DC motor reversal delay

Discussion in 'The Projects Forum' started by tracecom, Nov 4, 2014.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    I have built a uC controlled H-bridge circuit to control a DC motor. I have coded in a delay before starting the motor in order to eliminate the possibility of an instantaneous reversal of polarity. My question is what is the minimum delay time that I should code?

  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    that depends on the rotating mass, less mass, shorter time, more mass, longer time. thats why machine tools use a plugging switch or relay to detect zero apeed. a fixed mass could be done with a timer, a varying mass needs a zero speed detector.
  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    It's a small gear head motor with the output rotating at 40 rpm; the load is very light. Would 100 msec be safe?
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    If reversing, I would expect you would have to wait for most of a second. Why not measure the back emf from the motor using the ADC on the MCU, and reverse only when the back emf drops to zero. Another possibility is dynamic braking by switching the H-bridge into the appropriate state.
  5. MaxHeadRoom


    Jul 18, 2013
    If the process is always the same, all you need to know/time the stopping time of the motor, a DC motor generates a voltage as the same polarity of the applied power, this voltage is directly proportional to the rpm of the motor, on a small motor it should not even require a dead stop condition.