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.
The stator frame is the outer part of the machine and is made up of cast iron. It protects the enter inner parts of the machine.
The stator core is made up of thin silicon laminations. It is insulated by a surface coating to minimize hysteresis and eddy current losses. Its main purpose is to provide a path of low reluctance for the magnetic lines of force and accommodate the stator windings.
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 forgings of high-grade nickel chrome molybdenum steel forgings of high-grade nickel chrome molybdenum steel. The poles are created by the current flowing through the windings. They are used for high-speed applications as they have less number of poles. They also produce less noise and windage losses as they have a uniform air gap. DC supply is given to the rotor windings via slip-rings. Once the rotor windings are excited, they act like poles.