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Motor/generator question.

Discussion in 'Electronic Basics' started by Andrew Howard, Sep 12, 2003.

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  1. Does anyone know if its possible to use a motor as a generator (just an idea
    I had). If you got something else to spin the motor directly, would voltage
    appear at the motor's contacts?
    I have no idea about any of this, I was just wondering about it.

    Andrew Howard
  2. Many kinds of motors act as generators, also. The easiest kind to
    play with are permanent magnet field DC motors that have brushes.
    These include all sorts of toy motors from slot cars to radio
    controlled cars. Pager vibrators are very tiny examples.

    Stepper motors that have permanent magnet rotors (that have unpowered
    holding torque) make easy to use AC generators.

  3. ALL motors act as generators. This is actually part of the laws
    of physics. If you could find a motor which didn't act as a
    generator, then Lenz Law would be violated, energy would no longer
    be conserved, and you could build a perpetual motion machine.
    In normal use, motors consume less electrical energy when they
    are doing less mechanical work... and this is a "generator"
    effect. Spin a DC motor faster than it's normal speed, and it
    will pump energy backwards into its power supply.

    However, motors which lack permanent magnets won't act as generators
    if you simply spin the shaft. "Field coil" motors need help to get
    started. To make an "AC/DC" motor behave as a DC generator you have
    to connect a battery temporarily to the motor while you force the
    shaft to spin. This kicks it into operation, letting it power it's
    own field coil, and then the battery can be removed. (If the shaft
    should ever stop turning, the generator will need another "kick" when
    you start cranking it again.)

    AC induction motors are similar, they need a kick, but they also need
    a device which temporarily stores energy for part of a cycle, and then
    dumps some energy back into the motor coil for the rest of each AC
    cycle. In other words, connect the right value of capacitor across
    your induction motor and you've built yourself an AC generator:

    For hobby purposes, any small "permanent magnet" motor can be used
    as a DC generator.

    Try this: use some tape or a piece of wire insulation to connect
    the shafts of two small motors together. Use a battery to run one
    of the motors, then measure the voltage on the "generator". You can
    use this generator to light a flashlight bulb or an LED... or to
    run another motor.

    Hobbyists who build their own Wind Power generators love treadmills
    and other excercize machines. In particular, they love the large,
    multi-horsepower DC permanent-magnet motors in these machines, since
    they work great as high-amp DC generators, and the surplus catalogs
    are full of them. (A similar DC generator, bought new, might cost
    several hundred bucks!)

    SCIPLUS: DC PM motor/generators

    (((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
    William J. Beaty SCIENCE HOBBYIST website
    EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
    Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci
  4. Caliban

    Caliban Guest

    It's not only possible; it's actually employed on a very large scale in
    certain electrical systems for the purpose of reliability. E.g. in some
    systems normally AC generator #1 drives an AC motor which drives a DC
    generator which maintains batteries on trickle charge. If the AC generator
    suddenly trips off for whatever reason, then the batteries pick up the load.
    The batteries drive the (former DC generator and now) DC motor, which drives
    the (former AC motor and now) AC generator #2. The loads between AC
    generator #1 and the motor generator set never lose power.
    Your intuition is correct.
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