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.
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?
Re: Cause of motor failures#127533 08/15/0101:28 PM08/15/0101:28 PM
Originally posted by Dspark: >[b]6-15 cycles That's barely the blink of an eye. Nothing that you detected sounds like a power quality problem to me. I know its a short time but when it happens the current on one phase drops from ~124 amps to ~95 amps and on the other two phases it increases to ~167 amps. I only had the analyzer connected for two weeks so I can't say for sure how often these things happen. Wouldn't that combined with the negative sequence currents flowing from the voltage and current imbalance cause excessive heating?
Was the motor actually rewound several times? yes, I'll have to look at the records to see how many times.
What are they using for a spare while this one is out of service? How many pumps are there in all?[/B]
One deep well pump at this location.
Re: Cause of motor failures#127536 08/15/0111:12 PM08/15/0111:12 PM
Originally posted by resqcapt19: Is this motor on a VFD? Don(resqcapt19)
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.
Re: Cause of motor failures#127537 08/17/0109:47 AM08/17/0109:47 AM
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).]