AC motor stop

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

  1. jack mcleod

    Thread Starter Member

    Sep 4, 2007
    21
    0
    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
    5,694
    904
    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
    15,815
    282
    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
    21
    0
    Thanks, for the replies guys. Will experiment with inductive motors come spring. Still open to input.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    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.
     
Loading...