# Educate me on condenser motor hp



## SKIP4661 (Dec 3, 2008)

Won't hurt to upsize to a larger motor as long as the amp rating is the same or greater. That rpm is also strange a 1075 rpm should work.


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## Scrufdog (Jul 27, 2009)

i talked to a guy at R.E. Michel and he said its not good to go down in RPM. He had a bunch of 1075s. Whats the big difference in hp ratings? just amperage? As in, is there any great advantage to running a 1/4hp over say a 1/6hp motor (as an example only) if the RPMs are the same?


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## SKIP4661 (Dec 3, 2008)

The hp rating is the hp available for the load. A less hp motor may not have enough power to drive the load. Could result in an overloaded motor.


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## micromind (Mar 9, 2008)

First, the HP issue. 

Hp for a fan is related to RPM and air pressure. Higher RPM = higher HP. Higher pressure = lower HP. For a fan on an outside A/C unit, the pressure doesn't vary much. It's always pretty close to zero. On an inside unit, it can vary a lot. Duct size, filter condition, etc. 

Now for RPM. 

1140 is a NEMA standard for a 6 pole induction motor. The actual RPM with nothing connected to the shaft will be very close to 1200. Under full load, it'll be somewhere between about 1120 and 1170, depending on the design of the motor, voltage applied, etc. 

1075 isn't a NEMA standard (as far as I know), but more of an industry standard for a 6 pole PSC motor. As with the induction motor, the actual RPM with nothing connected to the shaft will be very close to 1200. PSC motors have more slip built in to them. They 'lug down' more under load. A two or 3 speed PSC motor will turn very close to 1200 RPM no-load regardless of the speed it's connected for. The lower speeds simply allow it to 'lug down' more. The actual speed of a 1075 RPM motor will be somewhere between about 1050 and 1150 under full load. 

Under actual conditions, the PSC motor (1075 RPM) will turn slightly slower than the induction one (1140 RPM). I'm an electrician, not an HVAC tech. They can answer better than I can, but I don't see where the difference in speed will cause problems with the unit. I could easily be wrong though. 

A 1/4 HP PSC motor will drive a fan slightly faster than a 1/5 HP one. a 1/6 HP one will drive it slower, and likely be overloaded. A 1 HP PSC motor will turn a 1/5 HP fan quite a bit faster than a 1/5 HP motor. 

Most likely, the existing motor is an induction type. It has a start switch built in to it. This switch is failing. That's why if you hit it, the switch jars closed, and the motor will start. A PSC motor doesn't have a start switch. 

I think you'd be OK with a 1/4 HP PSC motor. One thing that'll be different though, the PSC motor has much less starting torque than an induction one, so it'll start slower. 

Rob


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## Scrufdog (Jul 27, 2009)

thats to you both. I understand whats up now. Good stuff, thanks.


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## Scrufdog (Jul 27, 2009)

ok, maybe I'm not done. The motor I need to replace is mounted with a strap around it. I assume I can use any stud mount for it? Whats the difference between 48, 48Y and 48YZ?


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## micromind (Mar 9, 2008)

48 is a standard NEMA frame size. It's base-mount, the mounting holes are 4-1/4" X 2-3/4". The end of the shaft sticks out 4" past the first set of mounting holes. The shaft is 1/2" diameter, and 1-1/2" long. The center of the shaft is 3" up from the bottom of the base. 

The letter Y after the frame number means it's non-standard mounting. There isn't any designation as to what non standard means. It could be a non-standard base, or no base at all. Maybe end-mount, or a belly-band. 

The letter Z after the frame number means a non-standard shaft. It could be a different diameter, a different length, etc. 

Most HVAC motors are of the YZ frame type. They usually don't have a mounting base, and the shafts are usually longer than normal. 

Unfortunately, one 48YZ frame is not the same as any other 48YZ frame. There are several hundred different 48YZ frames in existence. Shaft length, diameter, and how it mounts are the big issues here. 

Usually, motors used in indoor units are open type, outdoor ones are enclosed. Most HVAC motors don't have cooling fans built in to them, they rely on the air movement of the fan they drive to provide cooling. 

Rob


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## Scrufdog (Jul 27, 2009)

sounds like I'll be pulling the old one to match up then. Thanks again.


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