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Instantaneous torque in a 3-phase motor

  1. May 21, 2014 #1
    This isn't a homework question because nobody assigned it to me. It is a theoretical question however.

    I understand that in a balanced 3-phase system with a constant load, instantaneous power is constant. Not "pretty constant" but a flat line. This suggests to me (warning: amateur) that instantaneous torque, in a 3-phase induction motor, might also be constant throughout each revolution, assuming a fixed load. But, given that slip is necessary, and that the rotor experiences a frequency of a few hertz, it may be hopelessly naive to imagine that torque isn't somewhat "bumpy."

    Again - this is instantaneous torque I'm thinking of, measured throughout a machine cycle or two, not torque vs. load or any of that usual motory spec stuff. ;-)

    I've exhausted the books I have at hand, and I feel as though I've seen every torque curve on the internet by now. Perhaps it's not an important question but it's burning a hole in my curiosity.
  2. jcsd
  3. May 21, 2014 #2

    jim hardy

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    Science Advisor
    Gold Member

    Look at an armature and observe the slots are skewed, not parallel to the shaft.

    That's done to relieve the magnetic discontinuity as the rotor slots pass under the armature slots, smoothing out torque and reducing audible "hum".

    try a search on 'squirrel cage slots skew' . There's a 1948 patent that looks interesting ,,, but on this computer i couldn't read it for some obscure software reason..


    old jim
  4. May 21, 2014 #3
    here is a youtube breakout of an AC motor that shows the skew.

    Last edited by a moderator: Sep 25, 2014
  5. May 21, 2014 #4
    You anticipated my follow-up too. Thanks.
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