Outside Fan motor replaced, caps replaced, still cuts out


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Old 08-28-09, 09:49 AM
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Outside Fan motor replaced, caps replaced, still cuts out

My daughter has a Carrier 48NHT030 central air system. Not ancient, but old. It stopped cooling recently, and I diagnosed a run capacitor. She called an HVAC guy and he came out, told her it was the fan motor (but replaced the cap as well as the motor)

Now the system starts from a cold start, works well for 20 minutes or so, and the fan motor stops. The motor has been replaced twice, and the last one a genuine Carrier motor.

My question is this. Other than the internal thermal breaker of the motor, is there a safety thermal switch that will shut off the motor prematurely? I am not willing to accept that three motors are bad.

I had her place a separate fan on top of the unit to blow cool air into the unit, and it caused the fan to start. So it's something thermal...but the fan motor shouldn't cut out, right?

I thought since there is a heating unit in the same frame, that there is a loop of sensors that are starting to degrade and causing nuisance cutouts.

What do you guys think?
 
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Old 08-28-09, 10:38 AM
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Has the fan blade ever been replaced. If so the pitch could be different and is overloading the motor. Also if the condensing coil is dirty the motor could also overload. I would try washing the coil with a hose.
 
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Old 08-28-09, 10:49 AM
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And what about the contactor? When the fan stops..does the compressor continue to run? Or does everything outside shutdown?

Are the connections to the contactor clean and tight...how about the contacts themselves?
 
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Old 08-28-09, 02:22 PM
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I would check the running amps of the condnenser fan motor to confirm an overload condition.
 
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Old 08-28-09, 06:33 PM
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Will a bad contactor provide enough resistance in the circuit to slow down the motor and overheat? I guess that makes sense. If the motor is running at a lower voltage all motors will overheat, right?
 
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Old 08-29-09, 11:12 AM
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It might..bad corroded connection and arcing at the contacts can cause resistance. What I was suggesting though was a weak coil on the contactor..that just gives up after it gets too hot. Thats why I asked about if the compressor was still running when the fan cuts off.

Best to check the voltage on the output of the contactor.
 
 

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