Types of Synchronous MotorPublished on 24/2/2012 & updated on Friday 11th of May 2018 at 06:23:53 PM
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- Non Excited Synchronous Motors
- Current Excited Synchronous Motors
Non Excited Synchronous MotorThe rotor is made up of steel. The external magnetic field magnetises the rotor, and it rotates in synchronism with it. The rotor is generally made of high retentivity steel such as cobalt steel.
Non-excited motors are available in three designs:
Hysteresis MotorHysteresis motors are single phase motors in which the rotor is made up of ferromagnetic material. The rotors are cylindrical in shape and have high hysteresis loss property. They are generally made up of chrome, cobalt steel or alnico. The stator is fed by single phase AC supply. The stator has two windings:
- main windings and
- auxiliary windings.
Reluctance MotorThe reluctance motor is based on the principle that an unrestrained piece of iron will move to complete a magnetic flux path where the reluctance is minimum. The stator has the main winding and the auxiliary windings just like the hysteresis motor. These help to create a rotating magnetic field. The rotor of a reluctance motor is a squirrel cage with some teeth removed to provide the desired number of salient poles. The reluctance becomes minimum when the rotor is aligned with the magnetic field of the stator.
When single phase AC supply is given, the motor starts as an induction motor. The rotor tries to align itself with the magnetic field of the stator and experiences reluctance torque. But due to inertia, it exceeds the position and again tries to align itself during the next revolution. In this manner, it starts to rotate. Once it reaches 75% of synchronous speed, the auxiliary windings are cut off. When the speed reaches synchronous speed, the reluctance torque pulls it into synchronism. The motor remains in synchronism due to synchronous reluctance torque.
Permanent Magnet Synchronous MotorsThe rotor is made up of permanent magnets. They create a constant magnetic flux. The rotor locks in synchronism when the speed is near synchronous speed. They are not self-starting and need electronically controlled variable frequency stator drive.
Direct Current Excited MotorDirect current excited synchronous motors need a DC supply to the rotor to generate rotor magnetic field. A direct current excited motor has both stator windings as well as rotor windings. They can either have cylindrical rotors or salient pole rotors. They are not self-starting and need damper windings to start. Initially, they start as an induction motor and later attains synchronous speed.
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