Double Revolving field theory-how the rotor develops torque

Double field revolving theory: According to the double field revolving theory, we can resolve any alternating quantity into two components. Each component has a magnitude equal to half of the maximum magnitude of the alternating quantity, and both these components rotate in the opposite direction to each other

1. Single-phase induction motors Constructional features, double revolving field theory, Equivalent circuit, determination of parameters. Split-phase starting methods and applications.

2. Double Revolving field theory Introduction If, however, the rotor is in motion in any direction when supply for the stator is switched on, it can be shown that the rotor develops more torque in that direction. The net torque then, would have non-zero value, and under its impact the rotor would speed up in its direction. The analysis of the single phase motor can be made on the basis of two theories: Double revolving field theory, and ii. Cross field theory.

3. Double revolving field theory – Double Revolving Field Theory, states that a single pulsating magnetic field of as its maximum value can be resolved into two rotating magnetic fields of (m/2) as their magnitude rotating in opposite directions at synchronous speed proportional to the frequency of the pulsating magnetic field .

4. According to double field revolving theory, an alternating uniaxial quantity can be represented by two oppositely rotating vectors of half magnitude. Accordingly, an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to half the value of the alternating flux and each rotating synchronously (Ns = 120f/p) in opposite direction.

5. As shown in above 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 direction respectively. After some time, when A and B would have rotated through angle +θ and –θ as in fig (b), the resultant flux would be,

6. 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 x Φm / 2 = - Φm . Which is shown in fig d)..

7. After three quarters of a cycle, again the resultant is zero as shown in fig. (e) and so on.

8. If we plot the resultant flux against θ between θ = 0° and 360° , an alternating flux is obtained. That is why alternating flux is considered to have two fluxes, each half the value and revolving synchronously in opposite directions.