Double Field Revolving Theory

Double field revolving theory states that the pulsating magnetic field produced by the single phase winding is equivalent to the phasor sum of two oppositely rotating magnetic fields each having a magnitude of 0.5φ_m with a synchronous speed of N_s=120f/P.

fig:2

Fig 2 shows two rotating magnetic fields,namely OA and OB,each having a magnitude of 0.5φ_m.OA is rotating in forward(clockwise) direction and OB is rotating in backward (anticlockwise) direction with the synchronous speed.

When the two rotating phasors reach the positions such as shown in fig 2,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=0 of the pulsating field shown in fig 1.Let us assume this position as reference point.

After 30° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.5φ_m  and the direction is upward. This condition is equivalent to ωt=30° of the pulsating field shown in fig 1.

After 60° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.866φ_m  and the direction is upward. This condition is equivalent to ωt=60° of the pulsating field shown in fig 1.

After 90° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to φ_m  and the direction is upward. This condition is equivalent to ωt=90° of the pulsating field shown in fig 1.

After 120° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.866φ_m  and the direction is upward. This condition is equivalent to ωt=120° of the pulsating field shown in fig 1.

After 180° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=180° of the pulsating field shown in fig 1.

After 210° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to -0.5φ_m  and the direction is downward. This condition is equivalent to ωt=210° of the pulsating field shown in fig 1 and so on.

Based on double field revolving theory,the torque-speed characteristics of single phase induction motor can be drawn as shown in fig 2.a,where torque-speed characteristics are shown,one due to forward rotating magnetic field and other due to backward rotating magnetic field.

fig:2.a

Here, OA=Forward starting torque

          OB=Backward starting torque

These two torques are equal and opposite. Hence,the net starting torque of single phase induction motor is zero.so,the single phase induction motor is not self starting.

The equivalent circuit of single phase induction motor is shown in the figure below (fig 2.b) :

fig:2.b

x