ECN Forum Forums Technical Discussion Electrical Theory and Applications Cause of motor failures

The information I'm looking for would be a chart of the temperature rise and long term effects on the motor insulation of a motor running with a voltage imbalance of .9 to 1.1 percent and current of about 5 percent. I know that the motor should be derated maybe 1 or 2 percent because of the voltage. I have a customer that has had several motor failures in the past two years (same motor). The motor repair shop reports that I have read say the problem is voltage spikes, transients, lightening, etc. I have run several surveys with a power quality analyzer and have not seen any RMS spikes, etc. even during lightening storms, (motor has lightening arrestors and surge capacitors). I have seen the voltage imbalance of 1% and current of
5%, plus waveform faults with peak voltages of 580 volts phase to neutral (480Y/277 volt system), but they only last one cycle, sometimes 2 or 3 of them within a 15 cycle time frame. I think it may be capacitors switching on the primary. There is also a system voltage drop of approximately 5% every 30 to 40 minutes as the result of another motor starting which causes a 20% drop in current on the motor in question followed sometimes by a current increase. I have
also caught a few partial phase losses which would single phase the motor for a short time. The question I have is could all of these things combined over time deteriorate the insulation to the point where a small voltage spike destroys the motor.

>I have also caught a few partial phase losses
Duration? Any explanation?

>could all of these things combined over time deteriorate the insulation?
They could. But you might try another repair shop next time. The problem seems to lie with this specific motor.

Dspark Duration? 6-15 cycles, a few times longer, no explanation. Most of the time just one phase.
The motor is a 100 hp. connected to a deep well pump. There has been two motor rewind shops involved that I know of. I Know that
if there is core damage to the motor that is not corrected it can create a hot spot in the windings near the core damage that could
result in premature failures. Do you think this is a possibility?

>6-15 cycles
That's barely the blink of an eye. Nothing that you detected sounds like a power quality problem to me.

Was the motor actually rewound several times?

What are they using for a spare while this one is out of service? How many pumps are there in all?

Is this motor on a VFD?
Don(resqcapt19)

One deep well pump at this location.

No it's not on a VFD. The motor originally had a autotransformer starter but the customer recently installed a solid state soft start. I don't see the advantage of the soft start in solving their problem.

The reason I asked is that we had similar problems on a 1 hp motor due to line spikes created by a VFD. This problem was solved by using an "inverter" rated motor.
Don(resqcapt19)

Inverter duty motors withstand the reflected wave phenomena which, if your soft-start unit has DC bus, could deliver nearly twice the DC bus voltage(2 x 800) to your motor. A cheaper alternative to an inverter duty motor may be load filters (reactor & capacitor unit) installed between the controller and motor. They are available from VFD manufacturers.They also make a special device mounted at the motor terminals to absorb the spikes.

[This message has been edited by Redsy (edited 08-22-2001).]