- Definition of Synchronous Motors: Synchronous motors are electric motors that rotate at a speed directly proportional to the frequency of the supply current.
- Non Excited Synchronous Motors: These motors use external magnetic fields to magnetize a steel rotor, achieving synchronization without additional electrical excitation.
- Hysteresis and Reluctance Motors: Types of non-excited synchronous motors that use different principles (hysteresis losses and magnetic reluctance) to achieve and maintain synchronous speed.
- Permanent Magnet Synchronous Motors: These motors use permanent magnets in the rotor to maintain a constant magnetic flux and require a variable frequency stator drive for starting.
- Current Excited Synchronous Motors: These motors need a DC supply to the rotor windings to generate the magnetic field and often use damper windings to start as induction motors before reaching synchronous speed.
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 in non-excited synchronous motors is made of steel. An external magnetic field magnetizes the rotor, causing it to rotate in sync with the field. Typically, the rotor is made from high retentivity steel, like cobalt steel.
Non-excited motors are available in three designs:
Hysteresis Motor
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.
Reluctance Motor
The reluctance motor operates on the principle that iron moves to complete a magnetic flux path with minimum reluctance. Like hysteresis motors, it has main and auxiliary windings to create a rotating magnetic field. The rotor is a squirrel cage rotor with some teeth removed to form salient poles. The rotor aligns with the stator’s magnetic field to minimize reluctance.
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 require a DC supply to the rotor to create the magnetic field. These motors have both stator and rotor windings and can feature cylindrical or salient pole rotors. They are not self-starting, so they use damper windings to start as induction motors before reaching synchronous speed.
