Double Revolving Field Theory

The performance of a Single-phase Induction motor is suggested to be Examined using the double Revolving field theory. It explains why the rotor Generates a torque once it Starts Rotating.

Double Revolving Field Theory

A Stationary Pulsating Magnetic field can be resolved into two Rotating Magnetic fields using the Single-phase Induction motor’s double Revolving field theory. Although both Magnetic fields are equally strong, they rotate in different directions. Each Magnetic field affects the motor differently,. And each Magnetic field’s Combined torque is equal to the total torque produced by the motor.

Mathematically, an Alternating magnetic field whose field axis is fixed in the space is given by,

B(θ)=Bmaxsinωtcosθ…(1)

Where, B_max is the maximum value of magnetic flux density which is Sinusoidally distributed in the air-gap of the motor.

This magnetic field is created by a properly distributed stator winding. That is carrying a Frequency-dependent current, and where is the space displacement angle from the stator winding’s axis.

(∵sinXsinY=(1/2)sin(X−Y)+(1/2)sin(X+Y))

∴B(θ)=(1/2)Bmaxsin(ωt−θ)+(1/2)Bmaxsin(ωt+θ)…(2)

The first term of eqn. (2) represents a revolving field which is moving in the positive θ direction and has a maximum value equal to 1/2 while the second term represents a Revolving magnetic field which is moving in the negative θ direction and has a maximum value also equal to (1/2)Bmax

Positively θ rotating magnetic fields are referred to as forward rotating fields, whereas negatively θ rotating magnetic fields are referred to as backward rotating fields.

The positive direction is the direction in which the single-phase induction motor is started initially. Both the magnetic fields rotate at synchronous speed in opposite direction. Hence,

Forward rotating field=(1/2)Bmaxsin(ωt−θ)

And

Backward rotating field=(1/2)Bmaxsin(ωt+θ)

It follows that a stationary magnetic field can be split into two Equal-magnitude Rotating magnetic fields that rotate at the same speed and in the opposite direction of the stationary magnetic field at the same frequency. The Double-revolving field theory of Single-phase induction motor is a theory that divides a Stationary pulsating magnetic field into two opposing rotating magnetic fields.

The two torques Generated are equal and in Opposition when the rotor is at rest. Therefore, at a complete stop, the net torque is zero. The torque caused by the Rotating Magnetic field acting in either direction of the initial Rotation, though, will be greater than the torque caused by the other Rotating Magnetic field if the rotor is given an initial Rotation by some auxiliary means in either direction. As a result, the motor produces a net torque that is parallel to the direction of the initial Rotation. As a result, the motor will continue to rotate in the same direction.