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Thread: Capacitors and Soft Starts

  1. #1
    Join Date
    Mar 2008
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    2

    Capacitors and Soft Starts

    Does a soft start motor control replace a capacitor on a large three phase motor? If not can you have both in the same circuit?

    Thanks

  2. #2
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    3,720
    Quote Originally Posted by fastfrank
    Does a soft start motor control replace a capacitor on a large three phase motor? If not can you have both in the same circuit?

    Thanks
    It might replace the capacitor depending on what it was there for, and NO! DO NOT allow them to be connected at the same time. The charging current of the capacitor appears as a short to the SCRs, so the di/dt can cause them to self-commutate (misfire) and it is usually a race to see if the caps blow before the SCRs short.

    But there are different reasons for having capacitors on large AC motors. If they are Starting Assist caps, then they need to be under contactor control to disconnect them when the motor reaches 90% speed. They work by providing VARs during start when the power factor is extremely low, which lowers the line current a little. But they can overexcite the motor if left on too long, hence the need for a contactor to take them off-line before the motor gets to speed. They would, however, be unnecessary if a soft starter is used because that will lower the line current itself.

    If they are "surge caps" they are meant to absorb any line voltage surges caused by switching them on or by other nearby equipment to try to avoid damage to the winding insulation. Similar to a station service lightning arrestor, except without the MOV. If that's the case, you can place them upstream of the soft starter, but I recommend still having them isolated off-line with a contactor during acceleration and bring them on only after the soft starter has finished ramping.

    If they are "PFC caps" (Power factor Correction), then the same issue of locating them upstream of the soft starter and isolation applies.

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