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Why permanent magnets are not used in Synchronous Motor?

  1. Jun 20, 2016 #1
    I am Telecommunication student but interested in motors.Especially, power generators.I was wondering why permanent magnets are not used in synchronous motor's stator or rotor? Is induction motor more efficient than permanent magnet used motor?
    What about in power plant generators, do they use permanent to make changing magnetic field?
  2. jcsd
  3. Jun 20, 2016 #2


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    Synchronous motor can be operted at various power factors by changing its field excitation. This is useful when it is to be used as a synchronous condenser for power factor correction. Hence, such synchronous motors can't operate on permanent magnets. But permanent magnet synchronous motors exist.
    In power plants, synchronous generators are used which are mostly fixed-armature rotating-field type. Rotor (field) is a rotating electromagnet. Field excitation of the alternator needs to be changed frequently (which is done using AVR) in order to keep the terminal voltage constant and to control the reactive power flow. Hence, permanent magnets are not used in power plant generators.
  4. Jun 20, 2016 #3
    Thank you for your answer. But how synchronous motor can produce power? if stator and rotor are made of coils windings?
    As far as I know power is generated through changing magnetic field.But I dont see the magnetic source in synchronous motors.So how does is really work?
  5. Jun 20, 2016 #4


    Staff: Mentor

    @cnh1995 gave you good answers. Here's another.

    Look at the generator rotor in the picture below. It is almost all field winding to make the electromagnet. Can you imagine making a permanent magnet that size? How would its size and weight compare with the electromagnet of the same field strength?

  6. Jun 20, 2016 #5
    Thank you for demonstration.
  7. Jun 20, 2016 #6
    So Synchronous motors are given external power to generate out power ?
  8. Jun 20, 2016 #7


    Staff: Mentor

    Your have the right idea, but the wrong words. Synchronous generators use external sources to power their field windings. But the field winding power is tiny compared to the mechanical power used to spin the rotor. Usually, a turbine provides that mechanical power.
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