Inrush Current and Motor Starters
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  1. #1
    Join Date
    Jun 2006
    South East Michigan, USA
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    Default Inrush Current and Motor Starters

    It has been advised by many (including myself) that one of the parameters used to size any contactor or motor starter in an RPC should be the amount of starting capacitance (based on the 550uF = 50 amps of potential inrush current from the caps ratio) that it is switching.

    Part of me thinks the practice of having any contactor or starter that is switching the starting caps having an amp rating for the inrush current it is switching may be overly conservative.

    A NEMA sized motor starter must have some inrush capability built into it. Starting a 3 phase motor powered by 3 phase from the utility will usually require about 5-7 times the FLA of inrush current anyway depending upon the motor code.

    For example, a NEMA Size 1 starter is rated at 7.5 hp @230V, but only 27 amps. While 27 amps should cover the FLA times the service factor of just about any 7.5 HP 3 phase motor, it would not be sufficient to cover the inrush current due to starting. (multiple times FLA) If the contactor is rated for switching a 7.5 HP motor on three phase, wouldn’t it have to be rated for the inrush current that is associated with a 7.5 HP motor as well?

    If you are using a NEMA size1 starter, does that mean you can only switch ~300 uF of capacitance per pole to keep the inrush under the 27 amp rating, or is there some additional inrush capability. If there is additional inrush capability, how much?


  2. #2
    Join Date
    Aug 2004
    San Ramon, CA USA
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    Yes, a contactor is indeed automatically sized to handle the inrush of a motor. So a NEMA size 1 contactor is sized for locked rotor current of that maximum (7-1/2HP 230V) motor, or approx. 127A, for 20 seconds (long enough for the OL relay to trip).

    But not necessarily a motor PLUS capacitors. Normally, i.e. a 3 phase motor starter, having capacitors downstream is actually a help to the contactor because they reduce the power factor so that the motor starting current is actually reduced. In an RPC however, where the caps are providing that phantom 3rd phase, I'm not clear what the effect would be on the contactor. However I doubt that oversizing is really all that necessary if you use a NEMA starter. If you try to use a DP (Definite Purpose) or closely sized IEC rated contactor it may be cause for concern. The bigger issue is when you go to shut down. Capacitors extend the arcing time when the contactors open, causing more of the contact material to vaporize and shortening the life.

    But again, NEMA sizing is serious overkill to begin with. I wouldn't bother with any additional.


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