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High quality adaptation of VFD: 1 to 3 phase; 15kw.

  1. Jul 7, 2014 #1
    Hi All,

    I have access to a 15kW VFD for little money. I only have single phase 240 vac at home, and need to drive a 10kW motor, 3 phase, 415v. I have been reading that 3 phase input VFDs can be run on single phase, but they must be derated by 50%. This would mean a 20 kW VFD. Also, it is a crude solution, and I am looking at doing it better.

    What I am thinking is to rectify the incoming single phase. This, I think, would give me ## sqrt2 * V rms ## ie ## 240 * 1.4 = 336 V ##.

    This would have much more ripple than the 3 phase produced by the VFD's 3 phase rectifier, but could be smoothed with appropriate caps. The voltage expected from a 6 pulse 3 phase rectifier would be, I think, ## 1.35 * V rms ## which is ##1.35 * 415 = 560 V dc ##

    My plan is to feed my dc onto the dc bus of the VFD so that the output stage won't see any difference from if it had been produced by its own rectifier.

    Can I employ a voltage multiplier to double it, and then regulate it down? How much would the internal circuitry of the VFD compensate if I just put the 336 v dc in?

    Golden words from those who knows would be greatly appreciated.
  2. jcsd
  3. Jul 7, 2014 #2
    What you want to do sounds dangerous!

    Your motor is rated for 10kW. Drawing that much power from 240V will take more than 41 Amps. Your motor should have a name plate on it. The name plate will include an amp rating. That should be how much it draws under full load with power factor and all figured in. Even still you need to account for high currents during startup (inrush) and any losses from rectification or inversion. That kind of stuff can be found in NEC tables.

    Let's say we have it all figured and come up with around 55 Amps for your motor. It's doubtful that you have a circuit that can handle that much current in your house. You almost certainly do not have receptacle in your house that can receive a 55 Amp or bigger plug.

    To answer one of your questions. It's not a good idea to multiply the voltage when your handling that kind of power. Your device would be a real beast once it's big enough to handle the power. It's better to use a transformer to raise the voltage before the rectifier. Remember that the transformer and the rectifier must have the proper power ratings to handle your VFD and motor. Any wires connecting anything at all must have the correct ampacity ratings.

    I don't recommend that you do this. This is what industrial electricians are trained for.
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