Single Phase Induction Motor - Working Principle & Operation

Construction of Single Phase Induction Motor :

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     The construction of a single phase induction motor is almost all same to three-phase induction motor. The only difference in the construction is its stator winding is made single-phase supply. The rotor construction is same as squirrel-cage type of three-phase induction motor.

     The stator winding of single-phase induction motor is usually of double-layer type. There is no electrical connection between stator and rotor of a motor. Due to use of single phase winding in the motor, the motor cannot produce rotating field to rotate the rotor, therefore the motor fails to start when the supply is given. Hence a single phase induction is not a self-starting motor. Some changes have to be done to the winding in order make self-starting of the motor. 

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Why single phase induction motor is not a self starting motor?

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     When single phase a.c. supply is given to the single phase stator winding, a sinusoidally pulsating magnetic field varying with time is produced. The field polarity reverses twice in each cycle but the field does not rotate.

    The pulsating ( alternating ) field is called stator field acts along the horizontal axis.This field will induce an e.m.f in the rotor bars due to transformer action. The resultant rotor currents produce a field which opposes the stator field ( as per the lenz's law ). The torque is produced by the reaction of rotor currents with stator field. As seen in Figure the two sets of forces ( forces on the rotor bars in top half are in downward direction,where as the forces on the rotor bars in bottom half are in upward direction ) will cancel each other and the rotor will experience no torque. Therefore, single phase induction motors are not self starting motor.

Single phase induction motor - Working Principle & Operation

Principle of Operation of Single phase induction motor :

     When the single-phase supply is given to the stator windings of a motor. It step-ups a magnetic field which is stationary in any one of the direction. When this magnetic fluxes will rotor bars, an emf will induced ( according to mutual induction ) in the rotor. As the rotor is made electrically conductive and circuited at both ends, this leads to flow of current in it.

     The direction of induced emf and current in the rotor conductors is shown in below figure. As seen from the figure the conductors placed on the upper side of the rotor ( shown by crosses ) will experience a force in downward direction. And the conductors placed in lower side ( shown by dots ) will experience a upward force. As seem that this two forces are in opposite to each other. Hence they will cancel each other and no torque will be produced on the rotor.    

Single phase induction motor - Working Principle & Operation

     As the net torque produced on the rotor is zero, the rotor fails to start. Mean while when a initial rotation is given to the motor in any of the direction. The rotor starts pickuping speed in that direction. Therefore, a starting torque is required to start the motor.

     Several methods are used to make a single-phase induction motor self-starting. The starting methods are used only to start to a motor. Once a motor picks-up it speed the method used can be disconnected or removed. 

     Now, it is interesting to understand how the motor continues to rotate with one single-phase winding on the stator once it is given an initial rotation. How the rotor develops torque when it is given an initial rotation can be explained with the help of any one of the two theories namely : 

(a) Cross field theory, and

(b) Double revolving field theory.

 Single phase induction motors are classified according to the methods of starting used for them.

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