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.