What Is a Synchronous Motor?
A synchronous motor is a motor in which there is no slip between the rotating magnetic field and the rotor. The rotation speed is determined by the number of poles in the motor itself and the frequency of the power supply.
Since there is no need to take the slip into consideration, it can be differentiated from ordinary AC motors. In terms of classification, it is treated as one of the induction motors. Synchronous motors require a DC power supply to the rotor in addition to the AC power supply.
Uses of Synchronous Motors
Synchronous motors are used in rolling mills in the steel industry because their speed does not depend on the load side.
The speed of synchronous motors is controlled by a resolver or other rotary position detector attached to the anti-load side of the motor. Originally, synchronous motors were used with ordinary AC motors because of their difficulty in speed control.
Principle of Synchronous Motors
In a synchronous motor, a rotating magnetic field is generated by the frequency of the AC power flowing through the stator, and the rotating speed of the rotor is determined by this rotating magnetic field (the rotating magnetic field and the rotor speed are the same).
On the rotor side, the DC power supplied through the brushes and slip rings flows to the coil wound on a piece of iron, which acts as an electromagnet. This piece of iron has a polarity (N-pole or S-pole), which changes depending on the position of the commutator. The rotor rotates when the polarity of this polarity and the polarity of the rotating magnetic field generated on the stator side repel (same polarity) or attract (different polarity) each other.
Rotor rotation speed N can be expressed as 120F/P. Unlike normal AC motors, no slip occurs between the stator and the rotor at this time. In order to supply DC power to the rotor side, a slip ring and brushes must be provided. The brushes are consumable parts and must be replaced periodically.