders (Nov 10, 2013)

Daniel Holzman said:


> Nap, I don't know is you are serious about the space time continuum, but I guess you either did not understand, or did not believe, the reference from Jacobs. At a given time, L1 is negative, the center tap (neutral) is 0 voltage, and L2 is positive. Since the voltage a device sees is the difference between the voltages on the two legs, the voltage from L1 to C is 120 - 0 = 120V, the voltage from L2 to C is -120 - 0 = -120, and the voltage between L1 and L2 is +120 - (-120) = 240V.
> 
> As to why the opposite ends of a single secondary transformer coil are at +120 and -120V at the same time, you have to read the Jacobs article for further information. And it has nothing to do with the space time continuum.


This thread is about 240 volts and has absolutely nothing to do with 120 volts or center taps. You're sidetracking yourself.

A load will see the exact same 240 volt sine wave from a 240volt NON center tapped transformer as it will from a 240 volt center tapped transformer with the center tap excluded from the circuit. It's the same thing.

There should be no discussion about a center tap here (and this is where the confusion lies). The center tap is a completely moot issue in a 240 volt circuit... and if the center tap is a moot issue then it automatically makes the discussion of "two 120 volt hot legs" a complete non issue.

In this discussion, there is L1 and L2... that is it.


On a side note,
L2 to C is 120volts (*+60 volts* with reference to 0 for 1/2 cycle, and *-60 volts* with reference to 0 for the other 1/2). It is not 120 volts to the zero reference.

L1 to C is the same but opposite


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## nap (Dec 4, 2007)

TimPa said:


> not for someone looking in and thinking that waveform is correct! if you are going to say "_this is how something is_" , it should be accurate.


So if you want to be picky;

The waveform is just fine. you are having problems with the reference scale.


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## Desertdrifter (Dec 10, 2009)

TimPa said:


> not for someone looking in and thinking that waveform is correct! if you are going to say "_this is how something is_" , it should be accurate.


For purposes of this discussion in a DIY forum, which most of this is beyond DIY and less than electrical engineering. It's fine. Going beyond Rms values in this regard is pointless. IMHO 


300 posts to topic on its way.


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## concrete_joe (Oct 6, 2014)

Bob Sanders said:


> This thread is about 240 volts and has absolutely nothing to do with 120 volts or center taps. You're sidetracking yourself.
> 
> A load will see the exact same 240 volt sine wave from a 240volt NON center tapped transformer as it will from a 240 volt center tapped transformer with the center tap excluded from the circuit. It's the same thing.
> 
> ...


whaaaaat. if you dont talk about the two poles of 120Vac(rms) then there is no discussion about phase. its only when you talk about CT and the two poles do people get confused about phasing.

the 240Vac(rms) coming out out of the secondary is one phase. stick probes anywhere you like, add taps wherever you like, w/o adding fancy stuff to it you have just one phase. in the physics of it all, no such thing as negative charge, you either have no charge, or you have some charge. the mag field pushes charge back and fourth (aka "AC"). from a reference perspective using zero is a good choice. you can certainly have a diff in charge, but each is still >=0, etc. for all intensive purposes, when you put a "+" or "-" in front of the voltage all that denotes a direction of the vector relative to the reference.

as i mentioned before, the mag flux slices the secondary coil all at the same time, therefore you don't have more than one phase there.


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## Desertdrifter (Dec 10, 2009)

Bob Sanders said:


> On a side note,
> L2 to C is 120volts (*+60 volts* with reference to 0 for 1/2 cycle, and *-60 volts* with reference to 0 for the other 1/2). It is not 120 volts to the zero reference.
> 
> L1 to C is the same but opposite


Whaaaa?

Could you graph that please?

So, you're saying that 240v is only 120v + for a half cycle and 120v - for a half cycle?

Speaking in RMS values here TomPa


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## mpoulton (Jul 23, 2009)

Bob Sanders said:


> On a side note,
> L2 to C is 120volts (*+60 volts* with reference to 0 for 1/2 cycle, and *-60 volts* with reference to 0 for the other 1/2). It is not 120 volts to the zero reference.


The rest of your post was true, but this isn't right. It's 120V RMS, which is about 170V peak (-170V on the other half of the cycle). Stick an oscilloscope on the AC line if you don't believe me, or Google...


