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DC motor/generator

Discussion in 'General Electronics' started by Daniel, May 12, 2005.

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  1. Daniel

    Daniel Guest

    Hi all

    I have a DC motor from a old RC car and I want to make it into a AC
    generator with an output of around 8V or greater. But when I run it from a
    drill the output that I get at the terminals is 0.8V. If I were to rewind
    the motor with a finer wire would that increase the output or just the
    current produced? Also is you have any other suggestions on how to get a
    greater voltage thats will be great.

    Thanks in advance
  2. DaveM

    DaveM Guest

    Assuming that you have a PM motor...your DC motor will generate a DC voltage
    when turned from an external force. The 0.8V AC that you're seeing is the
    ripple on the DC output from the motor.
    You won't get AC from a DC PM motor. In order to get AC from it, you'll
    have to rewind it so that you have the windings connected to a pair of slip
    rings, which will replace the commutator segments.
    Realistically, unless you want to do this for the learning experience, I
    suggest that you find a small AC generator that will give the output that
    you want. Cheaper and much less painful in the long run.
    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
  3. default

    default Guest

    It is unlikely your drill spins as fast as the motor did when running
    as a motor, so your output voltage will be lower. More turns of finer
    wire will raise the voltage (and lower the safe current).

    You can't get AC from a DC motor because the motor has a commutator.
    The commutator is a series of segments that essentially keep the
    armature changing magnetic polarity (make it act as if it were driven
    with AC) with respect to the field. Reverse the process, and DC comes

    Most small AC generators spin permanent magnet(s) in a stationary
    field (easier to make and no brushes or slip rings to worry about).

    If you want to tinker and have a source of parts, stepper motors make
    ac when you spin them. They come in a wide range of voltages and
    currents. They usually have a lot of poles so the output frequency
    will be high.

    Split phase capacitor motors (AC mains types) can be run as AC
    generators with outputs similar to the mains and relatively high

    Synchro Selsyns can be used to make AC. Armature and field have
    windings and armature has slip rings. They are a sort of rotating
    transformer. Feed DC to the armature/rotor (or field) and take AC
    from the Field (or armature) while spinning the shaft. Synchro's are
    dinosaurs that were used as computers and remote positioning
    applications in WW2.

    You don't say why you want AC, but if it is a frequency sensitive
    application, frequency changes with speed and the number of poles, so
    things like transformers or AC motors may not be happy with higher or
    lower frequencies than they are designed for.
  4. Stoj

    Stoj Guest

    Hi all

    In regard to my original post. The motor that is being used is a PM with a
    speed rating of 8100rpm. This is just going to be used for the purpose of a
    learning experience and so current is not important also I had made a
    mistake in the stating AC (like you have picked up it was meant to be DC).

    Thanks in advance
  5. Rich Grise

    Rich Grise Guest

    Then, as been mentioned, a drill will go nowhere near that fast - I'd even
    be surprised if a drill ran at 800 RPM.

    But if you want to futz around with mechanical stuff, you could make a
    pulley to run the with the drill, and a belt to go around the motor
    shaft. A 10:1 diameter ratio will give you a 10X speed ratio. It could be
    done in an afternoon with a wooden disk and fat rubber band.

    Good Luck!
  6. default

    default Guest

    Your drill may be more like 650 rpm. Electric drills are geared down
    for torque at the expense of speed. When spun at the same speed the
    motor turns, as a motor, it outputs a similar voltage. That's what
    "back EMF" is about.

    So, if current isn't important, winding with fine wire is the answer
    in raising voltage at lower spin speeds. Turns ratio should be
    reasonably close - double the turns and double the voltage for the
    same speed. Inductance and resistance increase and brush losses
    probably go up too.

    As Rich points out, mechanically increasing the speed with a pulley
    will work. With an electric drill, chisel, and some wood, it is
    fairly easy to "turn" (make it on a homemade lathe) a pulley. Making
    a hub to fit the pulley to the motor - generator could be a little

    Rubber bands are fine for slow speeds or ramping up to speed. "O"
    rings have less stretch and will last longer.
  7. What's the rated no load RPM (running as a motor) and how fast were you
    spinning it with your drill? Was that 0.8V output AC, DC or an AC ripple
    superimposed on DC?

    Its not going to be easy to convert a commutated (DC) motor into an AC
    generator. You are going to have to replace the commutator with slip
    rings or otherwise make some major changes to the motor.

    Why not run it as a DC generator and build an inverter?
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