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Thread: Dual motor coveyor controlled by 2 VFD's.

  1. #1
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    Dual motor coveyor controlled by 2 VFD's.

    Hello,

    New to the Forum here, but have lurked for years in search of fresh ideas and great advice, so thanks in advance!

    We are installing a conveyor belt that has 2 180 HP 480V Motors. Each motor will be controlled by an individual VFD. At times only one motor will be necessary to drive the conveyor.

    The conveyor belt is on opposing sides of the motors (i.e. top side for motor 1 and bottom side for motor 2), so our intention is to use the torque control feature in a Master/Slave scenario.

    My question is, do we need to worry and consider regen problems on the second motor if only the first motor is operating. If so, what is the best option for mitigating the problem?

    Please let me know if you would like or need more detailed information, and I appreciate any feedback or advice you can send my way.

    Best Regards!

  2. #2
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    Quote Originally Posted by snakeyjake View Post
    Hello,

    New to the Forum here, but have lurked for years in search of fresh ideas and great advice, so thanks in advance!

    We are installing a conveyor belt that has 2 180 HP 480V Motors. Each motor will be controlled by an individual VFD. At times only one motor will be necessary to drive the conveyor.

    The conveyor belt is on opposing sides of the motors (i.e. top side for motor 1 and bottom side for motor 2), so our intention is to use the torque control feature in a Master/Slave scenario.

    My question is, do we need to worry and consider regen problems on the second motor if only the first motor is operating. If so, what is the best option for mitigating the problem?

    Please let me know if you would like or need more detailed information, and I appreciate any feedback or advice you can send my way.

    Best Regards!
    Welcome Jake.

    A master slave arrangement sounds fine. I'd be inclined to operate the master in speed control and the slave in torque control with the torque reference taken from the master in order to get the two to load share.
    As for regen, as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator.
    For safety considerations, we normally have to fit a contactor between the VFD output and the motor - a physical disconnect .

  3. #3
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    Quote Originally Posted by Besoeker View Post
    Welcome Jake.

    A master slave arrangement sounds fine. I'd be inclined to operate the master in speed control and the slave in torque control with the torque reference taken from the master in order to get the two to load share.
    As for regen, as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator.
    For safety considerations, we normally have to fit a contactor between the VFD output and the motor - a physical disconnect .
    What he said. That's the way to do it and regen would not be an issue.

    But just to explain it a bit further, motors on VFDs can regen at any speed because the VFD is capable of giving it a "virtual" synchronous speed that is lower than the driven speed. But if nothing tells the VFD to do that, then nothing happens. If drive 2 is following the torque of drive 1, how would that ever happen?

    Rhetorical question, it wouldn't.
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  4. #4
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    Quote Originally Posted by Besoeker View Post
    Welcome Jake.

    A master slave arrangement sounds fine. I'd be inclined to operate the master in speed control and the slave in torque control with the torque reference taken from the master in order to get the two to load share.
    As for regen, as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator.
    For safety considerations, we normally have to fit a contactor between the VFD output and the motor - a physical disconnect .
    hmmm, you said " as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator". I think you are right but your reasoning is not. Idling motor will have voltage on its taps induced by its stator. But, the reason it does not become a regenerative source is that there is no path for current to backfeed because IGBTs are not firing.
    I am not an expert so I may be wrong though, but I don' think you need external voltage (applied by VFD) to make the induction motor generator.
    "Because it's there!"
    George Mallory

  5. #5
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    Quote Originally Posted by LMAO View Post
    hmmm, you said " as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator". I think you are right but your reasoning is not. Idling motor will have voltage on its taps induced by its stator. But, the reason it does not become a regenerative source is that there is no path for current to backfeed because IGBTs are not firing.
    I am not an expert so I may be wrong though, but I don' think you need external voltage (applied by VFD) to make the induction motor generator.
    remember this is an induction motor not synchronous motor with magnets so no voltage will be generated by simply rotating the shaft.....

  6. #6
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    Quote Originally Posted by LMAO View Post
    hmmm, you said " as long as to idling motor is not excited, i.e. doesn't have voltage applied to the stator, it should not become a generator". I think you are right but your reasoning is not. Idling motor will have voltage on its taps induced by its stator. But, the reason it does not become a regenerative source is that there is no path for current to backfeed because IGBTs are not firing.
    I am not an expert so I may be wrong though, but I don' think you need external voltage (applied by VFD) to make the induction motor generator.
    I think a reasonable assumption is that the OP was referring to the ubiquitous cage induction motor. Apply frequency and voltage. If it operates at below synchronous speed for the applied frequency it will motor. Drive it above applied frequency and it will regenerate. Apply neither and it will do neither.

  7. #7
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    Quote Originally Posted by mike_kilroy View Post
    remember this is an induction motor not synchronous motor with magnets so no voltage will be generated by simply rotating the shaft.....
    That was my point. Rotating shaft will generate voltage but there will be no power back-fed into grid/supply because VFD IGBTs are not firing (no path for curent for flow back).
    "Because it's there!"
    George Mallory

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