How to determine what the winding resistance should be on a refrigerator compressor

Thread Starter

Yami

Joined Jan 18, 2016
221
Hi guys,
I've got a ice cream display which when the compressor turns on it trips the circuit breaker. I have ruled out loose connection etc. and it all points to the compressor.
What I have found is that the run capacitor which should be 20 uF is 14 uF and the Start relay contacts have arched and corroded away. I am going to replace both of those. (I changed the capacitor and tried the relay after cleaning and sanding down the contacts, still the C.B kept tripping).

Anyway going to my original question
I measured the coil resistance of the compressor and I get:
Common ---- Run = 1.5 ohms
Common ---- Start = 5.4 ohms
Start ----- Run = 6.9 ohms

Does this look alright, 1.5 ohms seems low doesn't it? and I also checked whether the compressor was grounded and it wasn't.

Any help would be greatly appreciated.
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
That is about right, measuring resistance is not really a definitive test, the run winding is going to be the lowest.
If the start relay is now OK, it is either a shorted turn or a mechanical problem.
If the compressor is belt driven, you could disconnect this to test the mechanical issue.
Max.
 

Thread Starter

Yami

Joined Jan 18, 2016
221
Hi guys, just to give an update. I had a look at the data sheet of the compressor (NJ2212GK) and the resistance rating for the start winding is 7.25 ohms, the rated resistance value for the run winding is 1.95 ohms. So the readings I get are not 'so' off but there are a bit off. Could that be an issue.

I have replaced the starter relay and the run capacitor and still the breaker keeps tripping. The locked rotor value for the compressor is 36A, and the current MCB that was connected to it is rated at 10A. Just for the sake of it I tried a 32A MCB to see whether it was an issue with the MCB. Still it kept on tripping. I couldn't get enough time to even check the current draw.

Thanks
 

crutschow

Joined Mar 14, 2008
23,334
Sounds like the motor may have a shorted winding.
That's can't be determined with an ohmmeter.
 

BR-549

Joined Sep 22, 2013
4,936
"Does this look alright, 1.5 ohms seems low doesn't it? and I also checked whether the compressor was grounded and it wasn't."

Was there any resistance between windings and compressor frame?
 

Thread Starter

Yami

Joined Jan 18, 2016
221
"Does this look alright, 1.5 ohms seems low doesn't it? and I also checked whether the compressor was grounded and it wasn't."

Was there any resistance between windings and compressor frame?
Nope there was no connectivity between the chassis and the windings. I couldn't get hold of a Mega ohm meter though. I just checked with my DMM.
 

Thread Starter

Yami

Joined Jan 18, 2016
221
Hi guys, another update - I was able to borrow a mega ohm meter. I got a reading of around 300 M ohms across winding to chassis (point which had no paint on). So I can safely say that the compressor is grounded no?

Thanks
 

Thread Starter

Yami

Joined Jan 18, 2016
221
If that is a high voltage insulation Megger then 300megs is fine, for no ground fault.
Max.
Hi Max, yes its a high voltage meggar, I checked with 250V and also 500V. So should I assume that the compressor is ok?
 

BR-549

Joined Sep 22, 2013
4,936
How do you know that the compressor runs? How long does it take for the breaker to trip?
 

Thread Starter

Yami

Joined Jan 18, 2016
221
How do you know that the compressor runs? How long does it take for the breaker to trip?
It doesn't run at all, however once I heard it run for maybe less than 1 second before the breaker tripped. I didn't get that afterwards.
 

Thread Starter

Yami

Joined Jan 18, 2016
221
Compressors like that do fail, not the motor but the actual compressor pump. How old is the unit? Has it ever been recharged in its life time?
The unit is not that old, I would say maybe 2 - 3 years. I don't think it have been recharged (I'm not sure what it means) - this would be the first time that the actual chiller is opened after purchasing.
 

BR-549

Joined Sep 22, 2013
4,936
When did you buy? Is it warranted?

What is the voltage rating? What is your line voltage?
 
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