VFD to Power Supply Trouble - Page 2
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  1. #21
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    Johansen, no I did not. Any further advise on this? Willing to try.

    JST, the VFD has a parameter for PWM frequency, adjustable between 2 and 12kHz. Started at 4, went down to 2, then up to 12. As I increased the PWM frequency, the squealing harmonics in the power supply were audibly more pleasant, but I don't know if functionally so. At none of these frequencies did the power supplies output stay on for more than a second, and then I cut the power to it.

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    find some motor run caps, 10-50uf will work. delta, or y connection doesn't matter. values not critical. 10uf will be enough if you have 4khz or higher pwm frequency. larger caps can be connected in series or in y connection to make them electrically "smaller" in this application.

    you can't ground the neutral point of y connected caps in your case because the load reactor does not have any significant common mode inductance.


    so, again due to the common mode voltage, there is no guarantee the machine will work. most electronics have emi filters which have small capacitors connected line to ground.. and the caps after the line reactor will force a line to line sine wave voltage, but a vfd delivers in addition to the line to line voltage, a square wave voltage of +170 and -170 volts at the pwm frequency, from line to ground. maybe nothing will happen and the machine will work fine.. or maybe you blow up the emi filters in the power supply.

    one way around that voltage.. is to leave the machine ungrounded and wear rubber gloves.

    another solution is to run the vfd from an isolation transformer and you can ground the Y point of the capacitors after the load reactor.. leave the vfd and the transformer secondary ungrounded. turn it on and don't touch it.


    another way around it is just go ahead and ground the Y point of the capacitors.. through increasingly larger incandescent lamps and see if the drive faults or if the lamps do go dark with less amps flowing through them than the drive is rated for, set the pwm frequency to as high as practical.

    as i mentioned before i was able to sucessfully ground Y connected caps on a 400 watt drive with a total inductance of 600uH. i figured the drive should have blown up due to the 10 amps of current flowing, but it didn't.

  3. #23
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    Quote Originally Posted by I'llFixIt View Post
    ''' However, the power supply squealed like a pig and sounded like it might blow up. ...
    That's what a switcher supply will do if not fed from a sine wave. You might consider using a NOT switcher 24 volt supply.

  4. #24
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    Actually, the switcher never has a sine fed to it anyway..... it gets pulses at line frequency, at the top of the sine wave. There may have been some "magnetostriction" effect.

    However, the rectifiers may not be happy with HF pulse currents, so Johansen's caps make sense. You would be making a "sine filter", which might be best tuned to a few hundred Hz, possibly aroiund 1 kHz, to cut most of the HF currents. That will also cut output a bit, so be sure you still have enough sine wave peak to be within the SMPS input range.

    A small power resistor in series with the caps may help avoid a resonance peak if you happen to hit a strong harmonic frequency. Usually an ohm or less at 50W or thereabout is OK, but that really should be "designed".


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