- Synchronous Motor Definition: A synchronous motor is defined as a motor that runs at synchronous speed, determined by supply frequency and the number of poles.
- Stator Components: The stator includes a frame made of cast iron, a core of silicon laminations, and windings of enamelled copper.
- Rotor Types: The two types of rotors are salient pole, used for lower speeds, and cylindrical, used for higher speeds.
- Salient Pole Rotor: This rotor type has projecting poles and is suitable for medium and low-speed operations, featuring damper windings.
- Cylindrical Rotor: Made from solid steel, this rotor type is ideal for high-speed applications with a uniform air gap and DC excitation via slip-rings.
Synchronous motors run at synchronous speed. The synchronous speed is given by
Where, Ns = synchronous speed, f = supply frequency and p = number of poles. As we can see from the equation, the synchronous speed depends on the frequency of the supply and the number of poles.
The construction of a synchronous motor is very similar to the construction of an alternator. Both are synchronous machines where one we use as a motor and the other as a generator. Just like any other motor, the synchronous motor also has a stator and a rotor. We will look into the construction details of the various parts one by one in detail.
Stator of Synchronous Motor
The main stationary part of the machine is stator. The stator consists of the following parts.
Stator Frame
The stator frame is the outer part of the motor, made from cast iron. It protects all the internal components of the motor.
Stator Core
The stator core is made from thin silicon laminations with an insulating surface coating. This reduces hysteresis and eddy current losses. Its main role is to provide an easy path for magnetic lines and to hold the stator windings.
Stator Winding
The stator core has cuts on the inner periphery to accommodate the stator windings. The stator windings could be either three-phase windings or single phase windings.
Enamelled copper is used as the winding material. In the case of 3 phase windings, the windings are distributed over several slots. This is done to produce a sinusoidal distribution of EMF.
Rotor of Synchronous Motor
The rotor is the moving part of the machine. Rotors are available in two types:
- Salient Pole Type
- Cylindrical Rotor Type
The salient pole type rotor consists of poles projecting out from the rotor surface. It is made up of steel laminations to reduce eddy current losses. 
A salient pole machine has a non-uniform air gap. The gap is maximum between the poles and is minimum at the pole centres. They are generally used for medium and low-speed operations as they have a large number of poles. They contain damper windings which are used for starting the motor.
A cylindrical rotor is made from solid high-grade steel, specifically nickel chrome molybdenum. The poles are formed by the current in the windings. These rotors are used in high-speed applications because they have fewer poles and produce less noise and windage losses due to their uniform air gap. DC supply is provided to the rotor windings through slip-rings, making them act like poles when excited.
