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Why Single Phase Induction Motor is not Self Starting ?

Unlike three phase induction motors, single phase induction motors are not self starting. The reason behind this is very interesting. 

Single phase induction motor has distributed stator winding and a squirrel-cage rotorWhen fed from a single-phase supply, its stator winding produces a flux ( or field ) which is only alternating i.e. one which alternates along one space axis only. It is not a synchronously revolving ( or rotating )  flux as in the case of a two or a three phase stator winding fed from a 2  of 3 phase supply. Now, an alternating or pulsating flux acting on a stationary squirrel-cage rotor cannot produce rotation (only a revolving flux can produce rotation ). That is why a single phase motor is not self-starting.

  • However, if the rotor of such a machine is given an initial start by hand (or small motor) or otherwise in either direction, then immediately a torque arises and the motor accelerates to its final speed (unless the applied torque is too high). 

This peculiar behaviour of the motor has been explained in two ways (i) by two-field or double-field revolving theory and (it) by cross-field theory. Only the double field revolving theory will be discussed briefly. 

Double Field Revolving Theory

This theory makes use of the idea that an alternating uniaxial quantity can be represented by two oppositely rotating vectors of half magnitude. So, an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to half the value of alternating flux and each rotating synchronously in opposite directions.

As shown in Fig. (a), let the alternating flux have a maximum value of φm . Its component fluxes A and B will each be equal to   φm/2 revolving in anticlockwise and clockwise directions respectively. 

After some time when A and B would have rotated through the angles +θ and -θ as in Fig (b), the resultant flux would be 
= 2×(φm/2) sin (2θ/2) = φsin θ
After a quarter cycle of rotation, fluxes A and B will be oppositely directed as shown in Fig(c) so that the resultant flux would be zero. 

After half a cycle, fluxes A and B will have a resultant of -2×(φm/2) = -φm. After three-

quarters of a cycle, again the resultant is zero as shown in Fig(e) and so on. If we plot the values of resultant flux against θ between limits θ=0° to θ=360°, then a curve similar to the one shown in figure is obtained. That is why an alternating flux can be looked upon as composed of two revolving fluxes each of half the value and revolving synchronously in opposite directions. 

It may be noted that if the slip of the rotor is s with respect to the forward rotating flux (i.e. one which rotates in the same direction as rotor ) then its slip with respect to backward rotating flux is (2-s).

Each of the two component fluxes while revolving round the stator cuts the rotor, induces an earni, and thus produces its own torque. Obviously, the two torques (called forward  and backward torques) are oppositely-directed so that the net or resultant torque is equal to their difference.                
Hence, Tf and Tb are numerically equal but being oppositely directed, produce no resultant torque. That explains why there is no starting torque in a single-phase motor.

However, if the rotor is started somehow, say, in the clockwise direction, the clockwise torque starts increasing and, at the same time, the anticlockwise torque starts decreasing. Hence, there is a certain amount of net torque in the clockwise direction which accelerates the motbr to full speed.