Synchronous Motor

SYNCHRONOUS MOTOR

Basically a DC generator can be run as a d.c motor. Similarly, an alternator may operate as a motor connecting its armature winding to a 3 phase supply. It is then called a synchronous motor.

Characteristics of Synchronous Motor

Some characteristic features of a synchronous motors are given below:

1. It runs either at synchronous speed or not at all i.e., while running, it maintains a constant speed.

2. It is not inherently self starting. It has to be run upto synchronous speed by some means before it can be synchronised to supply.

3. It is capable of being operated under a wide range of power factors both lagging and leading. Hence, it can be used for power correction purpose in addition to supplying torque to drive loads.

Construction of Synchronous Motor

A synchronous motor is a machine that operates at synchronous speed and converts electrical energy into mechanical energy. A synchronous motor has the following two parts.

1. Stator

A stator which houses 3 phase armature winding in the slots of the laminated stator core and receives power from a 3‒phase supply. This construction is similar to those of a 3 phase squirrel cage or a wound rotor induction motor.

2. Rotor

The rotor is generally a salient pole rotor. The number of rotor field poles must equal the number of stator poles. In order to eliminate hunting and to develop the necessary starting torque when A.C voltage is applied to the stator. The rotor poles contain pole‒face conductors which are short‒circuited at their ends. The salient pole rotor excited by direct current to form alternate N and S poles. The exciting coils are connected in series to two slip rings exciter mounted r mounted on the rotor shaft.

Principle of Operation of Synchronous Motor

The fact that a synchronous motor has no starting torque can be easily explained.

Consider a 3‒phase synchronous motor having two rotor poles N_R and S_R. Then the stator also having two poles N_S and S_S. The motor has direct voltage applied to the rotor winding and a 3‒phase ac voltage applied to the stator winding. The stator winding produces a rotating magnetic field which revolves round the stator at synchronous speed N_S. The direct current is applied to a two pole field which is stationary so long as the rotor is not running. Thus we have a situation in which there exists a pair of revolving armature poles. (ie, N_S→S_S) and a pair of stationary rotor poles (ie, N_R→S_R)

Suppose at any instant, the stator poles are at position A and B as shown in Fig. 4.20 (a). It is clear that poles N_S and N_R repel each other and so do the poles S_S and N_S. Therefore the rotor tends to move in anti‒clockwise direction.

After a period of half‒cycle (1/2 f), the polarities of the stator poles are reversed but the polarities of the rotor poles remain the same as shown in Fig. 4.20 (b). Now S_S and N_R attract each other similarly N_S and S_R attract each other. Therefore, the rotor tends to move in the clock‒wise direction. Since the stator pole change their polarities rapidly, they tend to pull the rotor first in one direction and then after a period of half cycle in the other. Due to high inertia of the rotor, the motor fails to start.

Hence, a synchronous motor has no self‒starting torque ie, a synchronous motor cannot start by itself.

Making Synchronous Motor Self Starting

1. Using the damper windings as a squirrel cage induction motor

(i) A synchronous motor cannot start by itself. In order to make the motor self starting, a squirrel cage winding (also called damper winding) is provided on the rotor. The damper winding consists of copper bars embedded on the pole faces of the salient poles of the rotor as shown in Fig. 4.21.

The bars are short circuited at the ends of end ring arrangement to form in effect a partial squirrel cage winding. The damper winding serves to start the motor.

(ii) To start with, 3‒phase A.C supply is given to the stator winding while the rotor field winding is left unenergised. The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as a induction motor.

(iii) As the motor approaches the synchronous speed, the rotor is excited with direct current. Now the resulting poles on the rotor face poles of opposite polarity on the stator and a strong magnetic attraction is set up between them. The rotor poles lock with the poles of rotating flux. Consequently, the rotor revolves at the same speed as the stator field ie, at synchronous speed.

Equivalent Circuit of Synchronous Motor

Basically, the synchronous motor is connected to two electrical supply; a DC supply at the rotor terminals for excitation purpose and an ac supply at the stator terminals.

1. Under normal conditions, no voltage is induced in the rotor terminals by the stator field because the rotor winding is rotating at the same speed as the stator field.

2. In the stator winding, two effects are to be considered.

(i) The effect of stator field on the stator conductors is accounted for by including an inductive reactance in the armature winding. This is called synchronous reactance X_S. A resistance R_a is connected in series with synchronous reactance to account for the copper losses in the stator or armature winding.

(ii) The second effect is that a voltage is generated in the stator. winding by the synchronously revolving field of the rotor. This generated emf E_b is known as back emf. It opposes the stator voltage 'V'. The magnitude of EMF E_b depends upon the rotor speed and rotor flux ϕ per pole. The equivalent circuit of synchronous motor as shown in Fig. 4.22 and 4.23.

Synchronous Capacitors

An over excited synchronous motor running on no load is called as synchronous capacitor.

We have seen that, a synchronous motor takes a leading current when over excited and, therefore, behaves as capacitor. When a synchronous machines is connected in parallel with any induction motors that operate at lagging powerfactor, the leading KVAR supplied by the synchronous capacitor reduces the lagging reactive KVAR of the loads. Consequently, powerfactor of the entire system is improved.

Applications of Synchronous Motor

1. Over excited synchronous motors can be used to improve the powerfactor of a plant and other devices having lagging p.f such as welders and fluorescent lights etc.,

2. Low speed synchronous motors are used for drives such as centrifugal and screw‒type pumps, ball and tube mills, vaccum pumps, chippers and metal rolling mills etc.,

3. They are used to improve the voltage regulation of transmission lines.

Comparison between Synchronous Motor and Induction Motor