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DC shunt motor speed control

Discussion in 'Electronic Design' started by abhinav, Jan 31, 2005.

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  1. abhinav

    abhinav Guest

    hi there
    well, i have a little doubt here, kudos to any one who could clear it

    well i need to control a DC shunt motor above and close to base speed,
    and so i figured that field control to be optimum
    i figured out a strategy to adjust the field resistance by adjusting
    the duty cycle of a FET connected in shunt to it. I eeve figured out a
    way to vary the duty cycle of the FET in relation to the control
    voltage .
    now my problem is that i'm quite quite unsure of the system's
    behaviour during startup as the control voltage is generated of the
    speed of the motor
    Any pointers?
    any help would be gratefully accepted!
  2. Since your control system raises the duty cycle for lower speed, it
    should provide maximum duty cycle for start up. The pitfall is that
    there is an L/R time constant for the current in the highly inductive
    field windings and if you apply full armature voltage at the same
    moment you apply full duty cycle to the field, there will be a period
    of time during which the field current will be low, and the armature
    will draw lots of current that produces little torque. There may need
    to be a small time delay between the application of field voltage and
    the application of armature voltage to reduce this inrush of armature
  3. Tim Wescott

    Tim Wescott Guest

    John: you have that backward. As the field is decreased the motor speed
    will tend to increase because of reduced back-EMF. You'll also see a
    drop in the motor torque constant, so the field will go exactly the
    wrong way under heavy loads. You'll also have a case (starting at zero
    speed is the obvious case) where under heavy load decreasing the field
    voltage will decrease the available torque and let the motor slow down,
    so a monotonic speed/field relationship will cause positive instead of
    negative feedback.


    I would probably try to do this by monitoring the motor speed and
    putting a lower limit on the field current (or PWM) that I'd allow.
    This should make sure that you have enough torque without the armature
    going into runaway, and you'd presumably rise above the limit speed in
    "normal" operation. This will give you maximum field current at start
    up and during disturbances, and if the motor can spin at the desired
    speed at all you should be able to find a function that'll get you there.

    Of course your design will have to be fairly conservative to make sure
    that the lowest field current generated will actually get the motor
    no-load speed high enough. An alternative would be to sense the
    armature current and increase the field current anytime the armature
    current rises above some threshold. This should keep your motor
    accelerating any time it needs to, and if you're careful allow it to
    transition to field-current control when you get close to your target speed.
  4. Jim Thompson

    Jim Thompson Guest

    A number of years ago I designed and one of my uncles sold, in his
    golf cart shop, a "hot rod" kit for the people in Sun City who drive
    their carts on the streets to go to the grocery store.

    Series wound motor. Just switch in a shunt across the field, and top
    speed increases to 25MPH ;-)

    Before some liberal wienie panics... 4-wheel carts.

    ...Jim Thompson
  5. Damn, you are right, of course. Sorry for the mental lapse.
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