Resistance Value of Compressor??


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Old 04-18-10, 06:54 AM
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Resistance Value of Compressor??

While troubleshooting my friend's A.C. unit I determined that it was getting 240 volts to the compressor that was not coming on.

The contactor would vibrate about every minute or so for about 3-5 seconds and when it did, the voltage to the compressor would drop about 10 volts. This showed me something was trying to take place.

I disconnected the wires to the capacitor and tested it with my ohm meter. No shorts and it showed a charging kick. This is a test that is not fool-proof but a good indicator that all was well.

I removed the feed wires to the compressor and carefully zeroed the analog ohm-meter on the X1 scale. The reading showed zero ohms across the compressor input wires but there were no shorts to ground. I expected to see a few ohms of resistance at least.

I then did the same tests on a second unit he had and saw the same result. No readable resistance across the compressor feed wires.

My question to those in the know is if this (zero ohms) is the proper reading. If so, I can only suspect that the inductive resistance is the factor that allows the compressor to function properly and not appear as a short circuit to the circuit breaker.

The voltage drop across the contactor when it was vibrating may also have been enough to indicate that the contacts were shot. I guess I could have jumped them to be sure. That may be next.

The unit is 15 years old and was going through its first seasonal attempt to start. It may also need a "hard start" capacitor.
 
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Old 04-18-10, 07:38 AM
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Not a pro on compressors, but have used the analog meters almost forever. I would not have been disappointed with an ohm or two and I suspect that is what you were looking for, but I'm also not too concerned with zero, especially with your test of another unit. My thoughts are, your logic is fine, but not sure where the problem is.

Bud
 
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Old 04-18-10, 10:11 AM
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A good compressor will not have 0 ohms across the windings.
Common to Run will have the lowest value. Common to Start will have a higher value. Run to start will have the highest value.




After failing to start (and getting hot) the Common can open on thermal protection...


I would not trust that type of capacitor test.

A reading of starting amperage would probably indicate at or near the LRA listed on the unit nomenclature.


These are some good meters capable of measuring caps and amperage...




 
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Old 04-18-10, 10:16 AM
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Thanks for that fabulous description and information.
 
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Old 04-18-10, 01:08 PM
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Thanks Houston204 in 25 words or less .

Bud
 
 

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