Condensing unit fan turns slowly, sometimes

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mark32

mark32

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Currently in NJ
Hey guys!

I noticed one day that my house was not cooling off as quickly as it should. I meandered out to the backyard and saw that the condensing fan was spinning quite slow, probably not even half the typical rpm. The following day, however, it was running fine! Now, a week later, the same thing is happening, little cool air blowing inside, and the fan spinning slowly. What could the culprit be? A bad cap? A bad fan motor? I just put my hand on the motor, but it wasn't hot or overly warm.
 
My first check would be the cap. If your meter can read capacitance you can check it. It’s an easy one to diagnose. Discharge the cap safely, disconnect leads, then test.
I’m in Houston. I keep an extra capacitor at ALL times. The local ace hardware stocks several values. You can also order them online.
For those that are savvy, it beats paying the $500 invoice to the AC guy for replacing a $20 part.
Next would be to meg the motor.


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Most likely the cap. If you have a capacitor setting on your meter you can check it. It will have the rating in micro farads on it with a +- %.

Oops! Posted at the same time.
 
My AC did that several years ago-- it was thunderstorm season, and I think a surge got the starting capacitor for the motor. It was a 'double--' 2 caps in one container. Took the repairman about 20 minutes to swap it out, and I was back to cool air.
 
Thanks guys,

I changed the cap, but no difference. The fan motor is okay as the blades spin easily by hand. I'm unsure what the next component to check is?
 
The HVAC guys have a device they put on those condenser fan motors which actually creates the variable speed. I forget what the device is called but it does variable speed in response to load. Slower spinning at times could be normal. Google says the device is used for low ambient but not what it is called.

Newer ones I have seen coming with ECM type for variable speed.

Not cooling is a different problem. Typical those units get service twice / year, especially for gas pressure checks. If it runs but does not cool it would cost a fortune in electricity. You would see it in your bill.
 
ActionDave said:
If it is a new model with an ECM motor I would replace it with old style fan motor.
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If it were an ECM motor he wouldn't have had a capacitor available to change.

mark32 do you have full voltage to the motor? These are typically PSC motors which can have speed varied simply by varying input volts (well sort of). If you had a damaged conductor that was faulted to ground it may be running on 120 volts instead of 240, that would lessen the torque it can produce and would therefore run at a less than normal speed. Ground fault within motor could even do this, megger would detect that.
 
ptonsparky said:
No...No...No. Never sell tools! Store them so your heirs can go "WTF is this?!"
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My dad passed away little over a year ago. He was a mechanic, welder, machinist, and a few other things over the years. He has many "home made" tools that served a specific purpose that I will never know what a lot of them were even for. Like a tool for assisting pulling a bearing out of a specific part of a certain line of tractors, or even a specific model, or things of that nature. He had a metal lathe and could do a lot of different things with that. We were fixing something of mine one time and having trouble getting something either apart or assembled (can't remember) and he ended up making a tool to help us with the task.

I would have kept the lathe but don't have time to learn how to use it. My sister's husband is going to try to learn to use it though so he took it.
 
Saw a This Old House episode where this was happening (or something like it) because the exchange inside was frosting up? Maybe take a peek at those coils next time this happens?
 
Joe.B said:
Saw a This Old House episode where this was happening (or something like it) because the exchange inside was frosting up? Maybe take a peek at those coils next time this happens?
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Some AC units intended to run when it is cold outside may either run condenser fan at lower speed or even shut completely down via a thermostatic control, but I don't think froze up evaporator coil (if that is what you are talking about) would matter. If the unit has no such controls the condenser fan is on whenever the compressor is on.
 
Sorry about your Dad kwired, my old man passed away almost five years ago, we were tight. Anyway, I know on one of the occasions, when I noticed the fan was turning slowly, it was not even hot out, it was like 60 degrees outside. (My wife had made a soup and the kitchen was warm, so she turned the a/c on). So maybe this unit has this function built into it? I guess we'll see as soon as it warms up!
 
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