Synchronous motor is an AC motor which operates at constant speed irrespective of the applied load (whether it is full load or no load) and converts electrical energy into mechanical energy.
In order to understand the construction and working principle of synchronous motor, we must know the two most common terms used to describe the windings of machine.
Field winding applies to that winding which produces main magnetic field in a machine. In case of synchronous motor, field winding is applied on the rotor.
Armature winding applies to the winding where main voltage is induced in the machine. In case of synchronous motor, armature winding is applied on the stator.
Construction of Synchronous Motor:
The two main parts of motor are stator and rotor. Stator is the stationary part which carries the armature winding of motor. Three phase AC supply is given to the stator which produces rotating magnetic field. Rotor is also excited by dc supply which produces fixed magnetic poles (constant magnetic field). Synchronous motor is also known as doubly excited motor since its stator is fed by three phase AC supply while its rotor is excited by separate DC source.
It is the stationary part of synchronous motor. Stator core has a laminated construction and is buildup of steel. Steel is used to reduce hysteresis loss whereas laminations are done to reduce eddy current losses. A three phase armature winding is placed in the slots of stator core(can be seen in the below fig). Stator receives a power from three phase AC supply.
Rotor of the synchronous motor has a set of salient poles which are made up of steel laminations. Field winding is provided on the pole shoe of rotor. Rotor coils are connected in series with the slip rings. Dc current is fed to the rotor coils through slip rings in order to produce alternate N and S poles. The constructional diagram of synchronous motor is shown below.
Working of Synchronous Motor:
When we apply three phase AC supply to the stator coils (windings) which are wound 120degree mechanically apart in a space, they produce rotating magnetic flux. The revolving magnetic field produced rotates at certain speed known as synchronous speed Ns=(120f/p).
Dc current is given via slip rings to the rotor coils produces electromagnet. Hence rotor also acts like a magnet having alternate north and south pole. The below figure will clearly help you to understand that.
Now, let’s try to understand the working of synchronous motor by analyzing the above figure. In the above fig, we have a rotating magnetic field produced due to the excitation of stator having north(Ns) and south (Ss) poles. Rotor also acts like a magnet has north and south pole. By analyzing the fig at the given instant (rotating stator field and rotor field at arbitrary positions) we can say that north Pole of the rotating stator field will attract the south pole of rotor while stator south pole will attract the north pole of the rotor. Since stator field is revolving, it will try to pull the rotor poles along with it by the phenomenon of magnetic attraction. As a result, rotor is also going to rotate along with the stator field.
Since stator magnetic field is rotating at synchronous speed(Ns) and rotor magnets are also rotating with it, hence speed of the rotor will also be Ns. Thus, rotor rotates in synchronism with rotating stator field. This is how synchronous motor works.
However, this is not as simple as that. Since the stator field is rotating, therefore rotor magnetic field will try to lock up with the stator field but it will not happen due to high inertia of rotor and also due to the high speed of rotating stator field. Hence rotor field doesn’t magnetically locked with the stator field due to which motor fails to start. In other words, we can say that the motor has not enough starting torque since it requires a lot of torque to pull out a rotor from stand still position to rated rpm. Thus, synchronous motor is not self-starting.
Synchronous motor runs at constant speed irrespective of the applied load. The speed of rotating stator field is so high that it is unable to rotate the rotor from its initial position due to its (rotor) high inertia. In simple words, we can say that whatever be the starting position of rotor, synchronous motor is not self -starting.
In order to make synchronous motor self starting, the rotor is rotated by some external prime mover to bring it in synchronization with the rotating stator field. For detail, kindly read the following article: How we make synchronous motor self starting?
That’s all. Hope this will helps you.