VSD causing motor fault

These are multiple failures on a new installation that has been done for the company I work for. Over the past 12 months since the installation was done we have had a number of motors that have become faulty. The quantity is way over anything I have experienced before. One drive had 3 motors within a month. Before the installation motors were lasting up to 25 years!! As we had numerous motors on one drive become faulty we exchanged the drive to ensure the drive wasn't damaging the motors. We have since had other motors on different drives with the same faults. The motors that we are using are ABB high efficiency motors. My feeling is that the control system that was installed is the problem but obviously the installers say that is not the case and try to blame the motor manufacturers. ABB have investigated the motors and we have had them checked by our usual supplier, and no manufacturing faults can be found. We are using the Danfoss fc102 drives (HVAC) and using them to drive pumps at variable speeds. The pumps are not in constant use as they are supply pumps for our process system pumping beer. The faults that we get are short cct faults and the windings are slightly out of balance. When the motors fail they are not hot. Is it possible that the head preassure from the tank is trying to turn the motor faster than the control system is trying to drive the motor? Anybody any ideas, theories or similar experiences and give us some idea where to look for the problem?

My only question is: how far are the motors from the drives ?? cables run thru metal conduits ??

Did ABB inspect and analyze the bad motors? If so, what was wrong with them?

What's the distance between the drive and motor and what is the carrier Frequency?

Open one motor and check if the bearings are good. On some high efficiency motors you can see bearing damage due to current running in the rotor to the housing. found some critical burner marks inside them. it looks like sparking on the balls. prevention is possible by 'grounding' the bearings with special copper rings.
As these he motors are not within the old signature you will need other VFD or another setting on them as the return signal coming from motor is different. Same is when there is a long cable between them.

sinse the typical 3 phase voltage in the UK is 380 volts,
do you have line and load filters installed?
are the motors rated for inverters?

i ran across a similar problem with a triac power pack,
the grounding was providing a feedback loop through the circuits - its been 20 years ago, so my memory is a little dusty on that page.

regards,
james

To summarize and add to a bit:

Are the motors Insulation Class B? Must be Class F or better. Also preferred to be MG1 P31 but I'm not sure you can get this endorsement on a metric motor.

The longer the wire distance between the drive and the motor, the more stressed the motor becomes. At 400V and a carrier (PWM) frequency of 8khz or less, at 7.5kw, you will need extra line conditioning equipment if the wire distance is more than about 25 meters. If 75kw or more, you can go out to 100 meters before additional equipment is needed

Is the motor insulation failing or is it the bearings?

Do not be tempted to simply add reactors ahead of and behind the drive without determining the cause of the problem. Line reactors drop AC voltage and can cause other problems if not used advisedly. And, further, line reactors on the supply side of the VFD will not affect the motor life either way.

Finally, is your 400VAC power network a grounded wye source, floating delta source, a high resistance grounded wye, or corner grounded delta. These will not only affect the motor life but the VFD life, as well.

We are using the Danfoss fc102 drives (HVAC) and using them to drive pumps at variable speeds. The pumps are not in constant use as they are supply pumps for our process system pumping beer.

Is it possible that the head preassure from the tank is trying to turn the motor faster than the control system is trying to drive the motor?

Since you say they are not hot when they fail my first theory that you are running too slow is out - too slow the fan cannot cool motor.
With pumps you want to ensure pump runs fast enough to pump some liquid to ensure seals are kept cool.

What is head of pump discharge pipe? I suppose if a high head and CENTRIFUGAL pump at low speed setting the head could drive pump backwards similar to a setup with multiple pumps in parallel and no check valves.

Uhh are these centrifugal or positive displacement pumps?

Since this is a new setup are you sure you have pumps motors etc properly matched to the load?

Dan Bentler

Could you supply more info on the problems found with the motors ? i.e, how have they failed...

Regards

John V

I'm guessing that these are V/F drives. Could there be residual flow in the pipes causing the motor to be already turning when the drive starts? Is the voltage boost level set too low? Is the ramp rate too quick?

Any of these might cause the motor not to sync with the output frequency of the drive leaving it at full current and not going anywhere.

Nick

Just one note to the above posts: It is unlikely that the pump overhauling the motor or the pump spinning forward at start would cause motor failure. And the fault would be High DC Bus Voltage, not Short Circuit.

If the pump were spinning backward at start, the fault would be Overcurrent.

I'm not familiar with the Dan Foss drives but, almost certainly, they will have a parameter option labeled Flying Start, Catch a Spinning Load, or some other such name which can be enabled. When enabled, the drive delays the start by a half second or so and checks the motor for speed and direction. It then starts at that speed and direction and brings the motor back to the proper speed reference without faulting. That's a very nice feature that was added to most VFD's in the early 2000's.

@DickDV

As you have written the longer the wire to the motor. The motor becomes more stressed. Can you please explain? I mean how does it affect the motor.

The VFD produces rectangular pulses with a maximum height (voltage) of whatever the DC bus voltage is. But, due to the series inductance in the wires, the highest frequency components cause the vertical sides of the rectangular pulse to overshoot and undershoot. This extra overshoot and undershoot voltage puts additional stress on the motor insulation.

This overshoot voltage can easily double the voltage of the original pulse as the leads get longer. That is why, on 480V NEMA motors, the sinewave test voltage of 1200VAC is inadequate for inverter duty motors. MG1 Part 31 specifies 1800V and with high frequency rectangular pulses (not a sine wave) as a proper test for inverter duty motors.

Thanks. But what can be done to reduce that? Is this the reason filters are installed?

As always, Dick Dv gave you excellent info. I may be able to add a couple of things:

Good grounding is essential. Have you checked this at the motor and the drive?

Close coupling of motor and drive (short power leads) is always better.

Bearing damage described above (referred to as bearing fluting) is uncommon, and is also minimized by short power leads and good grounding. You can purchase grounding bushings and motors with insulated bearings if that is your problem, but it sounds like you have winding failures.

I've used reflective wave traps (RWT) at the motor to minimize insulation damage from spikes (referred to as DV/dt damage). As dick says, they do cause some losses and aren't a universal cure. Nothing is magic!

It is unlikely that the control system is at fault. This looks like an expmple of Jenkins Law: The part of a system that is least understood is usually thought to be the cause of any problem.

With centrifugal pumps there is usually a minimum speed below which the pump can't produce enough head to pump any flow. Therefore, I suggest that you have a minimum Hz set at your drive or controls that will keep the pump above that speed.

Finally, good practice says you should have a check valve on each pump if they discharge into a common manifold. That would prevent reverse rotation, but it doesn't really sound like that is your problem.