Inexplicable motor failures 3

[eeprom]

Hi,
I have two pumps, both 500 Hp, running on VFDs or across the line in bypass mode. Both motors have a history of failing windings. Both motors are inverter rated. There are no RTDs on either. Both motors are within 150ft of their VFDs, and neither motor is using VFD cabling. Both motors have two breaker in series. Breaker 1 is in a GE MCC and has a GE microversatrip with long and ground protection. Both long time settings are about 1.15X with a trip time of 5 seconds at 6X, both ground trips are set to 100A. Breaker 2 is an ABB LI breaker, with the long set at 1.2X FLA and the I set to 8X plug rating, which comes out to 6400A. Downstream of the ABB breaker are contactors for VFD operation and across the line starting. The VFD also has fuses internal.

Recently, motor #2 tripped twice on overload while running at a very low mechanical load. The trip was occurring in breaker 1 in the GE breaker in the MCC. The relay indicated the overload element had tripped. We meggered the motor and everything appeared to be fine. I did not get the megger readings, but there were in the hundreds of megs.

We started the pump (motor #2) up again, in bypass mode, and kept it at low load while we measured the phase currents. They were about 105% of the FLA, and there was some imbalance, about 2%. The pump was running at about 1/5 or 1/6 of it's rated load, but the motor was drawing more than it's FLA.

I asked to switch pumps so we could see what that current reading on motor 1 would be for the same mechanical load. However, that motor (motor 1) tripped upon starting, also in bypass mode. Again it was the GE relay that tripped, not the ABB. I did not see this, but I was told it tripped immediately. According to the GE relay settings, to trip immediately had to be caused by the ground fault element. The GE relay has no short time element, and the instantaneous element has been turned off.

We meggered motor 1 and it came in around 500 megs on all phases. We meggered again including the cables from the output contactor down to the motor and got the same. Phase to phase measurements were all similar, around 0.6 ohms DC resistance, but the meter used is not very accurate at that low of a reading. This is a fluke 287, and I don't think it's accurate to 0.1 ohms at that load. The calculated impedance of the motor windings, operating at pf of 0.84, results in a per winding impedance of 0.492/_32. Therefore I believe the fluke readings are reasonably valid.

We then tried hand rotating the pump and it rotated easily. There is no binding.

This is as far as I got. I can detect no ground fault, but it appears we are tripping on a ground fault. Motor 1 won't start because it faults. Motor 2 is running in its service factor while operating at 20% of its load. What I believe is that the motor windings are being deteriorated by spiking from the VFDs. As the winding insulation breaks down, some of the windings become shorted, and it then takes more current to create the same torque. The increased current causes heating, further breaking down the insulation, and then eventual failure.

This is happening to both motors. It seems it has be to related to the VFDs. However I cannot find anything wrong. What I am hoping for from the forum is someone who can help me calculate how spiking and reflected waves can shorten motor life. Or, point me in another direction.

Thanks for your help.

 

[Heaviside1925]

OP,
A little more information.
At what point in the system are you measuring power?
And
Have you gotten an IR camera on the motor to confirm heating?

 

[waross]

150 feet and you didn't mention any filters?
Try a simple impedance test;
Excite each winding with about 60 VAC and check the current.
An unexpectedly high current may indicate turn to turn shorts that an Ohms test may not show up.
You may not always be able to trust a megger reading on AC.
We had a cable that passed an Ohms test and a 250 Volt megger test but was a dead short to ground on 120 VAC.
Meggering at 500 VDC, we could see a high reading on the megger at slower cranking speeds, abruptly drop to zero as the cranking speed was increased.
For you younger folk, it's getting tougher to even find a picture of these old-timers.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[eeprom]

Heavyside, the current is being measured at the motor disconnect.

Waross, I don't have drawings so I've been tracing things out by hand. The VFDs have some sort of filtering on them, I can't tell if it's upstream or downstream. There are 6 fuses in the VFDs, which I am assuming go to two harmonic cancelling transformers. I have not identified any load filters, but at 150ft (that's a conservative distance), I would not expect to use load filters. That doesn't mean there aren't load filters.

I haven't yet tried to energize the motors with low voltage, as you suggest. The megger was done at 1000VDC, and it's a crank megger. I already know the motor is failing. I want to know how to prevent it in the future, and I am certain the failure is caused by the VFDs, I just don't know specifically how. If there is no load filter, then I believe that is a good option.

 

[Heaviside1925]

OP,
I am wondering if you are not having two separate issues, failing insulation on the motor windings and failing insulation on the wiring between the VDF and motors, it's just an issue wasn't noticed until breakers started tripping. I am assuming separate runs of RMC to each motor. If the wiring between the VDF and motors is THHN, you might want to replace it with XHHW or at least pull it and inspect it.

 

[3DDave]

Better windings?

