# Continuity between phases



## MrGus480

I am troubleshooting a 480 V 3 phase A/C condenser that tripped a circuit breaker; I'm showing continuity between the A and C phases on the unit. I have resistance tested the 2 fan motors and they are good. I tested the compressor and got 65k ohms A-B and C-A, and 3k ohms B-C. The compressor is currently disconnected but I'm still showing continuity. There is a 240v crankcase heater that I haven't tried disconnecting yet that I believe may be the cause for the continuity, and I'm pretty sure the winding in the compressor is the cause for the tripped breaker.


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## joebanana

65K ohm's seems high for a motor winding. The heater would explain the 3K. Of course you're going to get continuity, it's a load. Got a megger?


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## MrGus480

I do.


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## gpop

The numbers make no sense. 

Its no surprise to see low ohms between 2 legs especially as the controls are low voltage so somewhere there has to be a transformer. 

As for a 240v crank case heater that makes no sense. why wouldn't they use a 480v crank heater. If it is correct then that means a second transformer or a second feed to the unit which would be on a different breaker. 

Tell us the proper readings so we can calculate loads. It may be a motor then again it may be the heater is shorted but with out real numbers its nothing more than a guessing game. If you honestly got 65k across the compressor something is wrong with the motor or your meter. 

Also tell us when the breaker tripped. Did it trip when the unit was off, starting, running.


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## telsa

It wouldn't happen to have a control transformer that runs from L2 to L3?

Call out a trained service tech.

Thank me later.


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## telsa

BTW, it's as common as dust for such units to have a single-phase fan while having a 3-phase compressor.

This is no time or place to learn about HVAC units.


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## MrGus480

joebanana said:


> 65K ohm's seems high for a motor winding. The heater would explain the 3K. Of course you're going to get continuity, it's a load. Got a megger?


I have a megger at work. I was using a fluke 1117 multimeter with auto ranging and it showed k ohms when I put my leads on the compressor wires. They were disconnected from the contactor. The crankcase heater is being served by a transformer taking 480vac from the A and C phases and stepping it down to 240. 

I think I'm chasing a ghost anyway, our refrigeration mechanic confirmed bad compressor when he told me he saw blackened oil when he pulled pressure gauges off the unit.


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## MrGus480

Fluke 117. Excuse my fat fingering.


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## gpop

MrGus480 said:


> Fluke 117. Excuse my fat fingering.


Not being rude but was you touching the tips when you took the reading? 

It wouldn't be the first time i have ohm-ed myself and found the reading odd.

As for 3k ohm on a transformer input that is odd i would expect 3 ohms max.


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## MrGus480

Crap, I was. I'll retest in the morning. I know better than that.


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## gpop

MrGus480 said:


> Crap, I was. I'll retest in the morning. I know better than that.


I wouldn't stress it. If you ohm lower than the motor its open so its done. It just confirms what the mechanic is thinking.


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## macmikeman

Don't megger between windings, meg em each to ground. And listen to Tesla. Disconnect everything and test your continuity again. And then megger the feeders to each other after you are sure you got all the connected stuff disconnected. Non destructive ohm testing the windings with your multimeter not a megger, see if one is odd compared to the other two .


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## MrGus480

I'll give that a try and report findings tomorrow. I want to thank everyone for their input.


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## MrGus480

telsa said:


> BTW, it's as common as dust for such units to have a single-phase fan while having a 3-phase compressor.
> 
> This is no time or place to learn about HVAC units.


I'm well aware. This particular unit has 3 phase condenser fan motors.


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## MrGus480

Continuity tests with megger show infinite resistance to ground, fluke didn't ring out when testing to ground either. Retested across compressor leads with fluke using alligator clips and got the same numbers as yesterday; possibly due to resistive qualities of oil and refrigerant. 

The contactor for the compressor was stuck in and conductors show signs of arcing, soot deposits on the insulation near terminations.


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## just the cowboy

Rule of thumb.
On a 3 phase unit readings should be close to equal, reading across all 3 phases A-B B-C A-C. Readings may run from .5 to 100 ohms but should be equal. 
If not equal it is bad, most common is one set is open or half or twice the others.


Cowboy


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