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USING AN AC motor as an alternator.

Discussion in 'Electronic Repair' started by klem kedidelhopper, Nov 17, 2012.

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  1. My friend has a 40HP 3 phase motor which he wants to try to use as an
    alternator. He seems to think that if he powers this monster from a
    diesel engine he can generate enough useable AC power to run his small
    factory and heat it as well. Even if you could get some AC out of the
    motor I don't think this is possible. Any thoughts? Lenny
     
  2. He needs a magnetic field. I don't think such a motor has permanent -- or
    electro- -- magnets.
     
  3. you can do it, but the performance sucks.

    I've made generators using induction motors attached to engines, just to
    see it work.

    they act really weird, and work at some weird RPM nowhere near the value
    of the faceplate produce around their rated input voltage at about 60Hz.

    There's usually enough residual magnetization in the rotor or laminations
    to self excite, but not always. Runnign 12 volts across the windings from
    my engines starter battery for a second would always work when they
    didn't.

    The last test with a 3/4 HP 1725 RPM motor with about 200uF of caps across
    the power cord that would usually run it seemed too be able to run about
    400 watts of light bulbs, but only when run at 2200 to 2800 RPM. The
    output frequency did stick to about 60Hz, somehow. The output voltage
    regulation wasn't that bad, and was probably "regulated" by some sort of
    saturation or leakage in the motor design itself.

    I can't explain how the whole thing worked, but trying to drive an
    inductive load would make the thing stop acting as a generator, and the
    engine would then rev up as the load seemed to go away.

    Also, if you're dealing with single phase motor used as generators this
    way with 1:1 speed coupling to the engine, you get a very strange flicker
    or beating to to output voltage. I could see light bulbs pulsate in
    intensity, which was horribly obnoxious.

    then I realized that it was because I was using a 4 cycle engine, so the
    power stroke was brighter than the rest of them. a 15Hz flicker seemed
    about right. It still doesn't really jive with what the tachometer said
    for 2200 to 2800 RPM. I was using a wrap-around the ingnition cable unit
    from Sendec.

    Anyways, it was an interesting project.
     
  4. mike

    mike Guest

    The secret to a successful engineering project is the ability to generate
    many ideas and QUICKLY eliminate the bad ones. Most ideas are so
    incredibly bad that you don't need a detailed analysis to reject them.
    Gross approximations will do just fine.

    So, let's head down that path. Stop when you get an answer you can't
    live with.

    Assume that a 40hp motor can produce 40hp worth of electricity.
    That's about 30kW.
    Will 30kW do the job? Is it in the right format?
    Is 3-phase power what you want? Remember that you may need much
    more than 30kW peak to start a large machine. If the diesel dies
    or you have a brown-out
    under the peak load, things get ugly.

    Pick a round number. Say that it takes 90kW worth of diesel fuel to
    produce 30kW of electricity. I don't know what that works out to in
    dollars per kilowatt-hour, but does that number compare favorably
    with your other electrical energy alternatives? The good news is
    that, if you can capture it and it's at a temperature
    you can use, you have 60kW of heat for the factory,
    summer and winter.

    By this point, it should be obvious that the generator costs way more
    than power from the grid. I think my estimates were very generous.

    So, if you can stand the cost and the power is in the format you can use,
    you get to determine exactly how the motor is wound and start trying
    to separate out the field and power it.

    I'm betting you don't have to do all that work to reject the idea.
     
  5. Guest

    It seems alot more efficient when the grid is down.

    J
     
  6. Rich Webb

    Rich Webb Guest

    What *kind* of motor? If it's a typical squirrel-cage induction motor,
    your friend is SOL.
     
  7. this exactly matches what I came across, with a single phase induction
    motor. do you know why such a great speed overrun is needed to hit 60Hz
    and why the motors seems to try to output only it's rated Hz and input
    voltage?

    My generator project was going to have a marine alternator to make 24
    volts, but somehow I ordred the wrong type (12 volts) and shelved the
    project after my bank of batteried from a charger mishap.
     
  8. Guest

    "do you know why such a great speed overrun is needed to hit 60Hz
    and why the motors seems to try to output only it's rated Hz and input
    voltage? "

    The field actually rotates in the rotor. It does the same thing when it is a motor, when you load it and it pulls more current, you are fighting the magnetic field's desire to not rotate in the rotor. At 60 Hz it should be 1,800 RPM but it isn't. It's 1,750. That is actually nominal but it is accepted as the normal loss for motors that run machines rather then say, clocks.

    This is the same shit only opposite. It's also a hell of alot less eficientwhen not made for the purpose. In a way it's like running an internal combustion engine backwards, just not as bad.

    J
     
  9. Guest

    Rather THAN say
     
  10. josephkk

    josephkk Guest

    Not at all. But exciting it and regulating the output is a bit tricky.
    Common as wind power systems.

    ?-)
     
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