- 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 how the rotor field is produced. The two groups are non-excited rotors and current-excited rotors.
- Non Excited Synchronous Motors
- Current Excited Synchronous Motors
Non-excited machines use steel, hysteresis material or permanent magnets. Current-excited machines use a DC field winding on the rotor.
Non Excited Synchronous Motor
The rotor in non-excited synchronous motors is steel or a hard magnetic alloy, with no DC field winding. A stator magnetic field magnetizes a hysteresis or reluctance rotor so it can lock in step. Hysteresis rotors often use high-retentivity alloys such as chrome steel, cobalt steel or alnico.
Non-excited motors are available in three designs:
Hysteresis Motor
Hysteresis motors are often single phase motors in which the rotor is made up of ferromagnetic material. The rotors are cylindrical and use high-hysteresis alloys. Typical materials are 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. The rotor has no extra start winding. 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, hysteresis torque holds it in step. 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 turns to the lowest-reluctance position in the stator magnetic field.
When single phase AC supply is given, the motor starts as an induction motor. Reluctance torque tries to align the rotor with the stator field. Inertia carries it past that alignment, so it keeps turning. On some single-phase designs the auxiliary circuit is opened near 75% of synchronous speed. Other machines keep a permanent split capacitor. Near synchronous speed, reluctance torque pulls the rotor into step and holds it there.
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. Many inverter-fed PMSMs are not line-start and need an electronically controlled variable-frequency stator drive. A line-start PMSM with a cage can pull in from the supply.
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 line-start as synchronous machines, so they use damper windings to start as induction motors before reaching synchronous speed.
