Universal Motor and PWM Control

Discussion in 'The Projects Forum' started by luisferp, Nov 23, 2013.

  1. luisferp

    Thread Starter New Member

    Nov 11, 2013
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    0
    Hi! I'm working on a project to control the speed of Universal Motors (DC series Motor).

    My objective is to control it via PWM using one Mosfet.

    The circuit is pretty simple, and it's already working. But, as mosfets are sensitive to overvoltages, I started doing some measurements with my multimeter (I don't have a oscilloscope) and got surprised with some results, mainly the voltage over the motor that was about 165V (with motor turned off), ok, that's expected, but when I turned the motor on, got about 900 to 1150V over it. Does anyone know what's happening?

    Motor specs:
    Drill motor
    127V AC, 60Hz, 480W

    I made a simple diagram of my circuit just to ilustrate the idea.

    [​IMG]
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    For starters I would expect to see a much larger cap, especially when working with a Universal motor.
    A series motor such as this type is very noisy electrically, the customary method of control for these motors is SCR phase angle control.
    Max.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    +1
    Think about it - the motor is drawing maybe 4A or so. How much time do you get from that cap at a 4A draw? Not even close to a cycle. So the cap gets completely drained before the diodes come around to conduction again. It might as well not be there at all.

    At least part of your problem is due to sending large voltage swings across an inductor.
     
  4. luisferp

    Thread Starter New Member

    Nov 11, 2013
    4
    0
    I agree. That was the bigger capacitor here in hands.

    But, anyway, when testing the circuit without the cap (directly from the rectifier bridge), I measured about 550V over the motor.

    It's hard to say, but I believe the multimeter is showing me some kind of mean value of the voltage. In this case, I don't know how the capacitor (250V specified) and the mosfet (IRF840, Vds max = 500V) are still alive.

    Max, I tested, some weeks ago, the phase angle control, worked too. But I think it's more difficult to use it with PWM, and I think it gives much less torque under low speeds.
     
  5. luisferp

    Thread Starter New Member

    Nov 11, 2013
    4
    0

    Yes Wayneh, I see your point.

    A capacitor to filter the voltage from de rectifier bridge is not a necessary component for the circuit to work, maybe it could help with the noise produced in the motor, I don't know.

    And to reduce the overvoltages from de motor, I thought that only the parallel diode (600V, 8A, ultra fast) would serve.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    I suspect you are seeing a lot of electrical noise coming back from the motor to get an accurate reading.
    True that low end torque is poor with SCR for Universal motor control, but without some kind of feedback in place, you may see that in any case..
    Max.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,792
    1,103
    Try adding a snubber (R and C in series, rated for peak voltage) directly across the motor terminals, in parallel with the diode.
     
  8. luisferp

    Thread Starter New Member

    Nov 11, 2013
    4
    0
    I will try adding the snubber circuit, but won't be able to test it, only during the week. After it I'll post the results.

    Thanks everyone!
     
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