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Thread: Disconnect for VFD's

  1. #1
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    Disconnect for VFD's

    I have two 25HP 460/60/3 motors that running pumps and being controlled by a VFD. Both pumps will operate at the size time. What size should the disconnect be?

    Paul

  2. #2
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    Quote Originally Posted by pault587
    I have two 25HP 460/60/3 motors that running pumps and being controlled by a VFD. Both pumps will operate at the size time. What size should the disconnect be?

    Paul
    As per 430.250 each motor has a FLA of 34A.
    At 115% as per 430.110 each motor could be protected by a 60A Disco.

  3. #3
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    I have two 25HP 460/60/3 motors that running pumps and being controlled by a VFD. Both pumps will operate at the size time. What size should the disconnect be?
    if it is a single VFD what you have to sum the total motors.

    each motor @ 34 amp each

    so add both motor came up with 72 amp

    then i add 125% add to this

    came up with 90 amp you can use 100 amp disconnect switch but use 90 amp fuse as long the VFD can detect the overload for each motor other wise may have to add o/l block on it. and add N/O&N/C concat aux switch to shut off the VFD if the o/l kick out [ the voltage spike will destory the VFD if not carefull ]

    Merci, marc
    Marc
    master electrician
    Wisconsin and Paris France

    "Pas de problème, il marche n'est-ce pas?" (No problem, it works doesn't it?)

  4. #4
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    Where are these disconnects located in the wiring scheme? If they're between the VFD and the motor, you'll need to do something to insure they'll never be opened while the motor is running. If the VFD is powering a motor, and the output circuit is suddenly opened, it'll cook the VFD instantly.

    Usually you can get 'preload' switches that fit inside the disconnect, and wire them into the E-stop circuit. Don't pull this control wire in the same conduit as the VFD output wire, even if it's shielded. To do so will almost certainly cause the drive to operate erratically, and could cook the control board.

    You might not need a disconnect between the VFD and the motor at all. Usually, I fight pretty hard to not put them in. See 430.102 (B) exception (a). "where such a location of the disconnecting means is impracticable or introduces additional or increased hazards to persons or property." FPN #1, "some examples of increased or additional hazards include.....motors associated with adjustable speed drives....."

  5. #5
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    So if your motor is out of sight, code requires a disconnect,is there a exception, or one in the works .or how do you get around it???

  6. #6
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    Quote Originally Posted by KIRKBRO
    So if your motor is out of sight, code requires a disconnect,is there a exception, or one in the works .or how do you get around it???

    I think what Micro is saying is that 430.102(B)Ex(a) provides you with the exception as clarified in the FPN.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  7. #7
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    430.2 if the disc is before the VFD. Note that the OP did not tell the size of the VFD. It is common to have a vfd that is larger than the motor, so the rated input current is what I calculate from, not the motor amps.

  8. #8
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    When does the locked rotor amps come into the sizing of the disconnect? I think it was table 430.151 but I don't have the NEC with me right now. To clarify, there is one disconnect feeding both VFDs.

  9. #9
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    Quote Originally Posted by pault587
    When does the locked rotor amps come into the sizing of the disconnect? I think it was table 430.151 but I don't have the NEC with me right now. To clarify, there is one disconnect feeding both VFDs.
    You size for the sum of the ASD's normal current. Unless you have a bypass contactor in which case you would need to select your fuse or CB in accordance with the inrush current requirement.

    This would be a bad practice though as you really should have individual disconnects with special high speed fuses for IGBT protection fro each ASD's. The ASD manufacturer ususaly list their recommended protective device and its size that is usually in accordance with the NEC requirements.

  10. #10
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    Good call, I did mention that they have bypass.

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