From a basics approach the windings can have insulation fail from moisture, they can fail from too much heat, they can have insufficient mechanical support causing fractures in the insulation that can allow water ingress to allow shorts, or just abrasion from movement within the windings.

If the idea is that it is voltage spikes, then how high is the voltage spike?

What I see missing from the diagnosis is the oscilloscope check on the waveform(s) being received by the motors.

 

[waross]

You should be looking at the current reported by the vFD display.
Some clamp-on meter algorithms are widely inaccurate when measuring VFD wave-forms.
It is unlikely but possible that 1000 VDC is not high enough to cause the insulation breakdown.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

By the way, have you mentioned the voltage?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[eeprom]

The voltage is 480V, three phase. I cannot run the drives, that's why the motors are running in bypass. The drive on the motor in question has blown fuses, presumably due to the same motor failure that has caused the main breaker to trip. The other motor is in service, in bypass, and cannot be turned off.

Based on what I know so far, I believe that there is something about the drive circuit that's causing the motor insulation to fail. My best guess is that a load reactor is needed.

 

[jraef]

The creation of reflected waves and standing wave spikes is very complex, so the distance being “only” 150ft should not be taken lightly. You say 6 fuses in the drive, that implies it might be a 12 pulse drive, a technology not widely used in the US because of it being less effective than 18 pulse, but has virtually no effect on the output side. I mention it because this may be an indicator of this drive system being old, which may mean it has issues. In addition, it’s possible that someone has turned up the carrier frequency to reduce motor noise, without concern for what that might do to the motor.

Bottom line, there is a lot of potential for issues here. It really needs the attention of someone with a lot of drive experience.

If you are looking to keep these old drives and avoid future motor damage, I would be looking for dv/dt filters, not just load reactors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[electricpete]

> Recently, motor #2 tripped twice on overload while running at a very low mechanical load

> They were about 105% of the FLA, and there was some imbalance, about 2%. The pump was running at about 1/5 or 1/6 of it's rated load, but the motor was drawing more than it's FLA.

How did you determine the loading on the pump? I'm suspecting you might have an actual overload on that motor #2, with some error in reading your mechanical loading. The overload is indicated both by the trips and the current reading. There is no current unbalance to cause the trip. And lack of current unbalance also tends to make the motor stator fault scenario less likely. Perhaps rotor bar circuit problem could cause this, but it would be unusual for vfd motor to have a rotor problem (they typically arise from starting abuse). What does the motor drive?

By the way, were the motors running fine for awhile and then out of the blue just started doing this? Or did it start after new installation or recent maintenance?

Also if you're suspecting actual motor failure, you need to get a bridge reading along with your megger reading. Fluke multimeter is not accurate enough as you mentioned. Typically you're looking for a 4-wire resistance measurement (to rule out contact resistance), with an instrument accuracy of 0.001 ohms, preferably from motor T-leads rather than remote location.

 

[edison123]

The motors are fine since you are able to run them in VFD bypass mode. If the OL is tripping and you measured 105% current, then your pump is not under loaded. You could do Baker surge comparison test to check the winding turn insulation.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Hi, eeprom,
Please attach a photo of the motor nameplate.

Both motors have a history of failing windings

Provide a bit more information about the maintenance history of these motors.
Do the problems occur from the very beginning or only after rewinding?

Both motors are inverter rated.

Very often, the only measure that a service center takes in the case of "inverter duty" motors is to use "inverter duty" wires, which is not enough.
ACW

 

[Strong]

I did not see in the OP what type of pump. If centrifugal type, then motor load drops with pump load (reduced flow).

Walt

 

[eeprom]

I have traced this down to phase imbalance, I think. The loaded motor is drawing 634, 591, 587 A, and the voltages are within 2 volts.

As far as I can tell, the only reason for imbalance would be winding differences. I believe that the reason the motor is tripping is that the ground fault relay sees this imbalance as a ground fault. During start up, assuming the imbalance is similar, the residual ground current would be 300A, higher than the trip setting.

I believe the winding differences were the result of insufficient cooling from running the motors too slow while they were on vfds.

 

[waross]

As far as I can tell, the only reason for imbalance would be winding differences.
I believe the winding differences were the result of insufficient cooling from running the motors too slow while they were on vfds.

Check for turn to turn shorts.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[eeprom]

That's exactly what it seems to be. I would need to do a surge test, and I don't have tools for that. The motor is going to the shop

 

[waross]

Do an impedance test with 120 Volts and with toaster as a ballast resistor. Look for greater differences than seen with a DC meter.
There is still a chance that the lower voltage will not cause a breakdown.
It may be nice to know before the motor goes to the shop anyway.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

No motor with severe winding turns failure or ground fault will continue to run.

And variable speed motors invariably have external bowers for cooling.



Muthu

http://www.edison.co.in

 

[eeprom]

Waross,
What works the toaster test reveal? Just a higher resolution impedance test?

Also, what's an acceptable phase current imbalance? I would expect less than 2% variation from average current.