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Motor Speed and Torque Relationship

  1. Aug 14, 2013 #1
    I have a question/assumption that I'm not sure whether or not it is correct. Imagine I have an electric motor, if I have this motor spinning a load at a certain speed, can I assume that there is constant torque? That is, as long as the load isn't changing.

    However, if the load changes and becomes more difficult to spin yet the motor compensates and continue running the load at the same speed as before, this would require the torque to increase?

    This question originates from a variable frequency drive that can either control torque or speed of a motor. I was originally thinking that both variables could be controlled at the same time, however, now I feel like only one variable can be controlled at a time. Since at a constant load, the speed and torque will be locked together? Is it safe to assume that a motor can only control speed or torque one at a time?
  2. jcsd
  3. Aug 14, 2013 #2


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    I don't know about a variable frequency drive, but for dc motors there are controllers that can maintain speed in spite of a changing load, and also modify the speed in spite of the load, up to the limits of the motor.

    For an idealized motor, max torque occurs when the motor is stalled (until it overheats), and zero torque occurs at max rpm, and the torque versus rpm "curve" is a straight line. Maximum power occurs at 1/2 of max rpm.
  4. Aug 14, 2013 #3
    However, you're saying that if the load is changing and the speed is remaining constant, that would mean the torque is changing which also mean there is a change in power output?

    And I really like the second part you included. The inverse relationship between speed and torque would suggest that the power out putted would be greatest at half speed, half torque.
  5. Aug 15, 2013 #4


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    Scroll down about 2/3rds of this page to see some graphs of an idealized dc motor.

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