Synchronous Motor: Construction, Working Principle & Applications

A synchronous motor is an AC motor, which is identical to the alternator or synchronous generator. Similar to the dc generator, the synchronous generator can be made to run as a synchronous motor when driven electrically. The synchronous motor converts the electrical energy input into mechanical energy.

In the synchronous motor, the rotation of the rotor (or shaft) is synchronized with the frequency of the supply voltage. As the name suggests, in a synchronous motor, the magnetic field of the rotor and the stator run at synchronous speed.

Construction of synchronous motor

It consists of the stator, which is a stationary part, and the rotor, which is a rotating part. The stator consists of a core and the slots to hold the armature winding similar to the Synchronous generator.

Since it is a 3-phase electrical motor, the three-phase winding is wounded in the armature slots. A three-phase supply is given to the stator winding.

The rotor can be either salient pole type or non-salient pole type. The rotor is wounded with the field winding, which is excited from a dc supply. For a detailed explanation, check the construction of the alternator.

Features of synchronous motor

1. The synchronous motor maintains a constant speed when running, which depends on the supply frequency. The speed can be varied only when the supply frequency is varied, irrespective of load.
2. Synchronous motors are inherently not self-starting. It has to be made to run near the synchronous speed by some external means before it can be synchronized to the supply.
3. The synchronous motors can be operated under a wide range of power factors. Hence it is used for electrical power factor correction purposes.

Working of synchronous motor

The synchronous motors work depending on the interaction between the magnetic field of the rotor and that of the stator. The synchronous motor works on the principle of Magnetic Locking.

When a three-phase supply is fed to the stator winding, a magnetic flux is produced in the stator, which is called a rotating magnetic field. This rotating magnetic field has constant magnitude and rotates at synchronous speed.

The speed of the synchronous motor is given by the formula,

   

where

N_s is the synchronous speed of the rotating magnetic field

f is the frequency of the supply voltage

P is the number of poles in the rotor

Let us consider a two-pole stator as shown below. Both the poles are rotating at synchronous speed in a clockwise direction.

When the rotor winding is excited by a dc supply, a magnetic field is set up in the rotor. The rotor poles are marked as N_R and S_R in the figure shown below.

With the rotor position shown, let the North(N_S) and South(S_S) poles of the stator be at points A and B respectively. As you could observe, the north pole of the stator(N_S) and the North pole of the rotor(N_R) is closer in fig(a). Similarly, the south poles of both rotor and stator and closer to each other. As like poles repel each other, the rotor will try to rotate in an anti-clockwise direction.

But half a period later, the poles in the stator rotate and interchange their positions, that is N_S will be at point B and S_S will be at point A. It is shown in fig(b). Under this condition, N_S will attract S_R and S_S will attract N_R. Now, the rotor will tend to rotate in a clockwise direction. (It is just the reverse of the first direction.)

Hence, due to continuous and rapid rotation of stator poles, the rotor will be subjected to a torque that is rapidly reversing. Owing to the large inertia of the rotor, it cannot respond to the rapidly reversing torque and thus remains stationary. Hence the synchronous motor is not a self-starting motor.

Now, instead of being stationary, consider the rotor is rotating in the clockwise direction. Since the rotating magnetic field produced by the stator winding rotates at the synchronous speed, the rotor is made to rotate near the synchronous speed with some external means.

When the rotor reaches near the synchronous speed, the dc excitation is turned on. When the opposite poles of the rotor and stator come near, they attract each other. This establishes magnetic locking and hence the motor continues to rotate with unidirectional torque.

Starting of synchronous motor

From the working of the synchronous motor, it is clear that the motor will not start by itself. So How to start?

In order to start, the synchronous motor is mechanically coupled to either a three-phase induction motor or a direct current shunt motor. Initially, the DC excitation is not given to the rotor winding.

The rotor is speeded up to synchronous/near synchronous speed by the external prime mover and then excited by the dc source. At this moment, the rotor gets magnetically locked into position with the stator. Since the rotor poles get engaged with the stator poles and both run in the same direction synchronously.

Because of this interlocking of stator and rotor poles, the motor should either run synchronously or not at all. When magnetic locking occurs, the power to the external motor is cut off after a short period of time.

Applications of synchronous motor

Synchronous motors find extensive application in power factor improvement, constant speed motor operation, and balancing voltage regulation.

1. An overexcited synchronous motor having no load connected to its shaft has a leading power factor. It is widely used for power factor improvement. Especially in the case of induction motor, which offers a lagging power factor.
2. High-speed synchronous motors(above 600 rpm) are well suited for loads where constant speed is required. Examples include centrifugal pumps, blowers, compressors, rubber and paper mills, etc.
3. Low-speed synchronous motors(below 600 rpm) are used for drives such as centrifugal pumps, ball and tube mills, vacuum pumps, rolling mills, etc.
4. When large inductive loads are present, the voltage at the end of a long transmission line varies greatly. Because of the line capacitance, when an inductive load is abruptly disconnected, voltage tends to rise significantly above its normal value. This voltage rise can be controlled by connecting a synchronous motor along with a field regulator.

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