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## nap (Dec 4, 2007)

Bob Sanders said:


> On a side note,
> L2 to C is 120volts (*+60 volts* with reference to 0 for 1/2 cycle, and *-60 volts* with reference to 0 for the other 1/2). It is not 120 volts to the zero reference.
> 
> L1 to C is the same but opposite


bob, this is an accurate representation of a 120 vacrms 60 hz waveform.


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## Bob Sanders (Nov 10, 2013)

concrete_joe said:


> its only when you talk about CT and the two poles do people get confused about phasing.


There is no CT involved a 240 volt circuit. It is completely and totally moot. If you leave it in it is 240 volt single phase. If you take it out it is 240 volt single phase. The CT has NOTHING to do with it.


Do you understand that they make 240 volt transformers with no CT? If you measure the wave coming off of one of these and compare it to a wave coming from L1 and L2 of a CT transformer the two waves will be *EXACTLY *the same. There is no difference. It's just one big 240 volt sine wave because in BOTH cases it's coming from one single 240 volt winding.
Just because a transformer has a tap in it (center or otherwise) doesn't make it at all relevant to the circuit in question.

Once again, the CT is completely and totally irrelevant in a 240 volt connection, and has no bearing what so ever.... REMOVE it from your thoughts.


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## ddawg16 (Aug 15, 2011)

Wow.....just wow

Anyway, maybe this will help.

Since so many people are getting hung up on the 'center tap' and L1/N/L2...etc.

Consider this. You can have a transformer with as many taps as you want. It doesn't mater how many you have....it's still single phase. Why? Because it has a single phase feeding it. If you look on the line side of the transformer, there are 2 wires. Only 2 wires. If you feed a transformer singe phase, you get single phase out of it. The only exception I know of is a rotary phase converter. Or, in simple terms, running a 3ph motor with a single phase to get 3 phases out of it.


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## concrete_joe (Oct 6, 2014)

Bob Sanders said:


> There is no CT involved a 240 volt circuit. It is completely and totally moot. If you leave it in it is 240 volt single phase. If you take it out it is 240 volt single phase. The CT has NOTHING to do with it.
> 
> 
> Do you understand that they make 240 volt transformers with no CT? If you measure the wave coming off of one of these and compare it to a wave coming from L1 and L2 of a CT transformer the two waves will be *EXACTLY *the same. There is no difference. It's just one big 240 volt sine wave because in BOTH cases it's coming from one single 240 volt winding.
> ...


CT having nothing to do with pole-to-pole 240Vac(rms)?? yes, i agree.

it was relevant because the phasing question comes from seeing the two 120v poles on a scope, which some think are two phases of power.

but ok, CT in my head is gone, the mag flux is still the same, one and only one phase of power.


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## Desertdrifter (Dec 10, 2009)

nap said:


> bob, this is an accurate representation of a 120 vacrms 60 hz waveform.


Don't go screwing things up showing a "true" representation of a 120 sine wave. 

Post 299


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## nap (Dec 4, 2007)

Then this should be 300. Nothing new; just want to be the one.


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## concrete_joe (Oct 6, 2014)

clw1963 said:


> Don't go screwing things up showing a "true" representation of a 120 sine wave.
> 
> 
> 
> ...


which it a signal referenced to the CT of the xfrmer, all from one phase.


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## zappa (Nov 25, 2011)

zappa said:


> Hear ya go Bob....I swear that I didn't break the case open and shove another winding in there.



It looks like I missed a few pages. :whistling2:

What's all this talk about neutrals and center taps? Doesn't anyone remember this image of a simple wall wart. Two wires in, two out, and there is that all important inverted reference needed to produce power.


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## zappa (Nov 25, 2011)

nap said:


> let's just stop right there. Grounding has nothing to do with any of this.
> 
> 
> and again, still wrong. Did you miss the fact this is a waveform for a 240 volt supply at a residential service:
> ...


So lets say that you connect L1 to the first peak. Time wise, L2 has to be directly underneath L1 at -240 but there is nothing down there to hook it to except air?? There can be no potential without the missing waveform. :huh:


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## nap (Dec 4, 2007)

zappa said:


> So lets say that you connect L1 to the first peak. Time wise, L2 has to be directly underneath L1 at -240 but there is nothing down there to hook it to except air?? There can be no potential without the missing waveform. :huh:


Huh? connect L1 to the first peak? Explain what you mean. What are you referring to as L1?




You do not connect anything to "a peak" You connect the leads from the scope to L1 and L2. 

your statement of not having anything to connect it to directly under the peak makes no sense. A sine wave is a representation of the voltage between two reference points at any moment in time. As time progresses the display point moves to the right. Since the voltage is continually varying you get the upward and downward motion of the display point.

