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Torque in Gear system

  1. Feb 5, 2009 #1
    What will be the torque in a gear system after the motor get off. If so wat time it will take to stop.

    For example say if a motor is operating at a torque of 0.075Nm with 6000 rpm and the output gear rotate with a torque of 25Nm with 18 rpm. After some time motor is not experiencing any torque also. motor power supply is cutoff. so motor will rotate at no load. So how much time it will take to come to rest.

    Motor used universal motor.
  2. jcsd
  3. Feb 6, 2009 #2
    Assumption: steady state

    If motor is turned off now, it has a decelerating torque equal to whatever it was giving in at steady state, use T=I*alpha, that gives the deceleration, use equation of motion with initial angular velocity as omega(something that you had) & final angular velocity as 0, you get the time.

    Its a simple mechanics problem, not some electric motor related engineering thing(not being sarcastic, am just tellin)
  4. Feb 7, 2009 #3
    What you have told is unfortunately wrong.
    After the motor has been turned off, the decelerating torque cannot be equal to what it was giving in steady state ( Just because there is no more supply to give the torque).
    The decelerating torque is actually the frictional torque contributed by friction on shaft and between gears.
    Rest of what you have told is right.
    But what I dont know is how to calculate the frictional torque.
    After that, it is just T= I*alpha as you told.
    So if anybody can help with frictional torque, you are most welcome...
  5. Feb 8, 2009 #4
    steady state torque from the motor = torque requirement of load.

    When motor goes off, torque from motor = 0, load torque on the shaft = still steady state torque from motor. friction was already present, so it was a part of steady state torque, just assume friction stays constant & doesnt vary with shaft speed.

    Think of it like this, if you are holding the shaft with your hand & the motor is running, motor provides as much torque as your hand puts in. When power is turned off, you are still holding the shaft with same force as before(same load), you are retarding it more rapidly. I guess, you can imagine which system ll stop earlier, a motor with load at shaft or a motor with free shaft(in this case, friction at bearings(& a bit of air drag) causes the motor to slow down).

    First, I meant the numerical value of torque, not the source of torque. Second, one should be a bit more polite if confused. :smile:
  6. Feb 8, 2009 #5


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    Staff: Mentor

    There may be a language barrier there...
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