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motors and squarish waves

Discussion in 'Electronic Design' started by David Lesher, Jun 9, 2009.

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  1. David Lesher

    David Lesher Guest

    Take an induction motor. Feed it crappy AC, maybe
    spiky, maybe square-wavish.

    Will it care? One side says the inductance will
    limit the higher frequency current. Another PoV
    is the inter-winding capacitance will increase the
    current, and thus the I^R losses.

    What saith the s.e.d wisdom?
     
  2. James Arthur

    James Arthur Guest

    Stepper motors don't much care. Chopped drives commonly feed
    steppers ~20-80 volt rectangular waveforms.

    With typical 20KHz chopping you get a few uS feedthru pulse
    from the inter-winding capacitances, and hysteresis loss in
    the iron--lossy if you switch the flux too often--but all-in-
    all very workable.

    Cheers,
    James Arthur
     
  3. James Arthur

    James Arthur Guest

  4. Tim Williams

    Tim Williams Guest

    No, that's the myth. Deep down, they're all PWMed. ;-)

    Tim
     
  5. Rich Grise

    Rich Grise Guest

    I've done PWM (rectangular waves) with a PMDC motor, and it
    worked like a champ. I didn't bother to look at the waveform,
    because the purpose of the device was a motor that maintained
    torque down to 0 RPM, which it did, so I shipped it. :)

    Cheers!
    Rich
     
  6. Rich Grise

    Rich Grise Guest

    Oopps! Missed the "induction" part.

    Then again, one time I was working with a "Modified Sine" inverter, which
    produced a string of positive and negative pulses, PWM'd to average out
    to 120 RMS. They did it the lazy man's way - two output transformers
    with their secondaries in series; they just drove them with two
    overlapping square waves.

    It ran a bench grinder OK, but weighed almost 100 pounds and cost a
    fortune.

    Cheers!
    Rich
     
  7. David Lesher

    David Lesher Guest

    I took two quarters worth of the topic many years ago. It was a senior
    level course; maybe EE 412.

    It, of course, only touched the highlights.
     
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