Motor amperage

[BREEZY]

Good morning,
Just trying to get some clarity for a customer.
I have a 3 phase 40 amp motor for a compressor fed from a starter, the starter is fed from a 60 amp fused disconnect.

When the motor is running I have 40 amps/28amps/21amps on the load side of the disconnect. It seems as if the motor is not drawing enough amperage and the motor could be failing. Sometimes when the compressor is turned on the lights in the area get dimmed and the compressor wont start. Shut it off and turn it back on and it works fine again.

I noticed that the contacts are a little beat up and looking to get those cleaned up, the heaters themselves look fine but I did not remove to check behind them to see if there was any melting.

I am not too familiar with these types on installations and what could be the source of the problem so any info can help. I was going to order a contact kit and replace the internal parts of the starter, but if it is in fact the motor failing this will be for nothing. What is the correct way to see if the motor itself is failing?
Should I test voltage across each leg of the load side of the disconnect to make sure the proper voltage is there when motor is running? Also test voltage on starter when motor is running?


Any info would help.

Thank you!

 

[kwired]

voltage readings kind of important, both loaded and unloaded.

Testing voltage from supply side terminal to load side terminal of each pole while loaded (fall of potential test) can tell you if you have excess resistance in the disconnect or starter which could end up giving you performance like you describe if there is.

If voltages remain stable between loaded and not loaded chances are the motor has problems. Voltage may drop when loaded but should still remain fairly equal when it comes to phase to phase readings. If voltages are not equal you have a supply problem.

If this happens to be fed from a rotary or static phase converter it gets more complicated as the manufactured phase in those will normally be sort of unstable and seemingly unpredictable if you don't understand them very well.

 

[retirede]

Motor starter contacts aren’t designed to be messed with. They always look bad. That’s a result of making and breaking motor current. If you dress them up and make them nice and shiny, you’re only reducing their life. Measure the voltage drop under load, and if excessive, replace the contacts.

 

[ptonsparky]

Voltage readings not just phase to phase, but across the contacts. Line to Load. It's called Fall of Potential. A meter capable of millivolts is needed. Look for large discrepancies from one to the next and in excess of about 100.

 

[ptonsparky]

While you're doing the FOP test, do it from Line conductor to Load conductor. Then move one lead up, or down. Note the reading. Repeat.

The same method is used for checking connections at fused disconnects and CBs. Generally an overheated condition is visibly noticable but not so much in the early stages.

Simple hand held infrared thermometers are available, inexpensive and will work.

 

[BREEZY]

Thank you all for your replies,
The customer at the moment is also bringing out the manufacture rep for the compressor itself to see if there are any issues on their end. At the moment I am too busy to get back over there to do more testing but intrigued to find where the issue lies. Seeing as I was not too familiar with the tests to perform. As for voltage on the line side I did test that and I had 208/210/211 from phase to ground. I will have to go back and do voltage testing on load side and FOP test on disconnect and starter. This info has helped me greatly and always willing to expand my knowledge that's why I came here. After I do more testing I will come back with the results

Currently the 60 amp disco is fed from a 3-pahse 100 amp breaker, the 3-phase 60 amp fused disco feeds the starter then feeds the motor

Thank you all,
Breezy

 

[retirede]

Those phase to ground voltages seem strange. Did you measure phase to phase?

 

[BREEZY]

Those phase to ground voltages seem strange. Did you measure phase to phase?

I apologize I believe the voltage to ground was 120volts and between phases I had the 208/210/211volts

 

[Jraef]

When the motor is running I have 40 amps/28amps/21amps on the load side of the disconnect. It seems as if the motor is not drawing enough amperage

It's not that it is "not drawing enough" amperage, a motor draws what it needs to, so the other 2 legs showing 21-28A is probably what the load actually needs. But the real problem is that severe IMBALANCE in current.

The voltage is pretty good; <1%, so it's not that. A current imbalance that high wouldn't be anything mechanical (that you wouldn't
notice from the compressor walking across the floor from vibration). If it were a bad contact, it MIGHT cause that much of a difference, but it would have to be RESALLY bad. The FOP test would show that up glaringly so it's worth doing to eliminate that possibility, but I don't think that's going to be it..

So usually that much of an imbalance means there is a winding insulation failure going on in the motor. You need to put a megger on it.