# Air Compressor blowing start capacitors?



## micromind (Mar 9, 2008)

My guess is that the start capacitor is not being dis-engaged once the motor is up to speed.

If it is indeed a capacitor-start motor (not PSC; Permanent Split Capacitor), then the start capacitor is energized only during starting. Once the motor has reached about 1/2-2/3 speed, the start capacitor and start winding are de-energized, usually by a centrifugal switch inside the motor. 

If this switch fails to de-energize the start circuit, the capacitor will last only about 2 or 3 minutes. 

Start capacitors are designed to be used for only a short time, they won't last long if used continuously.

If you have a clamp-on amp meter, and can get it around one of the capacitor leads (safely!), there should be current only for a second or two. If there's still current after the motor has come up to speed, the start switch is bad. 

Some motors use a potential relay instead of a switch, it's a small box with 3 leads, usually one of the leads goes to the capacitor. 

To get to the start switch, you'll usually need to remove the back of the motor. The flyweight part is on the shaft, the switch part is on the endbell. 

Rob


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## BBC3 (Mar 15, 2010)

Thanks for the great reply. Yes the capacitor is a simple 2 lead 360v 60MicroFarad rated start only capacitor. I did notice that the first an only run after I repaired it, the thing seemed to run like a demon, and MAY have run a bit hot. It also seemed to fill the air tank to shut off capacity prettty quick. So I think you hit it.

There seem to be 4 main components to the unit. Electric motor, pump unit, start capacitor, and finally there is a small box outside the motor and pump that I believe is a "safety switch" they are calling it. Could that be the switch you are referring to i wonder?

2 more questions please. I am assuming the start capacitor is only useful for starting the unit one it is charged with air, as a load assist type starter, and that in theory I could run the unit without one as long as it fully discharged, or to a level low enough not to tax the electric motor on startup?

(2) Why is the capacitor rated so high? To try to understand MicroFarad rating vs. voltage, I am assuming the MFs are a measure of how quickly the capacitor will reach full potentcy for that "dailed up" voltage, and how quickly it can discharge? And why the 360V rating for a unit that operates at 110V? Is it just extra volts as a means to add the kick to turn over under a load? And that this extra voltage doesnt pack the amperage to hurt the motor?

Thanks






micromind said:


> My guess is that the start capacitor is not being dis-engaged once the motor is up to speed.
> 
> If it is indeed a capacitor-start motor (not PSC; Permanent Split Capacitor), then the start capacitor is energized only during starting. Once the motor has reached about 1/2-2/3 speed, the start capacitor and start winding are de-energized, usually by a centrifugal switch inside the motor.
> 
> ...


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## micromind (Mar 9, 2008)

A single phase motor is bi-directional, meaning it can turn in either direction. To get it to start, a second winding can used to cause a phase shift and make it turn in one direction. 

If there's a capacitor, it's in series with the start winding, and it'll cause more of a phase shift, resulting in more starting torque.

The problem is that once the motor is up to speed, the start and run windings will work against each other. So the start winding must be dis-engaged. 

If the start capacitor is not working, the motor won't know which direction to turn, and it'll draw a LOT of current, and overheat quickly. 

A capacitor is like a battery, but it can be charged and discharged VERY quickly. And it can withstand millions of charge cycles. 

The voltage rating is the maximum it can operate at. It'll work just fine on a lower voltage. 

The microfarad rating is how much energy it can store. The higher the mfd, the more of a charge it can store. 

The mfd rating of a start capacitor is tuned to the motor windings and voltage. If it's too big or too small, the motor won't start very well. 

The voltage rating is also related to the design of the motor. Since this is a capacitive-inductive-reactive circuit, the voltage across the capacitor can be higher than the input voltage. 

In short, the motor needs the capacitor to be the correct mfd in order to start, and it must be taken out of the circuit once it's running.

Rob

P.S. The 'safety switch' on the pump might be a low-oil shutdown. It could be the pressure switch that tells the motor to start when the pressure gets low, and to stop when it's high enough.


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## BBC3 (Mar 15, 2010)

Just a final update. I was able to repair the unit. Everyone makes this basic model with a combined type moter / compressor pump type scenario. It turns out the all use a capacitor that is both start and run capable. I found this out only after replacing a start only capacitor which blew in less than minute and running the unit super fast/hot. There is no cutoff switch anywhere on these units. It is a combo capacitor ( really kinda hybrid in nature) that these type compressors have, and thus explaining the HIGH (40-60$) price tag. On a further note, I noticied where Mi-T-M, who also makes that unit for various companies to sell under other names, has ceased production. They are remarkable units in that there is no conventional BRUSH-type motor, and thus should run forever given that one keeps the compressor motor oiled.

I also noted that home depot sells a Rigid brand unit for about 250 with a 3 year warranty. It is also an oil free unit. Thats probably a negative given an oil unit will last a lot longer properly maintained.



micromind said:


> My guess is that the start capacitor is not being dis-engaged once the motor is up to speed.
> 
> If it is indeed a capacitor-start motor (not PSC; Permanent Split Capacitor), then the start capacitor is energized only during starting. Once the motor has reached about 1/2-2/3 speed, the start capacitor and start winding are de-energized, usually by a centrifugal switch inside the motor.
> 
> ...


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## Yoyizit (Jul 11, 2008)

BBC3 said:


> And why the 360V rating for a unit that operates at 110V?


I used a series capacitor to drop 120vac down to lower levels by using its reactance. A 200 vdc capacitor would pop within minutes. A 400 vdc cap held.
The peak value of 120vac is 170v so the 200vdc unit should have held.
???


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## BBC3 (Mar 15, 2010)

I am not sure what a series capacitor is? Perhaps that means one with multiple voltage addressing? Or the range I was looking for. I can tell you this. When I used a start only capacitor, the 360volt capacity equalled running like a bat out of hell:yes:. It charged up about 3-4 times faster. Obviously too hot, and killed the capacitor. I was only back here to tell you all, that there is no centrifical switch associated with these units, and the capacitors employed are definitely not start only capacitors..:blink:



Yoyizit said:


> I used a series capacitor to drop 120vac down to lower levels by using its reactance. A 200 vdc capacitor would pop within minutes. A 400 vdc cap held.
> The peak value of 120vac is 170v so the 200vdc unit should have held.
> ???


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## a7ecorsair (Jun 1, 2010)

Yoyizit said:


> I used a series capacitor to drop 120vac down to lower levels by using its reactance. A 200 vdc capacitor would pop within minutes. A 400 vdc cap held.
> The peak value of 120vac is 170v so the 200vdc unit should have held.
> ???


What is the Peak to Peak value for 120V RMS?


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## Yoyizit (Jul 11, 2008)

a7ecorsair said:


> What is the Peak to Peak value for 120V RMS?


2x1.414x120 = 340v, assuming a sine wave.


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## franko35758 (Apr 28, 2013)

*air compressor*

I bought a central pneumatic air compressor at HARBOR FREIGHT.
4 hp-10 gal. 
I only use it for filling a portable tank to keep in my truck for emergencies
with my lawn care business.
it's only two y/o and does not have more than 10 hrs on it.,if that.
It quit running and will not start. it only hmmmms and spit.
the techs at harbor freight say's it's the starting capactor.
they were really great about helping me and sent me two that should have worked. not the same as the old one, they said they have changed
the models up.
I installed one of the capacitors and it is still hmmms and spits!
I rewired and the same.
I paid 130.00 $ for it and would like to get it fixed. thanks for your support. [email protected]


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