DC motor reverse polarity question

Discussion in 'General Electronics Chat' started by zeta_no, Aug 11, 2009.

  1. zeta_no

    Thread Starter New Member

    Jan 20, 2009
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    0
    Hi,

    I have a 12V DC motor and I want to know if it's ok to drive it at 12V and suddenly reverse the polarity to -12V with a H-bridge let say.

    If a DC motor is an inductive charge, I suppose we can do so.

    I just want to make sure before testing for real.

    Thank you,

    Olivier Henley
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Suddenly reversing polarity to a DC motor is very hard on it.
    Allow the motor to stop first, and then reverse direction.
     
  3. zeta_no

    Thread Starter New Member

    Jan 20, 2009
    7
    0
    Humm,

    Is it less critical if it happens for a very short time?

    Because, I did simulations for a PID controlling position of a DC motor. When I optimize my PID constants, the voltage behaviour is always the same. To rapidly reach a target position, the voltage peaks real high (here I make it saturate at 12V because my motor is rated as such) and then to quickly stop its motion, the voltage peaks real low (sature also at -12V). But all this occurs really fast, as I simulate a small step response in the order of the precision of my encoder.

    Nevertheless, I plan to feed the controller continuously, always reaching new positions. So I suppose it would deteriorate the motor quite fast.

    Should I avoid controlling position in favor of speed or just use another kind of motor?

    Thanks,

    Olivier Henley

    P.S: Maybe, this question is better suited at CNCZone, but if you have advise for me I would appreciate.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The worst case load on a DC motor is full power reverse at full speed. Much worse than slamming into a stall at full power. Your motion controller should have proper acceleration ramps that minimise motor load or at least keep it within a safe limit.

    If you messed with your PID settings so the controller slams the motor in reverse to stop it really quick you need to change your settings back, so it decelerates at the proper rate to a gentle stop.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Define "Short Time"

    If operating in the 50kHz range, going to brake on the H-Bridge might be all the PID needs for the speed change, and will be much less harsh on both the H-Bridge drivers and the motor.

    "Brake" is switching the H-Bridge to essentially connect both leads of the motor together, creating a shorted generator. R/C Car speed controllers do this automatically when going from full speed forward to reverse, depending on the speed, it brakes very well.
     
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