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Tuesday, 12 November 2019

Single Phase Induction Motor Working Principle



In general the construction of single phase motors is similar to a three phase squirrel cage induction motor except for the stator winding. The stator contains a set of windings producing several pairs of poles. When connected to an alternating current supply, the polarity of these poles would alternatively become north and south due to which a pulsating torque is produced locking the rotor stationary i.e., the torque produced will try to rotate the rotor in both the direction.

If the rotor is given a push in any direction, it would pick up speed and continue to run in the same direction. Hence, a single phase motor is not self starting. The auxiliary methods used are splitting the single phase into two phases or shading the pole to obtain a directional torque.


The peculiar behavior of the single phase motor can be explained into two ways :

(a) by cross field theory
(b) double field revolving theory

To overcome the problem and to make the single phase induction motor self starting it has to be feed in either from 2 phase or three phase supply the field produced by the supply may be made to revolve for producing self starting capability.

Cross-Field Theory : Consider a single phase induction motor having 1-Φ winding on stator and squirrel-cage rotor.

Assume that the rotor is now given an initial rotation in the clockwise direction. An e.m.f. called rotational e.m.f is induced in the rotor winding due to the stationary stator field. The direction of induced e.m.f. in the rotor conductors is shown in Fig. E.M.F. induced in the rotor conductors is in one direction on one side of the vertical axis and in the opposite direction on the other side of the vertical axis.

As the rotor circuit is closed the voltage so induced will produce a rotor current and a rotor e.m.f. wave whose axis is displaced by 90 degrees electrical from the stator axis. The frequency of the rotor-induced e.m.f. is high and, therefore, the rotor reactance is also high. The rotor current will lag the rotor induced e.m.f. by about 90 degrees. The field produced by rotor current, Φr known as cross-field will have a time-phase difference of 90 degrees with the stator field Φσ. Tηvσ the stator flux (Φs), and rotor flux Φr are in space time quadrature. These two fields will produce a revolving field (as in 2-phase supply) which will rotate in the direction in which the rotor was given an initial rotation. Thus the torque produced will be in the same direction as that of rotation.

Double Revolving Field Theory :

This theory is based on the fact that the alternating flux produced by stator winding can De represented as the sum of two oppositely-rotating vectors of half-magnitude. The summation of the vectors is a vector that changes in length along the horizontal axis.

Let Φm = maximum value of the alternating flux
A = B = Φm/2 component fluxes of Φs, revolving in clockwise, anti-clockwise respectively.
Φr = stator resultant flux at any instant.

From fig, (a) it can be seen that the magnitudes of Φr at intervals of 0o, 45o, 90o, 135o and 180o are respectively equal to 0, 0.707 Φm , Φm , 0.707 Φm and 0. At θ = 0o, the two component fluxes A and B are shown in opposite directions. After a time interval of 45 degrees, A and B have rotated in clockwise and anti-clockwise directions respectively, Fig (c). At the interval of one-fourth cycle i.e., 90 degrees, Φr is maximum Fig (d). At q = 135°, fluxes A and B will have a resultant of 0.7074 Φm, Fig. (e). After half cycle, again the resultant flux is zero as shown in Fig (f) and so on. If the component vectors A and B are drawn for one cycle. It will be observed that each of the component flux vectors will rotate by one revolution.

Torques developed by two components A and B are acting in opposite directions, each component develops a torque that tends to rotate the rotor in the direction in which the field rotates. The resultant torque is the summation of the torques produced by the two components A and B as shown in Fig. It may be noted that torque-speed curve is drawn for a speed range of— Ns to + Ns. From the resultant torque-speed curve the following can be observed.

a. Average torque at standstill is zero, and therefore, the motor is not self-starting (at Ni = 0, torque developed by the A and B components cancel each other).

b. When rotor is given an initial rotation in any direction, the average torque developed causes the rotor to continue to rotate in the direction in which it is rotated initially.

Brief description of two theories has been given to explain why a single-phase induction motor will continue to rotate in a direction in which the rotor is given some initial rotation. To make the motor self-starting, some starting device or method has to be employed. Single-phase induction motors are named according to the starting methods employed.

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