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DC motor

Discussion in 'Electronic Basics' started by aman, Apr 19, 2005.

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  1. aman

    aman Guest

    I can detect a short across the terminals of a DC motor(when I do a
    continuity test) before I even connect it. Can anyone tell me what the
    problem can be. Do you guys think the motor burnt away or what ?
  2. Lord Garth

    Lord Garth Guest

    Has the commentator ring slipped or are you using brushes that are too

    Pull the brushes and test the coils directly at the commentator. You could
    have some failed insulation in one of the coils.
  3. aman

    aman Guest

  4. Lord Garth

    Lord Garth Guest

    Looks sealed! So no. Since they tested this gear motor at 19 volts to
    measure a stall current of 1.76 A. I see no reason you can't connect
    it to 12 volt supply and limit the current too a few mA. Slowly
    increase the current while observing the voltage. If the motor is
    shorted, the voltage will only indicate 0 no matter the current limit.
  5. Jamie

    Jamie Guest

    i assume if its much less than 10.8 ohms ! then you must
    really have a short.
    because the data that shows does specify 10.5 ohms as the
  6. Rich Grise

    Rich Grise Guest

    Are you sure it's a "short", or are you just measuring a very low
    resistance? They do have a very low DC resistance, you know - the
    current flow is limited by what's called "back EMF", which is a
    voltage created by the motor's generator action while it's running,
    that opposes the applied voltage. You haven't said how big the motor
    is, or its ratings, or anything else.

    Just for perspective, I've never seen a motor fail short. If it's
    busted, it'll fail open or will have already burned up.

    How big is the motor, and on what resistance range were you? Did you
    zero the meter?

    Good Luck!
  7. Rich Grise

    Rich Grise Guest

    Get your hands on a 12V power supply and a 10 ohm, 25 watt

    Put the resistor in series with the power supply and motor:

    10 ohms, 25 watts
    +12 ----/\/\/----. A
    12V ret. --------' B

    Apply power, and see if it runs. The resistor will limit
    the current to 1.2A if it's short, which is somewhat less
    than the motor's rated stall current (1.76A, from the
    datasheet), so even if the motor is a dead short, it will
    not burn.

    If the motor runs, albeit slowly, then you've got a good

    If not, measure the voltage at point A (the + side of the
    motor) relative to point B (the - of the motor). If it's
    zero volts, you have a shorted motor. If it's more, I'd
    say it's safe to try the 19V supply - first with the
    resistor, and if it runs lamely, then leave out the
    resistor, because you've most likely got a working motor.

    Good Luck!
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