Whenever the AC supply is connected to the stator windings, line currents IR, IY, and IB start flowing. These line currents have phase difference of 120o 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 120o with respect to each other.
Let the flux produced by the line currents IR, IB, IY be φR, φB, φY respectively.
Mathematically, they are represented as follows:
φR = φm sin ωt = φm sin θ
φB = φm sin (ωt – 120o) = φm sin (θ – 120o)
φY = φm sin (ωt – 240o) = φm sin (θ – 240o)
It can be proved mathematically that these fluxes produce a rotating magnetic field in induction motor of constant amplitude and speed.
Speed of Rotating Magnetic Field
The rotating magnetic field in three phase induction motor (RMF) rotates at a constant speed called synchronous speed (Ns), which is given by,
NS = 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 same for RMF in synchronous motor.
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