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Selecting A Pre-charge Resistor Contactor


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#1 jraef

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Posted 09 October 2007 - 10:11 PM

I need some design help. I am trying to help someone design in a contactor for use in a pre-charge resistor circuit, where the contactor will only close AFTER the pre-charge resistor has been in the circuit for 1.2 seconds. The question is, does the contactor still need a capacitive duty rating? I say yes, because when it opens it still has to contend with the extra arcing as the capacitors discharge. One of my cohorts says no, because the inrush has already been taken care of by the resistors, so the contactor is just acting to bypass the resistor and when opening, the arc will transfer back to flowing through the resistors. I have searched long and hard for information on selection criteria for pre-charge resistor contactors, but can't find anything useful. Anyone have an opinion or suggestion?
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#2 marke

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Posted 10 October 2007 - 12:50 AM

Hello Jraef

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.

Best regards,

#3 jraef

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Posted 10 October 2007 - 01:01 AM

Excellent Mark, thanks.

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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