An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. In case of three phase AC operation, most widely used motor is three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.
A 3 phase induction motor derives its name from the fact that the rotor current is induced by the magnetic feild, instead of electrical connections. the operating principle of a 3 phase induction motor is based on the production of r.m.f.
working principle of a 3phase induction motor
The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation. The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2,4,6 or 8 poles. The synchronous speed a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.
Synchronous speed of rotation=(120* supply frequency)/ number of poles on the stator.
Thus the Three Phase Induction Motor is :
1) self starting
2) robust in construction
3) easier to maintain
4) less armature reaction and brush sparking because of the absence of commutatores and brushes that may cause sparks,
ques 1)why the exciting current of induction motor is so high as compared to the power transformer?
ans- unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current of the induction motor(30 to 40% of full load current)is much higher than that of the power transformer.
ques 2) How transformation ratio of induction motor is differnt as compared to power transformer?
ans- In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the tranformer ratio.
ques 3) How slip ring motors are started?
Slip ring motors are invariably started by rotor resistance starting.
ques 4) What is the effect of starting of induction motor on connected line?
ans- Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and neccesary to reduce the magnetic of stator current at starting and several methods are available for tjis purpose.