Why permanent magnets are not used in Synchronous Motor?

In summary, the conversation discusses the use of permanent magnets in synchronous motors and power plant generators. It is mentioned that permanent magnets cannot be used in synchronous motors as they need to be able to change their power factor, but they do exist in permanent magnet synchronous motors. In power plants, synchronous generators use rotating electromagnets for their field, which are controlled by an external power source. The conversation also touches on how synchronous motors generate power through changing magnetic fields.
  • #1
Akmalidin
69
6
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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  • #2
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.
 
  • #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 don't see the magnetic source in synchronous motors.So how does is really work?
 
  • #4
@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
 
  • #5
Thank you for demonstration.
 
  • #6
So Synchronous motors are given external power to generate out power ?
 
  • #7
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.
 

1. Why can't permanent magnets be used in synchronous motors?

Permanent magnets have a fixed magnetic field, which means they cannot be controlled or adjusted. This makes them unsuitable for use in synchronous motors, which require precise control of the magnetic field in order to maintain synchronization with the alternating current.

2. What is the main disadvantage of using permanent magnets in synchronous motors?

The main disadvantage of using permanent magnets in synchronous motors is that the magnetic field strength cannot be altered. This means that the motor's speed cannot be adjusted, as it is directly tied to the frequency of the alternating current.

3. Can permanent magnets be used in synchronous motors at all?

Yes, permanent magnets can be used in synchronous motors in certain applications, such as low power, low speed applications. However, for most industrial and high-power applications, permanent magnets are not used due to their limitations in speed and torque control.

4. How do synchronous motors without permanent magnets work?

Synchronous motors without permanent magnets use electromagnets instead. These electromagnets can be controlled by adjusting the amount of current flowing through them, allowing for precise control of the magnetic field and the motor's speed.

5. Are there any advantages to using permanent magnets in synchronous motors?

Yes, there are some advantages to using permanent magnets in synchronous motors, such as their simplicity and reliability. They also do not require a separate power source for the magnetic field, as electromagnets do. However, for most industrial and high-power applications, the disadvantages outweigh these advantages, leading to the widespread use of electromagnets in synchronous motors.

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