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over voltage of a 12vdc motor

Discussion in 'Electronic Design' started by CCE, Apr 30, 2007.

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  1. CCE

    CCE Guest

    I have a 12vdc motor from a power car window and I need to run it at a
    higher speed. The load on the motor is pretty low. What can I expect
    if I run this motor at 18 or 24 v?
  2. Tim Wescott

    Tim Wescott Guest

    The motor brushes get stressed by high currents; if you start the motor
    on higher than normal voltages they'll get stressed more, leading to
    faster (sometimes immediate) wear.

    The motor bearings get stressed by high speeds; running the motor on a
    higher voltage will make it go faster. Whether this causes problems
    depends on your bearings.

    Iron losses will be higher because the armature windings will be running
    at higher than normal frequencies, but if you're loading it lightly this
    may not be an issue.

    The brushes may bounce if you're going at 2x the normal speed, leading
    to quicker (but not immediate) wear and intermittent lower torque.

    If you're driving it from something that can limit the current as it
    starts up you may be fine, or you may run into brush or bearing
    problems. If it's a car window motor chances are that it's running
    pretty close to the limits of its design, but you can only tell by
    giving it a whirl.


    Tim Wescott
    Wescott Design Services

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  3. D from BC

    D from BC Guest

    I recall reading in a car manual that those's motors may have a
    overcurrent device (polyswitch?) type device somewhere.
    The device might trip with the added current from the added voltage.

    Those motors are probably designed for a short run time... Given
    enough time, continuous use and higher voltages will probably cook the
    D from BC
  4. Meat Plow

    Meat Plow Guest

    Eventual overheating and failure seems a safe assumption.

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  5. CCE

    CCE Guest

    The duration is pretty short, say 5 to 10 seconds every 3-4 minutes.
    This is for a hobby project and will only get used once a month or so.
  6. colin

    colin Guest

    Smoke. I expect the window winder motors are not continuosly rated.

    However I run those 9.5vdc cordless drill type motors at 12v,
    the normal bearings dont last long so I replace them with ball types,
    the brushes seem to last ok though.

    It all depends how long you run the motor for and how long you want the
    bearings brushes to last and how hot you let it get.

    Colin =^.^=
  7. Phil Allison

    Phil Allison Guest


    ** Count the contacts on the commutator and find how many poles it has.

    Allow 2.5 volts DC for each pole as a safe max working voltage.

    3 pole = 7.5 volts

    5 pole = 12.5 volts


    Go much above that and expect very short life from the commutator.

    Automotive heater fan motors typically have 11 poles, even though rated for
    nominally 12 volts, so can take 24 volts DC long as the current draw is not
    too high.

    Watch out for switch on surge currents that can damage the commutator at
    high voltages.

    ........ Phil
  8. CCE

    CCE Guest

    Thanks for everyone's input. Your posts have answered my question!
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