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Motor Terminology

  1. Jan 31, 2012 #1
    In terms of a torque-speed graph, for an induction motor the torque at 0% synchronous speed is roughly 150x that of full load torque and the motor only reaches 100% full load torque at roughly 95% of it's synchronous speed (depending on the rated slip).

    http://www.kalleload.net/uploads/dnyxjypnneix.jpg

    If full load torque by definition is the torque required to produce rated power at rated speed then what is rated power?

    Also what is actually considered full load? how is the load determined? My understanding is that looking at this graph you can, by knowing how much a load will slow down the rotor speed determine the torque the motor will produce to speed it back up am I wrong?

    Regards
    Dan
     
  2. jcsd
  3. Jan 31, 2012 #2

    jim hardy

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    ""If full load torque by definition is the torque required to produce rated power at rated speed then what is rated power?""

    Nameplate horsepower, 2 * pi * torque * rpm / 33,000

    Normal operation is to right of that dot on your curve.

    If motor operates to left of that dot it is overloaded and will burn up.

    There are four general speed-torque curve shapes for induction motors, A through D.

    Something like an elevator which requires lots of torque to get it started would likely use a D curve motor. Extreme starting torque but pretty bad speed regulation.
    A low inertia load like a centrifugal pump might use an A or B motor. Modest starting torque but fairly low slip hence ~constant speed.

    This AN887 from microchip is a good one, see page eleven.
    ww1.microchip.com/downloads/en/appnotes/00887a.pdf
     
  4. Jan 31, 2012 #3
    So essentially what full load torque is telling you is that that is the safe operational torque. So if a load requires high starting torque you allow for temporary overloading of the motor assuming it is capable of getting up to near synchronous speed at which point it will be in the region of 'Full Load'?

    And so to determine full load torque and so rated power, these are actually defined by what is considered safe operation i.e. operating the motor without it producing higher torque at lower speeds. So rated power would be the safe operational torque x rated speed

    Cheers
    Dan
     
  5. Jan 31, 2012 #4

    jim hardy

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    Yes.. Once you are aware of this, listen to an electric motor start.
    Good example is workshop grinder, a fairly high inertia load..
    It hums and slowly accelerates the wheels until they're nearly up to speed then you hear it "Whirr" as it snaps that last little bit up to full speed.
    That's when it crossed over peak torque into the stable region.

    I really advise that little experiment - for me, "feeling" something makes the equations come easier.
    You'll encounter "slip" in your motor courses and that intuitive feel helps you manipulate the formulas.

    And your mind will picture the speed-torque curve every time you hear an electric motor start.
     
  6. Jan 31, 2012 #5
    Excellent thanks!
     
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