Double Revolving Field Theory

   

     When single phase a.c. supply is given to the single phase stator winding, a sinusoidally pulsating magnetic field varying with time is produced. As the produced magnetic very two times in a cycle therefore, no torque will be produced on the rotor and hence rotor doesn't rotate. Therefore, single phase induction motors are not self starting motor.


     However the when an initial rotation is given to the rotor, it starts rotating in that direction which can be explained by the double revolving field theory.


     The magnetic field produced by the stator winding when an alternating supply is given is equal to the sum of the two revolving fields rotating at synchronous speed in opposite direction of equal magnitude. The magnitude of the each revolving field is equal to one half of the maximum value of the alternating field i.e.,  φ1m / 2  where φ1m is the maximum value of alternating field.


     Let us consider the two revolving fields as φf ( rotating in anti-clock wise direction ) and φ( rotating in clockwise direction ). The resultant φR of this two fields gives the value of magnetic field produced by the alternating supply ( i.e., alternating field ).



Consider the different instances as shown below :  


(i). The two fields φf and φb are shown opposite to each other at start, and resultant magnetic field φR = 0

Double Revolving Field Theory

(ii). After 90°, the two fields are rotated in such a way that both of them are now pointing in the same direction.  

The resultant magnetic field,

Double Revolving Field Theory

This instant gives maximum magnitude of original alternating field.


     So the continuous rotation of these two fields ( components ) gives the original stator magnetic field. This is purely alternating in nature.


     Now each separate component is rotating and hence get cut by the rotor conductors. Due to cutting of flux emf gets induced in rotor conductors which circulates the rotor current. The rotor current produces rotor flux.


     The rotor flux interacts with one component φf produces a torque in anti-clockwise direction and the rotor flux interacts with second component φb produces a torque in clockwise direction.

 
     If anticlockwise torque is assumed positive then clockwise torque produced by another is negative. The resultant of the two torques at start is zero. 

Double Revolving Field Theory


Following are important points about the resultant magnetic field :  


(i). At start the two torques are equal in magnitude but opposite in direction. This two torques tries to rotate the rotor in different directions. Hence net torque experienced by the rotor is zero. Therefore it is said that single phase induction motors are not self starting. 

(ii). When the rotor is given an initial rotation in any direction, the resultant average torque increases in the same direction and motor starts giving the performance similar to the 3-phase induction motor.



Also Check Out -

 1. Cross-field Theory of Single Phase Induction Motor 

 2. Why Single Phase Induction Motor is Not Self-Starting Motor  



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