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thothtp 06-04-2008 02:30 PM

Replacing a Condenser Fan Motor
Just found this forum, tons of great info! I just bought my first house over the past winter, and lo and behold discovered this spring that my AC Condenser Fan motor has seized up. When I turn on the AC at the thermostat, the Compressor fires up just fine, but the fan motor is locked up.

The Unit is an oldish (no idea how old unfortunately) Ducane unit, and I cannot find any info whatsoever on the motor itself. GE's website no longer lists the model number, so I assume it was discontinued long ago.

I took the motor to the local Grainger branch, and they are currently searching for an OEM part for me. In the meantime, I was able to find an A.O. Smith motor that exactly matches the specs, other then being about half an inch longer. I measured, and that should not effect anything, as I can mount the fan closer to the motor on the shaft. Here is a link to the motor.

Now here is my question. Assuming that I end up purchasing this motor, I need to figure out what to do about the capacitor. On my Unit, there is a single large cylindrical capacitor with 3 poles, and it appears that both the compressor motor and the fan motor are wired in to it. My question is, should I:

A: Wire the new motor into the existing Capacitor. The specs on the old motor nameplate list 5 MFD and 370 volt, the same as the requirements on the new motor. Is it safe to assume the old capacitor will work best?

B: Get the capacitor listed on Grainger's website as a required accessory. It's only $4. I would then wire up the new motor to the new capacitor, and keep the Compressor wired up to the old capacitor. Will that work? Will it damage the compressor somehow by not having the fan motor wired into the same capacitor? Will it even work without the fan motor wired up?

C: Figure out a new multi pole capacitor to wire both motors up to. I don't know which one I would need for this.

Thanks so much for the advice, I really appreciate it.

micromind 06-04-2008 07:41 PM

First, a bit of advice about capacitors. Safety stuff. Obviously,make sure the power is off to the unit. Capacitors are like batteries in that they store a charge. Most of them have resistors soldered across the terminals, to discharge them when they're de-energized. It takes about 5 minutes. Some don't. Sometimes the resistor comes loose from the terminal. Grabbing the terminals of a fully charged capacitor is....well.....unpleasant! Get a piece of insulated wire, strip both ends a bit, and short out the terminals. Short it to something grounded as well. (Like the metal its mounted to). If it is indeed charged, you'll get a nice SNAP, and you'll be very glad it didn't snap you!

How to connect the capacitor depends entirely on the new motor. You'll need to either just buy it, and look at its wiring diagram, or somehow get the wiring diagram for it.

If it shows only one capacitor wire, you can hook it up the same way the old one was, using the same capacitor. In this case, you don't need a new capacitor.

If it has two wires going to the capacitor, then the old capacitor won't work with the new motor. There are two ways to go from here. Either way, you'll need to buy a new capacitor for the new motor.

1) Simply remove the fan lead from the capacitor, and leave its terminal unused. Leave the compressor leads as they are. Connect the new fan motor capacitor as shown on the nameplate. It doesn't matter which lead goes where on a two lead capacitor. The two power wires from the fan motor go to the load side of the contactor.

2) Connect the new fan motor as above, and replace the compressor capacitor as well. To get its rating, you'll need to remove the existing capacitor and look at its rating. It'll likely be 15/5 or 20/5 or something like that. The old capacitor is, in reality, two capacitors in one can, and they share one lead. You already have the 5 MFD one for the fan, so you'll need to get the other one for the compressor. The MFD rating must match (the 5 MFD already does), the voltage rating must be the same or higher. For example, you can use a 370 volt capacitor to replace a 250 volt one, but you cannot use a 250 volt one to replace a 370 volt one. Round or oval doesn't matter, as long as you can get them mounted.

If you run into more difficulty, post back, there are several of us that can help.


thothtp 06-05-2008 08:13 AM

Is it at all possible for a bad capacitor to seize up a motor? I have no idea what caused the original to seize up, as I was not living here, although I am assuming it is just age. However, if for some reason the old capacitor could have caused that, then I don't want to risk burning out the new motor. Is that even feasible? Or will a bad capacitor just make the motor not run right.

Is there any downside (other then $4 and extra wiring) to getting a new motor capacitor, and leaving the compressor wired up to the old one?

Thanks again for the advice, I really appreciate it.

micromind 06-05-2008 11:57 PM

I don't see how a capacitor can cause a motor to seize up. A bad capacitor can cause it to burn up in time, usually motors seize up because the bearings get wet and rust, or get dry (run out of grease).

If you can get the endbells off, you might be able to replace the bearings, assuming everything else is still good. When you get it apart, it'll be obvious if the windings are burned up, not much else goes wrong with this type of motor.


thothtp 06-06-2008 08:16 AM

Unfortunately the OEM motor is a riveted together sealed unit that I cannot get apart unless I drill out the rivets. I am assuming the bearings just wore out eventually, either through rust or getting dry.

I ended up replacing it with the motor from Grainger I linked earlier, and using the old capacitor. Everything seems to be working great, and just in time for 96 degree weather! Thanks for the help guys.

ridgevilleac 06-08-2008 06:30 PM

If you have a 3 lead motor you can wire it the same as the old, BUT i would still replace the capacitor. It may still work but it is used up. It is always a good idea to replace the capacitor when replacing a motor. Micromind hit it on the head. A bad capacitor can burn up a motor. It just makes good logical sence to replace one with the other. If a 4 wire motor is purchased, the 2 brown leads need to be connected to a new 5uf capacitor strapped to the old one or somwhere as to not fall and short out. The other 2 leads usually black and yellow will be connected where the original motor is wired. Be sure to remove the brown capacitor wire on the old capacitor. Hope this helps.

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