Synchronous motors usually run at synchronous speed (Ns=120f/P), the synchronous motor speed is constant regardless of the load or 3-phase voltage. It is a machine that operates on synchronous speed and converts electrical energy into mechanical energy, Synchronous motor is an alternator and also operated as a motor.
Construction of Synchronous Motor:
The asynchronous motor has the following construction:
- Stator: It has a 3-phase armature winding in the slots of the stator core and receives power from a 3-phase supply.
- Rotor: Rotor in a synchronous motor is separately excited by a Dc source, the poles may be salient poles or non-salient poles
The stator and rotor have the same number of poles. It is a self-starting and auxiliary method is used for its starting.
- Now suppose the rotor is revolving, if the rotor position is such that N-poles of a rotor exactly rotates as the same manner of as N-poles of stator rotates, then they repel each other and torque produced will be anti-clockwise
- Stator rotates very fast but the rotor does not synchronize with the stator of a motor with the same speed and its direction becomes opposite with that hence the torque produced will be clockwise Since the rotor has to reverse torque so it does not start initially.
If the rotor of a synchronous motor is rotated with the same synchronous speed of stator then the opposite poles of the rotor will attract the poles of the stator and the opposite poles of stator and rotor will get locked with each other and motor will start running.
Synchronous Motor Starting Methods:
Following are the Self-starting Methods
Starting by Damper Winding:
It can become self-starting by providing damping winding in the rotor poles. Damper winding is the short-circuited copper bars inserted in pole faces of rotor poles when 3-phase A.C supply is given to a stator winding, it generates a rotating magnetic field, and this rotating magnetic field cuts the damper winding, an emf is induced and current flows through the damper winding and the motor start running.
Starting by DC Source:
It can be started when it is coupled with D.C supply or D.C compound motor, it is rotated at synchronous speed and then D.C excitation is given, when there is magnetic locking between stator and rotor, the D.C source is cut off as the motor starts running.
Application of Synchronous Motors:
- Synchronous motors which are over-excited can be used to correct the Power factor of plant
- They can be used to improve the voltage regulation of transmission lines.
- They are usually low speeds less than 300 rpm and its Power factor can be set up to unity and has high efficiency.
- Synchronous motors can be used in industries where constant speed motor is required.
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