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Thread: Single Phase current draw for a 3 phase output VFD: technical discussion

  1. #401
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    Quote Originally Posted by Ingenieur View Post
    you misunderstand the physics, you are not alone
    they do during the conduction period
    OK.
    Perhaps you'd care to offer your explanation, in your own words, of how a forward biased diode can put energy back into the supply.
    Si hoc legere scis nimium eruditionis habes.

  2. #402
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    Quote Originally Posted by Besoeker View Post
    It isn't and it doesn't. Think about how PFC works. The fact that it can exchange energy with the supply. Something a DC bucket can't do.
    You can persist with your claim. That doesn't confer validity.
    PFC caps improve power factor by exchanging energy with the load... not the supply. If the capacitor(s) exchanges energy with the supply, it is overcorrecting the motor load power factor. You end up with a power factor less than 1 but with current leading the voltage rather than lagging.

    In the case of a VFD, the active electronics of the PWM-inverter stage switch between the DC rails to facilitate the capacitor-inductor energy exchange... but it goes unnoticed because of the output frequency and PWM waveforms.
    I will have achieved my life's goal if I die with a smile on my face.

  3. #403
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    Quote Originally Posted by Smart $ View Post
    PFC caps improve power factor by exchanging energy with the load... not the supply.
    The supply is the common source/sink.
    Si hoc legere scis nimium eruditionis habes.

  4. #404
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    Quote Originally Posted by Besoeker View Post
    OK.
    Perhaps you'd care to offer your explanation, in your own words, of how a forward biased diode can put energy back into the supply.
    many texts on the subject
    doing your own learning yields better results



  5. #405
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    Quote Originally Posted by Ingenieur View Post
    many texts on the subject
    doing your own learning yields better results
    Could you kindly explain, in your own words, how a diode rectifier can put energy back in the supply which is what I politely asked?
    If you can't, that's fine too.
    We can agree and move on.
    Si hoc legere scis nimium eruditionis habes.

  6. #406
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    Quote Originally Posted by Besoeker View Post
    Could you kindly explain, in your own words, how a diode rectifier can put energy back in the supply which is what I politely asked?
    If you can't, that's fine too.
    We can agree and move on.
    I can, but I won't
    it is covered in depth in mant text on power electronics
    you can research it and study it at your own pace and math level

    the bottom line
    vfd's improve line pf
    rectifiers and igbt, etc, bidirectionally flow Q
    larger vfd caps improve pf further

    in my black box example
    starter/pfc caps vs vfd
    absolutely no difference in measurable and observable results



  7. #407
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    Quote Originally Posted by Ingenieur View Post
    I can, but I won't
    I am so astonished...................that you would decline such a golden opportunity to share your brilliant grasp of power electronics yet denigrate my claims on the subject..
    How about you tell us a little about your most recent design/install?
    Last edited by jumper; 06-22-17 at 02:39 PM. Reason: fix typos
    Si hoc legere scis nimium eruditionis habes.

  8. #408
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    Guys,
    As this descends further into acrimony (I don't care where from), I am closing the thread. As I said a while ago now, I am in agreement with the concepts put forth here on how the non-linearity of the principals involved make it difficult to nail this down into any exactitude, but I think we can all agree that attempting to size a VFD for a single phase feed at anything LESS than 2X the motor current rating is a recipe for trouble. THAT was the initial point of this thread; being able to give electricians, usually left with the responsibility for sizing circuits, a simple guideline for making a good choice and I feel it has been sufficiently addressed. I for one have NEVER used the 1.732 model for this purpose, but I do see it used quite often, especially in websites and by some "less than helpful" drive purveyors. That was why I brought it up, so it could be aired out and have the light of intelligent minds shine on it. So I think this thread has successfully accomplished that.

    Need we go on trying to convince one another of side details to the point of acrimony? I think not.
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