Motor troubleshooting help

[Jraef]

None of those tests would detect a shorted turn, which would greatly increase the current in one winding (therefore in two phase wires.) That seems to match the symptoms.
An ohm measurement using AC as the applied voltage or using an impedance bridge would conclusively diagnose this problem.
If you cannot measure one winding at a time (with delta corners being permanently bonded) you just have to realize that each measurement gives you a different series-parallel combination of all three windings.

The amperages seen could correspond to the combination of effects of voltage imbalance and a shorted turn.

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What I meant to say was that the simplistic L-G megger test would not catch that, not that a L-L would... sorry, I wrote that wrong.

 

[Ingenieur]

run it at a lower pressure
run it at no pressue
run it with the belts off
measure i for all 3

reciprocating type?
how many pistons?

 

[MTW]

I'm with MTW on this.

You have a significant current imbalance, about 22%. That's going to cause most SSOL relays to trip, because a severe current imbalance will cause extra motor heating even though the current is not exceeding the setting. That's what makes SSOLs better than heater type. So DO NOT "bypass" it! It's likely doing EXACTLY what it needs to do to keep that motor from burning up!

So the issue is, what's causing the current imbalance? Your voltage readings are showing that the current imbalance is not the result of a line voltage imbalance, so you ruled that out.

He only checked at the panel supply end, according to the OP. I wouldn't consider it ruled out just yet. I would want to see voltage readings at the load side of the starter, while loaded, before going further.

MTW

 

[petersonra]

They use CTs don't they?

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Cts are not heat sensitive.

 

[petersonra]

I'm with MTW on this.

You have a significant current imbalance, about 22%. That's going to cause most SSOL relays to trip, because a severe current imbalance will cause extra motor heating even though the current is not exceeding the setting. That's what makes SSOLs better than heater type. So DO NOT "bypass" it! It's likely doing EXACTLY what it needs to do to keep that motor from burning up!

So the issue is, what's causing the current imbalance? Your voltage readings are showing that the current imbalance is not the result of a line voltage imbalance, so you ruled that out.

When the IR guy said he "checked the motor", what does that mean? Megger test? If not, whatever he did was meaningless. If so, was he just checking L-G, not L-L? it could have an insulation breakdown turn-to-turn or phase to phase, but nothing going to ground. If all he did was the basic test of hooking all of the motor leads together to one megger lead and the other to ground, that would not find those other problems.

20% is not that bad. especially when the motor is as underloaded as it is. in fact, those readings suggest to me the motor might not have been loaded at all when the readings were taken and maybe when it is loaded the readings are different.

ETA: It appears the default setting in AB E300 electronic OL relays for current imbalance is 35% which is more than 50% higher than this situation.

 

[GoldDigger]

20% is not that bad. especially when the motor is as underloaded as it is. in fact, those readings suggest to me the motor might not have been loaded at all when the readings were taken and maybe when it is loaded the readings are different....

A good point. I think that to a first approximation the counter-EMF will be the same for all windings, and will have its greatest effect at low slip and low power. So under those conditions a small % difference in applied voltage can make a larger % difference is effective voltage (applied voltage minus counter-EMF.)

We might be looking for a condition that causes the OLs to trip only under certain loading conditions or only intermittently.

 

[Ingenieur]

do you have a pic of the name plate?
rated hp?
rated v ?
rated pf and eff?

 

[topgone]

Just replace the motor and get done with it, IMO.

 

[Jraef]

do you have a pic of the name plate?
rated hp?
rated v ?
rated pf and eff?

Just replace the motor and get done with it, IMO.

The OP said he is already done with it, he was just looking for ideas on why it might have been tripping.

 

[Cow]

While it was running I got readings of A- 29.7A, B- 23.5A, C- 18.9A.

This is a severe imbalance and is NOT normal despite what anyone may tell you.

I would have tested voltage right at the motor splices a-b, b-c, a-c. If voltage was within spec at the motor with a current imbalance like that, I would tell them to replace the motor.

