Page 144 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Now, consider the condition shown in Fig 2 (a). The stator and rotor poles are attracting each other. Suppose
that the rotor is not stationary, but is rotating clockwise, with such a speed that it turns through one pole-pitch by
the time the stator poles interchange their positions, as shown in Fig 2 (b). Here, again the stator and rotor poles
attract each other. It means that if the rotor poles also shift their positions along with the stator poles, then they will
continuously experience a unidirectional torque i.e., clockwise torque, as shown in Fig 2. The rotor (which is as
yet unexcited) is speeded up to synchronous / near synchronous speed by some arrangement and then excited
by the DC supply.

Advantages and disadvantages
Advantages
1 It is a constant speed motor and its speed remains constant from no load to full load.
2 By varying the field excitation, the power factor can be adjusted as desired.
Disadvantages
1 These motors cannot be used for variable mechanical load.

2 As this motor is not self-starting type, it cannot be started on load.
3 To operate this motor its field system requires DC supply.
4 This motor cannot be used for variable speed applications.
5 Hunting is produced in this motor.
Method of starting: As the synchronous motors are not self-starting, they have to be rotated at the time of starting
to their synchronous speed / near synchronous speed. A synchronous motor can be started by following methods:
1 By Pony motor method.
2 As an induction motor.
Starting by pony motor method: In this method, another small induction motor (known as pony motor) having a
pair of poles less than that of the synchronous motor to be started (or an oil engine) is mechanically coupled with
the shaft of the synchronous motor. This small pony induction motor is first started from AC supply mains and then
DC supply from the exciter of the motor is applied to the field of the synchronous motor. AC supply is not provided
to the armature (AC stator) of the synchronous motor at this time.

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