Countertorque in dc motors

Discussion in 'The Projects Forum' started by bjc, Mar 21, 2014.

  1. bjc

    Thread Starter New Member

    Mar 21, 2014
    What is the theory underlying the fact that certain dc motors can be turned, by its load, in a direction opposite to the applied torque of the motor without harm, whereas, I suspect this scenario would damage the basic ac induction motor?

    An example would be two separate, opposing winches pulling a common cable where one winch gathers cable against the torqure of the other winch (and motor) which being overcome by the first winch is forced to pay off cable.

    Also, does back emf have the same effects in ac motors as dc?

  2. MaxHeadRoom


    Jul 18, 2013
    I would challenge the first assumption, if the applied voltage to a DC motor is maintained and the motor is prevented by load to achieve a BEMF sufficient to oppose the applied voltage then the current will increase proportionately.
    A similar event occurs in an AC induction motor but not quite the same.
    On switch on, an induction motor is a transformer with a shorted turn secondary, as the 'secondary', the rotor, increases in rpm the secondary field eventually almost catches up with the revolving primary, within a few cycles, therefore off load and up to speed the induced current in the rotor is very small, when a load is placed on the rotor, the phase angle between primary and secondary fields increases and so does current.
    Last edited: Mar 21, 2014
  3. bjc

    Thread Starter New Member

    Mar 21, 2014
    Sorry, I forgot to mention that the motors in the example above are controlled by DC Drives (four quadrant).
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    That is usually done with a constant current source so the motors torque stays the same but not the RPM's.