AC motor, 2 caps, centrifugal switch

Discussion in 'General Electronics Chat' started by quivil, Nov 9, 2012.

  1. quivil

    Thread Starter New Member

    Nov 9, 2012
    I think this is an easy one for some one who knows motors.

    I have a lathe which stopped working with a loud pop. Capacitor, I thought. Yep. The 50uf cap was open. That's the running cap, I believe.

    Got a replacement, and put it in the circuit.

    Checked the 600uf cap, it seems fine. Bought another one just in case. Tested both, both within spec.

    Now, when I attempt to run the motor, it turns slowly and buzzes. If I give it a good push, it takes off at the proper speed. The running cap is doing its job.

    Somehow the starter cap isn't. It's either the centrifugal switch isn't closing when the motor is at rest (completing the circuit to bring in the starter cap), or there's a break in the wires somewhere.

    My question is, is there just one set of windings for this motor, or is there a second set of windings just for starting the motor? I know the running windings are fine, so if there's only one set, the windings are good.

    I'm going to test the centrifugal switch directly when the motor is assembled (by running a couple of wires through the case) to make sure it's making contact.

    Any other suggestions? I've tried both starting caps, and neither gets the motor turning at full speed.


  2. JDT

    Well-Known Member

    Feb 12, 2009
    There is a second set of windings for starting the motor. This is connected with the centrifugal switch. Check the switch and the continuity of the winding.
  3. PackratKing

    Well-Known Member

    Jul 13, 2008
    The start capacitor and switch, are in series with the start coils, which are the smaller coils in the motor, which are physically 90° removed from the run coils. If the switch contacts are fouled with anything, or the contacts eroded from normal use, it may need replacing. Do as JDT suggested.

    BillB3857 next post down is absolutely right.

    4 run coils = a 1725~ rpm motor - 2 run coils = 3450 ~ [ ~ More/less check the nameplate ]
    Last edited: Nov 9, 2012
  4. BillB3857

    Senior Member

    Feb 28, 2009
    If it is a wood lathe, my guess would be sawdust fouling the start switch. I've seen this happen a lot on table saws.
    shortbus likes this.
  5. quivil

    Thread Starter New Member

    Nov 9, 2012
    Thanks guys, that helps a lot.

    It's a metal lathe. The switch contacts look ok. Worn, but ok, and they're very isolated from the world.

    There are four wires going to the motor from the reversing switch, two red, two yellow, that look just like this:

    Could some one tell me where is the best place from which to measure the continuity of the coils? About what resistance should I expect? ~0, I assume...
    Raymond Massey likes this.
  6. quivil

    Thread Starter New Member

    Nov 9, 2012
    Found the problem. There were two wires crossed on the terminal block. They looked fine.

    It turned out, they had worn through each other, and one of the solid copper wires had been vaporized, leaving no connection. That's why the run capacitor was blown, and then the thing wouldn't work. From the top, you couldn't even see it.

    Thanks for the help all, it's working like a champ now!