The induction motor rotates due to the rotating magnetic field in induction motor, which is produced by the stator winding in the air gap between the stator and the rotor. The stator has a three phase stationary winding which can be either star connected or delta connected.

Whenever the AC supply is connected to the stator windings, line currents I_R, I_Y, and I_B start flowing. These line currents have phase difference of 120^o with respect to each other. Due to each line current, a sinusoidal flux is produced in the air gap. These fluxes have the same frequency as that of the line currents, and they also have the same phase difference of 120^o with respect to each other.

Let the flux produced by the line currents I_R, I_B, I_Y be φ_R, φ_B, φ_Y respectively.

Mathematically, they are represented as follows:

φ_R = φ_m sin ωt = φ_m sin θ
φ_B = φ_m sin (ωt – 120^o) = φ_m sin (θ – 120^o)
φ_Y = φ_m sin (ωt – 240^o) = φ_m sin (θ – 240^o)

The effective or total flux (ɸ_T) in the air gap is equal to the phasor sum of the three components of fluxes ɸ_R, ɸ_Y and, ɸ_B.

Therefore, ɸ_T = ɸ_R + ɸ_Y + ɸ_B

Prove that RMF in Induction Motor is Rotating

To prove that this RMF is rotating we will:

Step 1: We will find the values of total flux ɸ_T for different values of θ such as 0, 60, 120 , 180 ….. 360^o.

Step 2: For every value of θ in step 1, we will draw the phasor diagrams.

By observing these phasor digrams you can understand easily that ɸ_T keep shifting its position from 90^o to 30^o to – 30^o to – 90^o and so on. In other words ɸ_T rotates in the clock wise direction.

We will use the phasor ɸ_R as the reference phasor i.e. all the angles are drawn with respect to this phasor.

ɸ_T at θ = 0^o

 ɸ_T at θ = 60^o

ɸ_T at θ =120^o

ɸ_T at θ =180^o

Similarly we can prove that ɸ_T at θ =180^o is given by,
ɸ_T = 1.5ɸ_m ∠ – 90^o
The phasor diagram for θ =180^o is shown in Figure (d).

Speed of Rotating Magnetic Field in Induction Motor

The magnetic field in three phase induction motor (RMF) rotates at a constant speed called synchronous speed (Ns), which is given by,

N_S = 120f₁/P RPM

where  f₁ = Frequency of stator supply
P = Number of poles of motor.

Direction of Rotating Magnetic Field

The direction of rotating magnetic field depends upon the phase sequence of the AC supply connected across the stator winding. If we interchange any two phases of the AC supply, we will get new phase sequence, then the direction of rotating magnetic field in three phase induction motor will reverse. It will start rotating in reverse direction and so does the rotor.

Since a stator and its winding of three phase induction motor is the same as that of a synchronous motor. So all above discussion about RMF in induction motor is the same for RMF in synchronous motor also.

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