There are different types of synchronous motors based on the way they are excited
- Non Excited Synchronous Motors
- Current Excited Synchronous Motors
Let us take a look at the different types one by one.
Non Excited Synchronous Motor
The 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 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.
The combination of the two produces a revolving magnetic field from a single phase supply. They are self-starting and do not need additional windings. When single phase AC supply is given, a rotating magnetic field is produced. This rotating magnetic field induces eddy currents in the rotor. The rotor starts to move initially with a slip. When the rotor reaches synchronous speed, the stator pulls the rotor into synchronism. So initially the motor starts as an induction motor and later runs as a synchronous motor.
The 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 rotor 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 Motors
The 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 Motor
Direct 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.