Page 143 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Synchronous motor

LObjectives: At the end of this lesson you shall be able to:
• explain the working principle of synchronous motor
• explain the constructional details of synchronous motor
• state the different methods of starting a synchronous motor
• state the application of synchronous motor.

Introduction
A synchronous motor is a machine which converts electrical energy into mechanical energy while rotating at a
constant speed equal to synchronous speed. The synchronous generators can operate either as generators or
as motors. When operating as motors – by connecting them to three phase source – they are called synchronous
motors. As the name implies, synchronous motors run in synchronism with the revolving magnetic field. The
speed of rotor is therefore tied to the frequency of source. As the frequency is fixed, the motor speed stays
constant, irrespective of load or voltage of the three phase line. However, synchronous motors are not used so
much because they run at constant speed.
Construction
Synchronous motors are identical in construction to salient pole AC generators. The stator is composed of a
slotted magnetic core, which carries three phase winding. Consequently, the winding is also identical to that of
three phase induction motors.
The rotor has a set of salient poles that are excited by a DC current. The exciting coils are connected in series to
two slip rings and the DC current is fed into the winding from an external exciter.
In some synchronous motors slots are also punched along the circumference of salient poles. They carry a
squirrel cage winding similar to that in three phase induction motors. This damping winding serves to start the
motor.
Modern synchronous motors often employ brushless excitation similar to that used in synchronous generators.
A relatively small three phase generator called exciter, and a three phase rectifier are mounted at one end of the
motor shaft. The DC current from rectifier is fed directly into salient pole windings without going through brushes
and slip rings. The current can be varied by controlling the small exciting current, that flows in stationary field
winding of the exciter.
The rotor and stator always have the same number of poles. As in case of induction motor, the number of poles
determines the synchronous speed of the motor.
Working principle
When a three phase winding is fed by a 3 phase supply, then a magnetic flux of constant magnitude but rotating
at synchronous speed, is produced. Consider a two-pole stator of Fig 1, in which are shown two stator poles
(marked N and S ) rotating at synchronous speed, say, in clockwise direction. With the rotor position as shown,
S
S
suppose the stator poles are at that instant situated at points A and B. The two similar poles, N (of rotor) and N
S
(of stator) as well as S and S will repel each other, with the result that the rotor tends to rotate in the anticlockwise
S
direction.
But half a period later, stator poles, having rotated around, interchange their positions i.e. N is at point B and S
S
S
at point A. Under these conditions, N attracts S and S attracts N. Hence, rotor tends to rotate clockwise (which is
S
S
just the reverse of the first direction). Hence, we find that due to continuous and rapid rotation of stator poles, the
rotor is subjected to a torque which is rapidly reversing i.e., in quick succession, the rotor is subjected to torque
which tends to move it first in one direction and then in the opposite direction. Owing to its large inertia, the rotor
cannot instantaneously respond to such quickly-reversing torque, with the result that it remains stationary.

130

CITS : Power - Electrician & Wireman - Lesson 76-85