Directly Connected Pfc & Motor Failure
Posted 08 November 2006 - 08:53 AM
We have lately seen the failure of two screw compressor motors on Carrier chillers. The failures occurred a short while after an attempt to improve the power factor on the motors. The starting set-up on the motor is un-usual. The units are set up for Direct on line starting but you have six wires coming out of the motor. Three wires are permanently connected to the power lines while the second set of three wires connect to the power via a 3 pole contactor to close the windings in delta. I can provide a sketch but I am not sure how to insert it here.
Unfortunately the contractor undertaking power correction did not realize the set-up and assumed that the motor was set-up as a normal 3-wire direct on-line start and connected a delta capacitor to the contactor terminals Ė I can providee sketch. When the power breaker was closed the compressor motor started running straight away, before the chiller went through its normal start up sequence. I assume this happened because the capacitors formed some kind of a star point on the motor windings.
The power breaker was opened almost immediately after it was closed so that the motor only ran for a few seconds. As the contractor did not realize what and why the motor started up straight away he attempted connecting another set of capacitors on another chiller compressor motor obviously with the same result.
The capacitors were then disconnected and the chillers ran normally for a few days after which the motor winding insulation failed.
The compressor motors are cooled by the chiller fluid however even though this was not flowing through the motor (valves etc did not operate) I believe there was not enough time for any thermal damage to occur.
Line voltage here varies between 400-415VAC, 50Hz. Compressor motors are rated at 45kW 112Amps and the connected capacitors were rated at 500Vac 20KVar. I have asked for some more motor information but donít know if I will get any except for nameplate details
Any idea what could have happened?
Posted 08 November 2006 - 10:01 AM
Yes, this connection would have provided essentially a star connection for the motor which means that under normal conditions, it would have effectively reduced the motor rating to one third of it's nameplate rating. Because the star connection was made up of capacitors, you had in essence, a sereis tuned circuit on each phase and if the capacitve reactance was close to the inductive reactance, you could have had very high voltages generated at the capacitor terminals.
I would not expect this to cause an insulation breakdown after a few days, rather I would expect that it could happen immediately.
I strongly recommend that special capacitor contactors are used for switching the capacitors, sepate from the windings.
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Posted 09 November 2006 - 04:02 AM
Corona Inception Voltage decreases with time and temperature. Ergo, Insulation may take time to breakdown.
Exception would be if there was an insulation weakness at the git-go .... i.e., a spot where the insulation might be nicked or crinkled, and in reasonable proximity to the stator frame .. or a winding turn that poses a high, voltage gradient turn-to-turn.
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