3-Phase AC motor control help required

Discussion in 'The Projects Forum' started by abicash, Nov 14, 2013.

  1. abicash

    Thread Starter New Member

    Oct 18, 2007

    I have designed a PWM based 3-phase sine generator .
    PWM freq = 16KHZ using a Microcontroller and a Look up table for sine

    The bridge stage is 3 nos Dual IGBT modules with 10000uf/600v electrolytic capacitor across the input to the bridge and 3 nos 0.22uf/600v capacitors across each module.

    I have attached the waveform across R-Y phases and i see a lot of oscillations on the scope.
    On looking closely i see this to be the effect of BEMF from the AC motor.

    1)The waveform is seen on a CRO since surprisingly the oscillation doesn't show up on a DSO.The frequency which you see is a 30Hz envelope by a 8KHz PWM sine
    2)The rating of the motor is 1HP and the leads are quite long (close to 15 meters).

    I request someone to comment on this.Maybe pointers or desirably a solution.
    Please help
  2. tcmtech

    Distinguished Member

    Nov 4, 2013
    What are you intending to use the motor for?

    Is this a exercise in learning how to build a 3ph motor driver or something else?

    Just curious being that a used but good working 1 HP VFD unit can be bought for around $30 or less.
  3. MaxHeadRoom


    Jul 18, 2013
    Is this an AC induction motor or P.M.?
    Usually the typical 3ph VFD outputs a PWM'd Pseudo sine wave.
  4. crutschow


    Mar 14, 2008
    A small inductor in series with each output may damp the oscillations.