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three phase induction motor

Discussion in 'Electronic Basics' started by jimmy, May 12, 2004.

  1. jimmy

    jimmy Guest

    hi


    Does anyone know of a good resource on the internet where i can find
    information to answer the following question.


    and yes i have tried search Engines.



    Describe how three phase induction motor speed is controlled with the
    use of a variable voltage, constant frequency method


    cheers
     
  2. In any induction motor, the field coils produce some approximation of
    a rotating magnetic field that magnetizes and drags around the rotor.
    To achieve variable speed at fixed frequency (of rotation of that
    field) the rotor must fall behind the field rotation by various
    amounts (variable slip). This is an inefficient process, but usable
    torque can be produced, especially if the shorting bars in the rotor
    are deeply buried in the iron of the rotor, and/or are of higher than
    normal resistance. An induction motor optimized for this service is
    called class D.

    Explanation of Class A,B,C, and D induction motors:
    http://alperucar.f2o.org/induction_motor/

    Simple phase control circuits have been used to reduce the effective
    voltage applied to small induction motors to achieve variable slip
    speed control, but their efficiencies are terrible.

    Now that fractional horsepower 3 phase variable frequency drives are
    approaching $100 each, the variable slip method is rarely used.
     
  3. Bill Vajk

    Bill Vajk Guest

    Lowering the voltage allows for greater magnetic slippage, but
    also generates a lot more heat and inefficiency. RPM is not
    effectively/efficiently controlled in 3 phase ac motors by
    reducing the voltage. Frequency control is the accepted
    state of the art.
     
  4. Don Kelly

    Don Kelly Guest

    Look at the speed torque curve of an induction motor- Normal operation is
    between the peak torque and synchronous speed (slip =0). The operating point
    is where the load speed torque crosses this curve. If the voltage is reduced
    the peak torque is reduced approximately as the square of the voltage. The
    slope of the speed torque curve becomes lower (slip increases at any given
    torque) so that the intersection between this curve and the load curve
    occurs at a lower speed. This approach provides very limited speed control
    and isn't really very desirable. It is easy to stall if the voltage is too
    low. It is often used with a small single phase fan motor such as on a stove
    vent hood or a forced air flow for a fireplace because it is cheap.
    "Dimmers" designed for this purpose allow setting of a minimum speed limit
    so it is not possible to accidentally stall the motor (they also start the
    motor at full voltage).
     
  5. jimmy

    jimmy Guest

    cheers


    jimmyg
     
  6. evidently you didnt type the search phrase "variable frequency drive" into google.
     
  7. Rob Paisley

    Rob Paisley Guest

    ---------------------

    This method is not a variable voltage although it could be controlled by one.

    From Google: wound rotor motor "3 phase"

    http://www.ecmweb.com/mag/electric_woundrotor_motors_high/

    Rob.
     
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