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Thread: Condenser fan motors tripping thermal cut out.

  1. #1
    Join Date
    Aug 2017

    Condenser fan motors tripping thermal cut out.

    A good friend of mine has been having trouble with an old a/c unit. Recently had a refrigerant leak which has been repaied. The unit worked fine for about a week until 2 days ago when the condenser fan motor failed. The motor and capacitor were replaced. Now that the new motor is in, during the heat of the day the motor will shut off due to high temprature. The motor was replaced with the exact RPM and hp as the original. The old fan has been reused. With a thermocouple on the motor case it's running around 150-170 f. Shutting off at 180. We have checked voltage and frequency and they are fine. 240v 60hz. The amp draw on the motor is %75 of its nameplate rating. After verifying proper voltage, frequency and capacitance in the new capacitor we deduced there must be a problem with the new motor. Purchased another motor, from a different store and different brand. Same issue results. In about 20 min the motor will go from ambient to 180 and trip the thermal cutout. Now at a loss we upgraded the motor from 1/4hp to 1/3 hp, different brand and now larger motor. What do you know, exact same results. 240v, 60hz and %75 of name plate amp draw.

    At this point I have gone over the wiring multiple times, bypassed the AC wiring and powered the motor direct. Also powered it separately from another 240 source (dryer plug). Every result is the same. The motor appears to spin at the correct RPM. It's moving air and the house is cooling just fine. Only issue is the fan motor is shutting off due to thermal overload. I have noticed that the motor is humming. Sounds like a 60hz hum. Sounds like a 3p motor with a lost phase or voltage inbalance. Only thing is, its a 1p motor and voltage and hz stays steady. Amp draw is within spec. Refrigerant compreasor has no issues and does not get excessivly hot. Only thing I can think of is we got multiple bad motors. Only think I can think of to test next is to meg out the windings in the motor.

    Any one have any ideas as to what could be the problem here. Any troubleshooting steps that I missed? Any help is greatly appreciated as it HOT in Florida right now

  2. #2
    Join Date
    Feb 2003
    Seattle, WA
    I am closing this thread in accordance with forum rules. If you are not an electrician, then we are not permitted to assist you in performing electrical troubleshooting or repair work.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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