Single phase induction motor has distributed stator winding and a squirrel-cage rotor. When 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 behavior of the motor has been explained in two ways
(i) by two-field or double-field revolving theory and
(ii) by cross-field theory.
Only the double field revolving theory will be discussed briefly.
Double Field Revolving Theory
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
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.