I have no idea what you are talking about though., Your statements make no sense what so ever.


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## ddawg16 (Aug 15, 2011)

zappa said:


> It looks like I missed a few pages. :whistling2:
> 
> What's all this talk about neutrals and center taps? Doesn't anyone remember this image of a simple wall wart. Two wires in, two out, and there is that all important inverted reference needed to produce power.


Swap the wires on the output and it will look the same.


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## zappa (Nov 25, 2011)

ddawg16 said:


> Swap the wires on the output and it will look the same.


Yes it will....thank you!


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## zappa (Nov 25, 2011)

nap said:


> A sine wave is a representation of the voltage between two reference points at any moment in time.


Exactly what I'm getting at. Where is the second reference point?


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## zappa (Nov 25, 2011)

nap said:


> ok, check this:
> 
> the larger wave form is what you have when you connect to L1 and L2 with your leads from the scope. The red wave is what you get when you connect from the center tap to either L1 or L2. The blue wave form is what you get when you connect from the other L to the center tap.
> 
> all of this is going on in the time frame exactly as you see it there. There is no additive to come up with 240 volts and the only reason the 2 120 volt wave forms appear to be out of phase is you are using the center tap as the reference point. From center to one end will be + at the exact same time from the center to the other end will be -. That inclusion of the center tap is what is confusing people.


This image is clear as day. The blue and red is showing what happens electrically, what is producing the power, a representation of L1 and L2 placement. The black is showing just a simplified additive voltage result of L1 and L2. The black is what you would see on a scope when only using one probe and the associated ground clip.

The 240 winding is a balanced circuit and you need two probes to view what is really going on electrically.


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## Daniel Holzman (Mar 10, 2009)

Voltage is not "additive". Voltage represents the difference in potential between two points, thus is subtractive. You get 240 volts because +120 -(-120) = 240.


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## joed (Mar 13, 2005)

delete.


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## InPhase277 (Feb 9, 2008)

Where's Gregzoll to clear up all the confusion?


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## ddawg16 (Aug 15, 2011)

InPhase277 said:


> Where's Gregzoll to clear up all the confusion?


Actually, I was hoping Cletis would chime in ......I need a good laugh


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## Bob Sanders (Nov 10, 2013)

zappa said:


> This image is clear as day. The blue and red is showing what happens electrically, what is producing the power, a representation of L1 and L2 placement. The black is showing just a simplified additive voltage result of L1 and L2. The black is what you would see on a scope when only using one probe and the associated ground clip.
> 
> The 240 winding is a balanced circuit and you need two probes to view what is really going on electrically.


No.
The black is showing REALITY when L1 and L2 are used. You are using two scope probes (improperly I might add) to measure the SAME wave. It appears out of phase because you are looking at the SAME wave backwards is all.


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## nap (Dec 4, 2007)

zappa said:


> It looks like I missed a few pages. :whistling2:
> 
> What's all this talk about neutrals and center taps? Doesn't anyone remember this image of a simple wall wart. Two wires in, two out, and there is that all important inverted reference needed to produce power.


inverted reference to produce power?

You make no sense. If you read the primary and secondary windings they will be 180 deg out of phase with each other. That is a simple function of induction. If you read only the primary or secondary, there is no inverted phase because you will be reading a single sine waveform. 


but I'm not going through this again. You simply have no idea what is going on. 





> Daniel Holzman	Voltage is not "additive". Voltage represents the difference in potential between two points, thus is subtractive. You get 240 volts because +120 -(-120) = 240.


You have 120 volts (nominal) (170 peak) positive AND you have 120 volts (nominal) (170 volts peak) negative but you end up with a 120 volt supply. Your reference point is halfway in the middle




but how you obtain both 120 and 240 volts from the same transformer:

You have a center tapped tranformer. You are connecting a tap point halfway through the windings so you have cut the number of windings you are drawing power from in half. Half the number of windings (in the ratio of primary to secondary windings) (wraps of wire as opposed to a winding set) means half the voltage of the whole. 

