Selecting A Pre-charge Resistor Contactor
Posted 09 October 2007 - 10:11 PM
Posted 10 October 2007 - 12:50 AM
Provided that the capacitors are fully charged when the contactor is closed, it should not need to be capacitor rated for normal operation.
Opening is not an issue with capacitors unless there is significant inductance in the circuit.
The only issue is the abnormal operation such as voltage surges and transients. If the capacitors are line fed, then any disturbance on the supply will cause current transients and these can cause issues with the contactor if it is thermally rated.
I always used thermal rated contactors with a 30% overload capacity for the soft charge contactor on VSDs and never had any problems.
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Posted 10 October 2007 - 01:01 AM
I found an indirect reference indicating a 22% derate (which is what we apparently do with our PFC cap control contactors with built-in pre-charge resistors), so I think I'll go with your 30% to be sure.
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