Motor troubleshooting help

[ishium 80439]

I've got an issue with a motor that I'm hoping someone more knowledgeable than I can shed some light on.

It's a 3 phase 208V motor for a compressor. The nameplate rating is 38.4A. It is being fed with #8 THHN and the panel is approximately 100' away. I was called because they were having issues with it and the tech from Ingersoll-Rand requested I come out and and look at the feed side of things as he could not find an internal issue.

While it was running I got readings of A- 29.7A, B- 23.5A, C- 18.9A. Voltage was consistent at the panel lugs and the breaker at A-B 209.4, A-C 209.9, B-C 205.3. These voltages held steady both under load and unloaded. All connections were secure and there were no signs of overheating or corrosion. The panel this is being fed from is practically a dedicated panel with only a lighting circuit and a circuit for a tube heater in it.

To me it seems that it is obvious that the problem is in the motor but for some reason the engineers keep looking to the premise wiring as the culprit. What am I missing?

 

[gar]

180307-1120 EST

Measure the voltages at the motor. This may mean several places around the compressor. Such as, where the wires first terminate near the compressor, and directly at the motor.

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[ActionDave]

I would say about 80% of the time trouble calls on motors are are caused by a problem that has nothing to do with the motor.

You gave us voltage and amperage, those sound ok, but you didn't tell us what the problem is.

Check all connections, check the pressure switch, check the unloaded on the compressor head, check the check valve, check and make sure the leads aren't wired up for 480V.

 

[DrSparks]

You didn't tell us what the symptoms were

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[ishium 80439]

According to the operations manager the call originated because the the internal thermal overload kept kicking out and stopping the motor. That compressor has been there for about 3 years or so and had been running fine until a few months ago. It apparently started as an intermittent problem and got progressively worse.

Thanks for the replies so far.

 

[DrSparks]

According to the operations manager the call originated because the the internal thermal overload kept kicking out and stopping the motor. That compressor has been there for about 3 years or so and had been running fine until a few months ago. It apparently started as an intermittent problem and got progressively worse.

Thanks for the replies so far.

Faulty heater?

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[ishium 80439]

The overloads are currently internal and electronic. The plan they are going with is to bypass them and use a conventional thermal overload in their place. But how would that explain the disparity in the current draw?

 

[DrSparks]

The overloads are currently internal and electronic. The plan they are going with is to bypass them and use a conventional thermal overload in their place. But how would that explain the disparity in the current draw?

Didn't see that. The only thing I can think of is a bad winding

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[DrSparks]

Didn't see that. The only thing I can think of is a bad winding

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If one of the windings are dropping out it would essentially single-phase the motor and cause the overload to trip.

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[ishium 80439]

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.....

And in interest of full disclosure at this point this really is just curiosity for me. Once the wires hit the lugs on the unit I walk away and hand it over to the techs from the manufacturer.

 

[gar]

180307-1211 EST

ishium 80439:

In your first post you used the word "panel" twice, and lugs at "panel". Where is this panel? Is it the breaker panel (100 ft away from compressor) or some panel next to or on the compressor?

If the voltages are very close to one another at the compressor, then it is clear the problem is between that point of measurement and the motor or within the motor.

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[MTW]

I would suggest that you check the internal connections and voltages at the starter and motor. Your currents are imbalanced, and that will cause the overload to trip, even if they are lower than the overload setting value. IEC starters and electronic overload relays have differential protection built in, to protect for unbalances and single phasing.

If you have a weak connection, it will cause the voltage to vary, and the currents will reflect the unbalanced voltages. What voltages you measure at the supply end, may not be what the motor sees.

It could be worn starter contacts, loose or overheated connections, or bad terminations in the motor connection box. Just because the compressor service tech say's its good, doesn't make it so. Check the voltages inside the unit, at the load side of the controls and the motor.

MTW

 

[ishium 80439]

Yes, to clarify I was referring to the same panel- the one from which the compressor is fed. I first measured voltage at the main lugs and then on the load side of the breaker- these values were the same. From that breaker the wire is unbroken to lugs within the unit itself. (and for more clarity the 100' is probably closer to 80' and is the total distance including the portion of the run up the wall from the panel and back down to the compressor)

 

[ptonsparky]

The internal thermals fail often enough. Make sure the motor is cooling. Check for dirt or crude on the cooling fins, blocked ventilation etc.

 

[petersonra]

I like the idea of jumping out the internal OL and using an external one, at least as a test. It does not seem to me that the current levels you are seeing should trip an OL.

You mention the internal OL is electronic. Is it possible the setting is wrong?

 

[petersonra]

The internal thermals fail often enough. Make sure the motor is cooling. Check for dirt or crude on the cooling fins, blocked ventilation etc.

electronic overloads are not generally heat sensitive.

 

[Tony S]

Have you carried out continuity and insulation tests on the motor windings?

I'll be honest, it's where I would have started.

 

[DrSparks]

electronic overloads are not generally heat sensitive.

They use CTs don't they?

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[Jraef]

I'm with MTW on this.

A- 29.7A, B- 23.5A, C- 18.9A

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.

 

[GoldDigger]

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.

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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