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DC Motor overvoltage

Discussion in 'Electronic Basics' started by Emil Johnsen, Aug 19, 2004.

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  1. Emil Johnsen

    Emil Johnsen Guest

    I want to run a 3.6V DC motor from a 12V battery. The motor will not run
    continously, only 0.5sec at a time with several seconds delay before it will
    run again.

    Can I do this without damaging the motor?

    (I'd like to avoid using a regulator or PWM if possible, also the extra
    torque from higher voltage would be useful.)
  2. Rich Grise

    Rich Grise Guest

    If you know the motor current at normal operation (you can measure
    this ;-) ), just add a series resistor. Lessee, I think R = E/I,

    so your resistor would be (12 - 3.6)/(Imotor) ohms.

    Good Luck!
  3. Jamie

    Jamie Guest

    you can on some.. but i can bet you that the RMP's are really
    what you should at least do is pu some diodes in series.
    to drop the level.
  4. Emil Johnsen

    Emil Johnsen Guest

    If you know the motor current at normal operation (you can measure

    The motor draws ~3amp, so I will need a ~30W resistor, which means either a
    bulky wirewound resistor or an expensive film resistor. Since it won't run
    continously I can probably use a smaller resistor, but I don't know how
  5. Don Kelly

    Don Kelly Guest

    However, what is important is the current at startup in this operating mode.
    That will be higher than at normal load. In addition, the resistor while
    cheap will not help at no load or light load- (speed will be high and the
    speed will change more rapidly under load.
    Also- what kind of motor? shunt, permanent magnet, series? it makes a
  6. Emil Johnsen

    Emil Johnsen Guest

    However, what is important is the current at startup in this operating
    I'm guessing series, because this motor comes from en electric screw driver,
    but I'm not 100% sure.

    I'm going to use the motor as an actuator for a ball valve, obviously with
    significant gear reduction. I will leave the motor on for <0.5sec, during
    this time it will rotate the valve shaft 90deg and stall as it hits a
    mechanical stop. The motor will not see particuarly light loads.
  7. Doug

    Doug Guest

    My guess is that you stand a good chance of doing some kind of mechanical
    damage to the motor, particularly in light of the fact you will be
    inpacting it on a stop With 3 times the voltage, the starting torque may be
    more than the motor can handle.
  8. Don Kelly

    Don Kelly Guest

    If you are going to use a screwdriver motor -why not keep the gears that go
    with it? If it is a series motor, you can limit current with a resistor. I
    would suggest that as you are running at 3 times the voltage, use 2 times
    the measured locked rotor resistance in series- rate the resistor on the
    basis of the current under locked rotor conditions. This will limit the
    current to what the motor normally takes in this condition and stall torque
    to what it is normally. The speed will drop more under load than it would
    A screwdriver motor is expected to stall so you could reduce the extra
    resistance somewhat. Watch for arcing or sparking at the brushes- I think
    the windings will take the overcurrent for the time under consideration but
    the brushes may be a problem.
  9. I haven't tried it, but I would have thought the answer was No. And if
    it wasn't for your final proviso, I'd have suggested a simple 12V PWM
    approach. Apart from probably minimising risk of motor damage, that
    would allow you to secure extra torque.

    FWIW, that's what I was considering doing for my curtain controller
    (subject of recent threads). That too uses a screwdriver motor and
    gearbox. Mine was originally powered by 2.4 V (2 x 12 V Nicads). But
    that didn't quite cut the mustard. So I increased it to 3.6 V (with a
    permanent trickle charge sourced from an existing convenient 12 V DC
    supply). In initial tests that seemed to give too violent an action
    (especially dramatic when the curtains opened automatically at dawn
    <g>). But it turned out that the necessary longish wiring to the motor
    provided sufficient resistance to tame it.

    BTW, I originally tried my 2.4 V motor at progressively higher
    voltages from my bench power supply, but quit at 6 V.

    Did you also consider a limiting microswitch? I'd be interested in
    what stall-detection circuitry you are using.
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