A three-phase synchronous machine which converts three-phase electrical energy into mechanical energy is called a three-phase synchronous motor.

A three-phase synchronous motor is a constant speed machine, and it runs at the synchronous speed. The synchronous speed of a three-phase synchronous motor is given by,

$$mathrm{mathit{N_{s}}:=:frac{120mathit{f}}{mathit{p}}cdot cdot cdot (1)}$$

Where,f is the supply frequency andP is the number of field poles in the motor.

Like any other electric motor, a 3-phase synchronous motor also consists of two main parts namely stator and rotor. The stator houses a 3-phase armature winding and receives power from a 3-phase ac supply source. The rotor is a rotating part and carries field winding which is excited from an external source of DC power.

The most important drawback of a synchronous motor is that it is not self-starting and therefore an auxiliary mean has to be used for starting it.

Working of 3-Phase Synchronous Motor

Consider a three-phase synchronous motor having a salient-pole type rotor of two poles namely $mathit{N_{mathrm{2}}}$ and $mathit{S_{mathrm{2}}}$. Thus, the stator will also be wound for two poles which are $mathit{N_{mathrm{1}}}$ and $mathit{S_{mathrm{1}}}$. A direct voltage is applied to the rotor winding and a balanced three-phase ac voltage to the stator winding.

The stator winding produces a rotating magnetic field which revolves around the stator at a speed called synchronous speed ($mathit{N_{mathit{s}}}$).The direct current flowing through the rotor winding produces two field poles in the rotor and the magnetic field due to these poles is stationary so long as the rotor is not running. Hence, in this case we have a pair of revolving armature poles $left ( mathit{N_{mathrm{1}}}-mathit{S_{mathrm{1}}} right )$ and a pair of stationary rotor poles $left ( mathit{N_{mathrm{2}}}-mathit{S_{mathrm{2}}} right )$.

Now, consider an instant at which the stator poles are at positions A and B as shown in Figure-1. It is clear that poles $mathit{N_{mathrm{1}}}$ and $mathit{N_{mathrm{2}}}$ repel each other and so do the poles $mathit{S_{mathrm{1}}}$ and $mathit{S_{mathrm{1}}}$. Thus, the rotor tends to rotate in the anticlockwise direction. After a period of half-cycle of AC supply, the polarities of stator poles are reversed, but the polarities of the rotor poles remain the same as shown in Figure-2. In this situation, poles $mathit{S_{mathrm{1}}}$ and $mathit{N_{mathrm{2}}}$ attract each other and so do the poles $mathit{N_{mathrm{1}}}$ and $mathit{S_{mathrm{2}}}$. Thus, the rotor now tends to rotate in the clockwise direction.

Since, the stator poles are changing their polarities rapidly, they tend to pull the rotor first in one direction and after half-cycle of ac in the other direction. Because of the bidirectional torque on the rotor and high inertia of the rotor, the synchronous motor fails to start. Therefore, a synchronous motor has no self-starting torque.

Making a Synchronous Motor Self-Starting

A synchronous motor cannot start by itself. To make the motor self-starting, a squirrel-cage winding, called damper winding, is provided on the rotor. The damper winding consists of copper bars embedded in the slots cut on in the pole faces of the salient poles of the rotor, as shown in Figure-3.

These damper windings serve to start the synchronous motor by itself, which is explained below −

• Initially, a 3-phase supply is fed to the stator winding while the rotor winding is left open. The rotating magnetic field of the stator winding induces currents in the damper windings, and due to electromagnetic forces, the rotor starts moving. Thus, the synchronous motor is started as an induction motor.

• Once the motor attains a speed nearly equal to the synchronous speed, the rotor winding is excited from a source of dc supply. Now, the resulting poles on the rotor face the stators pole of opposite polarity, and a strong magnetic attraction is set up between them. Thus, the rotor poles are locked with the rotating poles of the stator. Consequently, the rotor revolves at the same speed of the stator poles, i.e. synchronous speed.

• Since the rotor is now rotating at the same speed as the stator field, the damper bars do not cut any flux, hence have no induced currents in them. Thus, the damper windings of the rotor are, in effect, removed from the operation of the motor.

• In this way, a synchronous motor is made self-starting. It must be noted that due to magnetic interlocking between the stator and rotor poles, the synchronous motor can only run at synchronous speed.