Using some simple numbers:


primary side is 100 volts. If you have an equal number of windings, you simply create the same voltage on the secondary (100 volts) (an isolation transformer does this). If you cut the number of windings on the secondary so there is a 2 to one ratio, you halve the voltage output of the secondary (50 volts) . If you halve the windings again, you have 1/4 the primary voltage as the output of the secondary. (25 volts)

This is what is happening when you have a 120/240 center tapped transformer. You essentially have three transformers in one. One produces 240 volts and the other two produce 120 volts each. You do not do anything to make the two 120 volt transformers add to each other to get the 240 volts. In fact, when you tap 240 volts from the transformer, the center tap is totally out of the picture. 


but back to your additive voltage; 


lets use actual voltages for a residential service. 

the 120 volts rms has a peak voltage of (rounded just a hair) 170 volts. Now using your formula, we get 170 - (-170) =340. Now, to get 120 volts rms we divide the peak to peak by 2* the square root of 2 and we come back to 120 volts. 

P to P/2*√2=120

Your formula of where you get 240 volts is simply misleading and does nothing to answer the question of how you get 240 volt supply from a 120/240 volt center tapped transformer.


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## concrete_joe (Oct 6, 2014)

its very simple, its sinusoidal single phase, period. keep searching for the magical magnetic fields and you might find that "other" phase :laughing::laughing::laughing:.


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## nap (Dec 4, 2007)

concrete_joe said:


> its very simple, its sinusoidal single phase, period. keep searching for the magical magnetic fields and you might find that "other" phase :laughing::laughing::laughing:.


I'm with ya on this one joe. :thumbsup:


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## cygnus23 (Mar 31, 2016)

So, I followed a link from Google that showed the 2 flipped sine waves, because I never really understood what single phase looked like as a sine wave. I knew 240 residential is single phase, but knew it contained two 120v lines and a neutral coming from the utility pole transformer. Even though I work with 3-phase 208 all the time in the entertainment industry, and re-wired most of my last house, it bugged me that I couldn't offer anyone an explanation of the waveform, even though I understood 208v, which seemed like it should be more complicated. I didn't understand why it didn't make sense. I popped into the thread around page 15 and read about 3 pages before it made sense. My allegiances towards different posters changed several times until I combined the ideas from a couple posts in my head and at once, a light bulb (12v DC, sorry) popped on. I will attempt to explain in layman's terms and clear up the confusion.

#1. The 240v is not created by adding together two 120v lines. Re-read this until you forget everything that is confusing you.
#2 There is one 240v winding that happens to have a center tap. Connecting this tap and either end of the CONTINUOUS 240v winding results in 120v. The 120v is a subset of the 240v winding.
#3 The two hot wires coming from the transformer are 240v when metered together because they are 240v. If you choose to meter one of them and the center tap, you get 120v. Because the winding is continuous and the tap is in the center, when you put a red probe on one of the hots and the black probe on on the center tap (neutral), the sine will start by going up. Placing the red probe on the other hot, the sine wave will start by going down, when referencing the same time domain. It would be easier to understand if the two sines were just represented as on top of each other, and added together to produce the 120v, but it wouldn't be scientifically accurate, and most importantly, RE-READ point #1. The 240v is not created by adding together two 120v lines. The coil is a 240v coil that just happens to have a center tap. Stop thinking about sine waves being added together, or cancelling each other out.
#4 The center tap is completely unneeded for 240v devices to operate, including the electric meter, because they are designed to run @ ~240v. They only require a ground for safety. The center tap (neutral) only exists for 120v devices. Period.
#5 Remember, the service is called 240v Single Phase. When the two hots are metered, there is only one sine wave. There are no other phases, and nothing reversed, or flipped over.

Hope this helps someone else, and thanks for the thread.
Bryan


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## beenthere (Oct 11, 2008)

> #1. The 240v is not created by adding together two 120v lines. The coil is a 240v coil that just happens to have a center tap. Stop thinking about sine waves being added together, or cancelling each other out.
> #4 The center tap is completely unneeded for 240v devices to operate, including the electric meter, because they are designed to run @ ~240v. They only require a ground for safety. The center tap (neutral) only exists for 120v devices. Period.
> #5 Remember, the service is called 240v Single Phase. When the two hots are metered, there is only one sine wave. There are no other phases, and nothing reversed, or flipped over.


Just to play the devil's advocate. 

Actually, the "240 volt" meter works on 120 volts, or 240.


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## zappa (Nov 25, 2011)

cygnus23 said:


> #5 Remember, the service is called 240v Single Phase. When the two hots are metered, there is only one sine wave. There are no other phases, and nothing reversed, or flipped over.


Hello, yes it is single phase. But that single phase is created with 2 mirrored waveforms moving away or towards each other at the same point in time. It's just the way a transformer works when you don't have another voltage reference such as a ground or center tapped neutral.


