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Induction Motor Torque Decreasing when Load is increased

  1. Mar 17, 2014 #1

    Having measured an induction motor on no load and obtained a torque figure, I have noticed that when load is increased, the torque continues to decrease. How it this possible as I am observing a reduction in rotor speed and increase in rotor currents and current drawn from supply?
  2. jcsd
  3. Mar 17, 2014 #2


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    Staff: Mentor

    Is this related to you other thread from today? https://www.physicsforums.com/showthread.php?t=743773

    If so, I can merge the threads.
  4. Mar 17, 2014 #3

    jim hardy

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    I'd need to know what you mean by "load" to make sense of the question.

    Are you measuring "slip" in % of synchronous speed, or RPM, or some other unit?

    What is your "torque figure" ? Result of a dynamometer measurement, or a motor rating, or result of a computer simulation?

    What do you mean by "on no load" ? It is counterintuitive to speak of "increasing load" and "decreasing torque" , unless perhaps you are trying to describe the torque-speed curve to left of peak .

    A picture is worth a thousand words - have you plotted the results of your experiment ?
    Posting same might clarify your inquiry.

    old jim
  5. Mar 17, 2014 #4


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    How does one measure torque with no load? At no-load speed there is no torque.

    Google "induction motor torque" and look at the images.
  6. Mar 21, 2014 #5
    The motor rpm will increase until the acceleration torque is positive [and more than “0”].That means all the time the torque produced by motor will be more than load torque. The acceleration will be 0 when motor torque and load torque will be the same.
    The motor torque vs. speed curve present 3 –significant-fragments. At first the torque will decrease as speed increases. The second portion the torque will increase, but this region it is a non-stable region since if the speed will decrease the torque will decrease more –more or less up to motor is stopping.
    If the load increases [and due to this the speed will decrease] and motor torque decreases then the motor is in this region of the curve.
    In no-load situation the speed is far from synchronous speed-slip very large- and any load will decelerate the rotor and reduce the torque.
    In normal situation the motor will work in the 3rd region where if rotor speed decreases the motor torque will increase and will be equal to the load torque at a less velocity-more slip.
    In my opinion this motor works in the second-instable-region. This could happen if the rotor is damaged: one or two bars are broken.
    By the way, even in “no-load”-that means no outside load-the inner side of the motor losses will create a parasite torque-sometime significant enough.
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