Why permanent magnets are not used in Synchronous Motor?

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Discussion Overview

The discussion revolves around the use of permanent magnets in synchronous motors and generators, exploring the efficiency of induction motors versus permanent magnet motors, and the operational principles of synchronous motors in power generation. The scope includes theoretical and conceptual aspects of motor design and functionality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why permanent magnets are not utilized in the stator or rotor of synchronous motors, suggesting that induction motors may be more efficient than those using permanent magnets.
  • One participant explains that synchronous motors can operate at various power factors through field excitation, which is not compatible with permanent magnets, although permanent magnet synchronous motors do exist.
  • There is a query about how synchronous motors generate power, particularly regarding the absence of a magnetic source in the motor's design.
  • A participant highlights the impracticality of using large permanent magnets for the generator rotor compared to electromagnets, raising concerns about size and weight.
  • Another participant clarifies that synchronous generators require external power for their field windings, but this power is minimal compared to the mechanical power supplied by a turbine.

Areas of Agreement / Disagreement

Participants express varying views on the role of permanent magnets in synchronous motors and generators, with no consensus reached on the efficiency comparison between induction motors and permanent magnet motors. The operational principles of synchronous motors also remain a point of inquiry without definitive resolution.

Contextual Notes

Participants mention the need for field excitation in synchronous motors and generators, but do not fully resolve the implications of this requirement on the use of permanent magnets. The discussion includes assumptions about the efficiency and functionality of different motor types that are not explicitly defined.

Who May Find This Useful

This discussion may be of interest to students and professionals in electrical engineering, particularly those focused on motor design, power generation, and the comparative analysis of motor efficiencies.

Akmalidin
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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?
 
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Akmalidin said:
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?
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.
 
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 don't see the magnetic source in synchronous motors.So how does is really work?
 
@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?

maxresdefault.jpg
 
Thank you for demonstration.
 
So Synchronous motors are given external power to generate out power ?
 
Akmalidin said:
So Synchronous motors are given external power to generate out power ?

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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