AC motor stop

Discussion in 'The Projects Forum' started by jack mcleod, Feb 18, 2008.

  1. jack mcleod

    Thread Starter Member

    Sep 4, 2007
    A shot of DC into an AC motor will stop the motor. Plan is to open one of the 240 V AC lines and insert a NC, SW in parallel with a diode. Opening the SW will send the DC shot to the motor; the regular motor SW is shut off at the same time. What should the PIV and A ratings of the two diodes be?

    1) max draw 8A, operating draw 3-4 A (table saw)
    2) " " 5A " " 2-3 A (radial arm saw)

    How long would it take (us or several sec.) for the varnish on the windings to melt? or would that even be a problem?

    I'm aware of the inertial hazard of the blade coming off - turn the saw back on quickly.

    advice and experience welcomed
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    I have never tried nor wanted to do that with a single-phase motor. A free-wheeling DC motor can be stopped in about a rotation or two, which would mean about 50 to 100 mS at 1200 rpm. I used that method with a high-speed winch to keep it from back lashing as the line was fed out. John
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    You could think about using a lower voltage through a transformer and rectifier. Possibly 24 volts @ 2 -3 amps. More current limiting that way. So no smoke from the coils, or spinning blades loose in the shop.
  4. jack mcleod

    Thread Starter Member

    Sep 4, 2007
    Thanks, for the replies guys. Will experiment with inductive motors come spring. Still open to input.
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    If it's breaking you want, why not simply swap the supply for a resistive load? With the supply removed and the load in place, the inductive motor becomes a generator. Kinetic energy of the motor and saw quickly becomes heat dissipated by the resistive load.

    All that is needed is a power resistor and a double-pole double-throw break-before-make switch.