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How to use DC to stop AC motor?

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 2, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    Greetings All,
    I have a grinder with a 1/4 HP reversable induction motor in it. It is
    not a capacitor start or run motor. It has a centrifigul start switch.
    When grinding tools it is often necessary to reverse rotation several
    times to grind different features on the same tool. This entails lots
    of waiting for the motor to slow down enough to throw the reversing
    switch. Though I could replace the AC motor with a DC motor and PWM
    drive (which I already have), I would rather put in some kind of DC
    injection and a timer to stop the motor faster. Is this something a
    person with limited understanding of electronics can build? If so, can
    anyone here provide me with a schematic or point me to a site or book
    which would?
    Thank You,
    Eric R Snow,
    E T Precision Machine
  2. I don't have a schematic for you, but all it takes is a single dry
    cell connected across the motor for a few seconds. If you can figure
    a way to interlock a relay to your forward, reverse control, so that
    the DC cannot be applied while the motor is powered, you can just
    connect the DC through that interlock with a momentary push button.
    You should interlock both sides of the Dc circuit, so you cannot get a
    shock off the battery. A D cell should power a hundred or so stops.
  3. John Larkin

    John Larkin Guest

    If the main motor winding were driven from 120 VAC through a diode and
    a power resistor (in series), you should get pretty good braking. We
    used to do this on tape deck reel motors, and it didn't take a lot of
    DC current to give nice drag.

    I'd suggest a 1N4005-type diode and maybe a 300 ohm, 50 watt resistor
    as things to try. There's probably a place in the reversing switch
    setup where you could arrange for the small DC to flow whenever the
    motor wasn't supposed to be running (no timer!) but that would depend
    on how things are wired.

    What sort of machining do you do?

  4. Eric R Snow

    Eric R Snow Guest

    Greetings John,
    It appears that I never replied to your message here. Sorry about
    that:( Anyway, the machining I do is general job shop stuff. So,
    almost anything except automotive and aircraft. Plastics, metals,
    ceramics, and composites all run through my shop. Mostly, these days,
    it's aluminum, brass, stainless, and acetron plastic getting done.
    Part descriptions range from door hinge finials to baseplates for
    fingerprint reading machines. I modify stepper motors and ballscrews.
    So, pretty varied. Also, some welding gets done. Some retired guy
    will need the handle for his lawnmower welded up and things like that.
    They get the "Good Neighbor" rate. And some of my weld jobs require
    tig welding of stainless parts and aluminum parts.
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