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## Tom738 (Jun 1, 2010)

cygnus23 said:


> ... I will attempt to explain in layman's terms and clear up the confusion.
> 
> #1. The 240v is not created by adding together two 120v lines. Re-read this until you forget everything that is confusing you.
> #2 There is one 240v winding that happens to have a center tap. Connecting this tap and either end of the CONTINUOUS 240v winding results in 120v. The 120v is a subset of the 240v winding.


I think this would be a more confusing way to think about it for most people, although maybe it helps people who don't understand it initially. Rewording it to say "The 240v is not created by adding together two 120v lines; instead, the two 120v lines are created by _dividing_ a 240v line" may help.

"One continuous winding that happens to have a center tap" is going to be really meaningless to a layman. But alternating current is probably a little meaningless too.

If they're at the level where they are thinking about the wave form, then a neutral reference voltage with +/- 120 V on either side of it seems like a pretty simple concept, but maybe I'm biased by time playing with oscilloscopes in college. 

Putting the sine waves on top of each other conceptually by making it 0 and 240V with a split in the middle feels less elegant with a more complicated graph, and I suspect it would create more problems with comprehension. But I could be wrong.

Ultimately, of course, whatever helps someone understand what's going on is worth sharing.


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## zappa (Nov 25, 2011)

zappa said:


> Hello, yes it is single phase. But that single phase is created with 2 mirrored waveforms moving away or towards each other at the same point in time. It's just the way a transformer works when you don't have another voltage reference such as a ground or center tapped neutral.


Now, if you ground either leg of the secondary 240 volt winding, with no grounded center tap of course, there WILL be only one waveform that is referenced to ground.

You need two voltage reference points to create power. If balanced, meaning ungrounded 240 like a standard residential service, you are using the two out of phase waveforms for the voltage reference as I explained above. If grounded leg, one waveform and the ground are your two references.

For the past 25 years I have been working with balanced and unbalanced (grounded) data and audio signals through transformers which is exactly the same theory but with much lower levels. I wouldn't be employed if I was doing it wrong all this time.


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## concrete_joe (Oct 6, 2014)

post #316, period.



zappa said:


> Now, if you ground either leg of the secondary 240 volt winding, with no grounded center tap of course, there WILL be only one waveform that is referenced to ground.


this is one and only one waveform.

poco has 1ph or 3ph, they do not provide 2ph by itself, 2ph is physically a part of 3ph.



Tom738 said:


> I think this would be a more confusing way to think about it for most people, although maybe it helps people who don't understand it initially. Rewording it to say *"The 240v is not created by adding together two 120v lines*; instead, the two 120v lines are created by _dividing_ a 240v line" may help.


in fact it is exactly that, "two" identical 120v secondary windings connected in series. how you make physical "tap" connection is up to maker.



cygnus23 said:


> So, I followed a link from Google that showed the 2 flipped sine waves, because I never really understood what single phase looked like as a sine wave. I knew 240 residential is single phase, but knew it contained two 120v lines and a neutral coming from the utility pole transformer.


no. it is two wires (aka "hot") that are 240ac between each other, along with a third (aka neutral or tap), and possibly a 4th (aka bare earth ground).


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## Jump-start (Sep 26, 2012)

Scott-T connection for 3 phase to two phase.


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## concrete_joe (Oct 6, 2014)

Jump-start said:


> Scott-T connection for 3 phase to two phase.


sure, but show me the generating side of the poco where only 2ph is being generated.

Niagra Falls was a 2ph generator for some time, but "_two-phase power was superseded with three phases and is not used in the industry_"

but true, poco could install a Scott T.




Conclusion:
The Scott T connection in theory would be suitable for supplying a three, two and single phase load simultaneously, but such loads are not found together in modern practice. The Scott T would not be recommended as a connection for 3 phase to 3 phase applications for the following reasons:
•
The loads of modern buildings and office buildings are inherently unbalanced and contain equipment that can be sensitive to potential voltage fluctuations that may be caused by the Scott T design.
•
A properly sized Scott T transformer will have to be a minimum of 7.75% larger than the equivalent Delta-Wye transformer. Properly sized, it would be a bulkier and heavier option and should not be considered a less expensive solution.
Applications requiring 3 phase to 3 phase configuration are best served by a Delta–Wye configuration. This configuration is the smallest and best suited for the unbalanced loads and sensitive equipment of today’s applications that could be affected by voltage fluctuations.


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