Nothing more on your part needs to be done at this point, no OL's need to be bypassed, etc. They just need to replace the motor. That's the first step.

 

[Jraef]

This is a severe imbalance and is NOT normal despite what anyone may tell you.

I would have tested voltage right at the motor splices a-b, b-c, a-c. If voltage was within spec at the motor with a current imbalance like that, I would tell them to replace the motor.

Nothing more on your part needs to be done at this point, no OL's need to be bypassed, etc. They just need to replace the motor. That's the first step.

That's what I was thinking. Yes, the A-B E300 OL relay is factory defaulted for 35%, but we don't know what I-R uses as a SSOL. When I was at Siemens a decade ago, it was the Siemens ESP200, which has no set value for phase current unbalance tripping. It changes based on the OL trip class you set it for by biasing the trip curve and the higher the imbalance, the stronger the bias. So for example if you set it for Class 5 or 10, the unbalance will severely bias the OL tripping curve to where (from my experience) the OL trips at around 50-60%% of its set value (people used to complain about that a lot). If you set it for Class 20, the same imbalance has less of an effect. So a lot of submersible pumps require Class 10, some (Franklin) require Class 5, and they would be very susceptible to what were considered otherwise "acceptable" levels of current imbalance. When I was there, Siemens had obsoleted the old ESP100s they got with buying Furnas, and the ESP100 didn't do this. The ESP200 was nothing like it, they used the chip set from the German SSOL and that had this "variable" imbalance system; people were NOT happy with it. So later, Siemens put in a switch so you could disable it and most submersible users do that.

 

[Cow]

some (Franklin) require Class 5, and they would be very susceptible to what were considered otherwise "acceptable" levels of current imbalance.

I know what you mean. Franklin has those "smart" starter pump panels with an LCD screen and menu for setting various trip points. Just two weeks ago I had to adjust one from the factory default of 5 percent because it kept tripping.

I know dairies don't like their water troughs running out of water because of extra sensitive factory defaults...

 

[kwired]

Compressor load very likely is higher when it gets near cut-off pressure. If you didn't happen to measure current at that time, you could possibly miss the situation that is triggering the trip. The unbalanced current with pretty balanced voltage is a sign you have motor winding issues though.

 

[Besoeker]

Thanks, that is exactly what I was thinking also so can't figure out why they are resistant to the idea of changing the motor.....

Quite.
It's not even a big expensive motor. Maybe access is physically difficult?

 

[MrJLH]

Take an IR scan of the breaker/starter.

Also have you started looking at the compressor?

What type?

I would start looking at the loading valve and even perhaps the air system.

 

[Besoeker]

Take an IR scan of the breaker/starter.

Also have you started looking at the compressor?

What type?

I would start looking at the loading valve and even perhaps the air system.

Don't think that would cause unbalanced currents. And that seems to be the obvious issue.

 

[topgone]

Don't think that would cause unbalanced currents. And that seems to be the obvious issue.

On second thought, methinks the problem evolved from overheating of the windings. It could be that there was small delayed setting of the loader valve causing very high currents on the motor --> then caused the inter-winding shorts --> which in turn resulted into unbalanced currents on the phases! Maybe a stretch but very possible.

 

[Cow]

On second thought, methinks the problem evolved from overheating of the windings. It could be that there was small delayed setting of the loader valve causing very high currents on the motor --> then caused the inter-winding shorts --> which in turn resulted into unbalanced currents on the phases! Maybe a stretch but very possible.

Which means you still have to replace the motor to have a solid baseline to troubleshoot from.

 

[Besoeker]

Which means you still have to replace the motor to have a solid baseline to troubleshoot from.

Yes, it seems the obvious course of action. Why the reluctance I wonder?

 

[retirede]

Yes, it seems the obvious course of action. Why the reluctance I wonder?

OP stated his responsibility was only to check premises wiring. There was a service professional on site for the compressor.

I’d guess he passed on the recommendation to him/her.