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reversing an alternating current motor..

Discussion in 'Electrical Engineering' started by DiezMon, Jun 7, 2005.

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  1. DiezMon

    DiezMon Guest

    Hello all,

    Go easy on me, I'm not an engineer ;)

    I googled this and couldn't find a definitive answer. I have an AC
    motor/gearbox that I want to use to drive a pulley system. The motor takes
    110-15volts AC.

    How does one reverse an AC motor? Is it as simple as DC where you can just
    reverse the polarity, and just insert a reversing DPDT switch in there?


  2. No. There are lots of different types of motor which work
    on AC, and only some are reversible. You'll need to give
    more details of the motor...
    Does it have a commutator, brushes, and windings on the
    armature (i.e. a universal motor)? These are reversible
    but some are not designed for it and won't work well.
    Does it have an external capacitor? These are reversible.
    Does it have a couple of thick single turn coils wrapped
    around the field winding core (shaded pole synchronous
    motor)? These are not reversible.

    Might also help if you said what the motor was originally
    designed for (e.g. something which required a synchronous
    motor) and what nameplate information there is on it.
  3. DiezMon

    DiezMon Guest

    I figured it wouldn't be that easy ;)

    I'm going to have to disassemble the thing and examine it a bit. I found it
    at a surplus store so there isn't much information on it.

  4. SQLit

    SQLit Guest

    Not only is the motor an issue. Some gear boxes are designed for load in one
    direction only.
  5. --

    -- Guest

    that 115 volts indicates it would be a single phase motor, and that means it
    is either a universal motor or a capacitor-start motor.

    If it is a universal motor which does not have a capacitor, but rather has a
    board with a capacitance simulator, can't help you. It can be done, but you
    need to know where to put the switch leads on the board.

    If it is a motor that uses a physical capacitor (little round can or disk) -
    there are three basic parts in the cap start motor - the rotor
    (moving/rotating electromagnet), the stator (fixed electromagnet), and the
    capacitor (it shifts the incoming amps in one of the magnets so as to cause
    the magnets to start out of phase, rather than lock up in opposition).
    The motor runs because the magnets are made to be a little out of phase
    when it starts, and it keeps running because the rotor momentum keeps the
    phases just a little off. Which way it shifts on start establishes which
    way it rotates. So you make the rotor magnet lag, it rotates one way. You
    make the stator magnet lag, it rotates the other way.

    I have reversed single phase motors by switching one of the leads of the
    capacitor, so as to change which magnet "leads" the other when the magnets
    build a field. E.g., if it is on the rotor lead(s), move it to the stator
    lead(s). It has been a while since I have done it personally (Once I showed
    the shop, now they do it as required).

    I think I moved the capacitor lead from the rotor wire to the stator
    wire -

    Note: If the gearbox has a uni-directional overrun brake, it won't go in
    reverse. Those brakes are fairly special and rare, however.
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