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Thread: VFD size

  1. #1
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    VFD size

    Do any body know were I can find in the code a back-up statement to justified that a 30hp VFD can not be installed in a 40hp pump? is my understood that the motor input and the VFD output need to match. HVAC company buck-up his statement in that they will not run the motor in a full speed but I see a BY-Pass option on the VFD and is clear that in a by-pass mode motor will run a full speed. I read in the code about sizing conductor,overload protection but nothing that clear says that HP need to match. I know that you can use one size up VFD but small look as a no no for me. Thanks in advance for your help.

  2. #2
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    Do the mfg install instructions tell you it is OK to put a 40hp motor on their 30 hp vfd?
    Tom
    TBLO

  3. #3
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    I did not actually open the VFD box due to I was waiting for the guys replace it for a 40hp drive!!! do you think that it is possible that a 30 hp VFD handle a 40hp pump?
    What about overload inside when it is running in a BY-Pass mode?
    I'm sure 40hp VFD will cost more money but I think that matching HP IS THE PROPER INSTALLATION!!!. Need something from the code to backup my statement .thx

  4. #4
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    I sure as heck wouldn't try to do it that way but if the drive manufacturer gives you their blessing.......

    I'm the opposite, I like to oversize the drive a little if I can...

  5. #5
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    You are required to install equipment as per it's instructions. Something about labeling and listing. If the drive mfg says their 30 hp setup is good for 40, go for it. I would think if it was able to handle the 40 hp...it would say 40 hp on the box.
    Tom
    TBLO

  6. #6
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    Quote Originally Posted by Cow View Post
    I sure as heck wouldn't try to do it that way but if the drive manufacturer gives you their blessing.......

    I'm the opposite, I like to oversize the drive a little if I can...


    Definite I agree with you !!! One size up in the VFD will not create any problem with overload settings but I not sure if you going more than one size up you will be abble to set-up the proper overload settings !!!

  7. #7
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    Quote Originally Posted by ptonsparky View Post
    You are required to install equipment as per it's instructions. Something about labeling and listing. If the drive mfg says their 30 hp setup is good for 40, go for it. I would think if it was able to handle the 40 hp...it would say 40 hp on the box.
    Right why would you limit your VFD with 30 if it can handle 40hp.

  8. #8
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    Well, some drive manufacturers have a general duty HP rating and a heavy duty HP rating. No way to know without checking into it.

  9. #9
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    this is a lot like using an undersized contactor. the thing is that if the motor is properly protected by the overload, the contactor won't likely ever have a problem, and neither will the VFD. If it needs more current than the drive can put out, it will just safely trip.

    whether this is code legal or not is dubious despite likely being safe.

    incidentally, this is not that unusual of a situation. a lot of places only stock certain size motors as spares and if they need a motor and don't have one the same size as was originally installed they will just install the next size up that they do have.
    Bob

  10. #10
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    You size your breaker to the VFD .... so you're setting the stage for the motor start-up current to trip the breaker.

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