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Efficiency of 3 phase AC Induction motors

Discussion in 'Electronic Design' started by [email protected], May 15, 2006.

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  1. Guest

    I have noticed that larger motors (40+ HP) are typically more efficient
    than smaller motors, and also there is not as much weight difference
    for 6 and 8 pole motors compared to 2 and 4 pole. I have several

    (1) Will a 40 HP motor rated at 90% efficiency have better efficiency
    at 20 HP? What about at 10 HP?

    (2) What sort of efficiency can be expected if a motor is run for short
    times at 2x or 3x nominal torque rating? What are typical duty cycles
    and maximum ON times for such conditions?

    (3) What could be done to make a 12 pole motor (about 5-10 HP) about
    the same size and weight as the same HP 2 or 4 pole motor?

    Thanks for any ideas.

    Paul E. Schoen
  2. Jim Backus

    Jim Backus Guest

    It's too long since I did electrical machine theory, but I think the
    motor could stall at that load.
  3. Guest

    No, the motor is most effiecent at rated load.

    Depends on the cooling, a small motor like this would likely burn out
    within 30 - 90 secs at 3x load, efficiency is very poor less that 15%
    The more poles you have the slower it runs, so for the same HP you have
    proportionaly more torque. This means you need a bigger shaft and
    bigger frame to handle it.
  4. ----- Original Message -----
    From: <>
    Sent: Monday, May 15, 2006 6:33 PM
    Subject: Re: Efficiency of 3 phase AC Induction motors

    I thought possibly a motor run at lower than rating would have less copper
    heating loss, but I seem to recall that motor current stays fairly constant
    over a wide range of HP output, so the VA is about the same but phase angle
    shifts closer to unity at the rated HP. This effect may not occur in PM
    BLDC motors and other designs.
    That jives with my initial thoughts. I am more familiar with transformer
    overloading, which might handle a 3x overload for about 3 minutes with a
    duty cycle of 10%. Of course a motor at 3x would be running at close to its
    breakdown torque, at which it would become 0% efficient with locked rotor.

    My conclusion is that it is probably best to specify a larger motor that
    would not be pushed any more than about 1.5x to 2x, for no more than about
    2 minutes. Hopefully, the efficiency at fractions of maximum HP will not be
    too bad. This for a vehicular application, where loads change drastically,
    and efficiency is a major factor. There is already a loss of efficiency
    because of extra weight of a larger motor.
    That is one of the best explanations I have heard. I had thought it was
    more a function of winding efficiency, with more overlap in higher pole
    numbers, but less so in larger motors with more stator slots. Is that also
    a contributing factor?

    Thanks for the information.

  5. Glen Walpert

    Glen Walpert Guest

    I noticed that the 3 manufacturers now road testing small fleets of
    hydrogen fuel cell vehicles (Honda, Mercedes and GM) are all using 60
    KW permanent magnet rotor synchronous motors. I have not been able to
    find any information on the stator design of this motor, but it would
    appear that the 18-phase Chorus design, with it's 3 times greater low
    speed torque capability than a standard 3-phase induction motor design
    in the same frame, is a natural for vehicular use if an induction
    motor is to be used. I am not sure if the induction motor advantages
    of high phase order can apply to permanent magnet rotor motors or not,
    but I suspect not because of the need to change stator field profiles
    with speed in the high phase order Chorus design, and the difficulty
    in adjusting the permanent magnets :).
  6. Tim Wescott

    Tim Wescott Guest

    In permanent magnet motors (commutated or brushless) there is a certain
    power lost just to spin the motor -- this is mostly friction, windage
    and core loss, but I couldn't for the life of me tell you which ones are
    more important. So for any given voltage there is always one load (and
    hence current) that gives the best efficiency.

    You'd have a similar effect in any field-wound motor, except the maximum
    efficiency point would vary with the field. I would expect that a large
    industrial induction motor would be most efficient at it's rated output,
    or possibly slightly less if it's designed for varying loads.
    From Merriam-Webster online

    Jive: 2 a : glib, deceptive, or foolish talk b : the jargon of hipsters
    c : a special jargon of difficult or slang terms

    Jibe: : to be in accord : AGREE

    (sorry for the compulsive proofreading)


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  7. Standard AC nduction motors only operate properly over an EXTTREMELY
    NARROW speed range for a given input frequency.

    The synchronous speed minus a few herts of slip frequency.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  8. Rich Grise

    Rich Grise Guest

    Would that be 60 hurts per second in the US, and 50 in the UK?

  9. At 3600 RPM, with a whip attached, that would be 60 Hurts per second. In
    the UK you don't get as many lashes. Of course I've heard there are lots of
    cute red-haired lashes in Ireland :)

    Hertz per second is actually a figure for acceleration. Naturally, I'll be
    using a V/F controller and I might go as high as 360 Hz if the motor does
    not become too inefficient.

    I have been know to slip with some frequency myself...

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