Preventing voltage spikes with AC motor switching off

Discussion in 'The Projects Forum' started by gloveofpower, Feb 15, 2016.

  1. gloveofpower

    Thread Starter New Member

    May 7, 2013
    12
    0
    A few weeks ago I bought a Leslie rotating speaker from the 70s. It uses a motor to spin its speaker to give that distinctive sound that everyone from the Beatles to Stevie Ray liked to use. Anyway my unit has various electrical problems so I've basically torn out the original amp and switching circuitry that toggled the motor on and off. The motor uses 120V AC power, straight from the wall. I have a momentary contact switch that interrupts power (aka a momentary off, normally closed switch). This switch is important since it slows down the rotation momentarily adding a human element to the sound. However each time the switch is engaged (ie the power to the motor is cut) all the amplifiers in the room will pop. I want to get rid of that popping.

    I did a small amount of research but I'm not sure where to begin. My first thought was to use a transient voltage supression diode (ie a flyback diode if you want to search that on wikipedia) but then it occurred to me that since AC power has both positive and negative voltage due to the sine wave, a diode probably isn't relevant here. I'm looking for any advice on how to stop those pops, which I'm assuming must be voltage spikes from disengaging the motor, which by nature is an inductive load. Thank you in advance for help!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,564
    2,379
    A R/C AC snubber could do the trick, Place one across the motor and also the switch contacts.
    Max.
     
    ebeowulf17 likes this.
  3. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    100% agree with Max that the snubber is the best starting point.

    I don't know if this always true, but in my very limited experience with AC motor switching noise, using an SSR (with zero-crossing activation and triac internals) instead of relay contacts has eliminated all measurable noise. It might be overkill, but if the snubber doesn't get it done, an SSR might be worth considering.
     
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