Rotating magnetic field in Induction Machine

Induction Machine

Induction machine is simple in construction and low in cost. And also Rugged/Robust structure, has good speed regulation, well design induction motor have efficiency up to to 90%. Power factor (P.F) 0.9, No commutation, armature reaction effects as AC.

Shunt motor is only preferred over induction motor due to it's speed control.
Induction generator drawn magnetizing current when supply hence it is not preferred for generation purpose.
Starting torque of induction motor is not high as compared to DC series motor.

Rotating magnetic field (RMF) :-

When magnet rotates then we say rotating magnetic field.
when a 3 phase supply ( 120 degree mutual phase displacement displacement with respect to time) is applied to a three phase winding winding phase winding (120 degree displacement with respect to space to space). It will produce a rotating magnetic field which rotates at the  synchronous speed with constant magnitude 1.5Φm with a specific direction which depends on phase sequence sequence. Rotating Magnetic Field is discovered by Nikola Tesla

ΦA = Φm sin(ωt)

ΦB = Φm sin(ωt - 120)

ΦC = Φm sin(ωt - 240)

As the windings are identical and supply is balanced, the magnitude of each flux is Φm.

Case 1 :- At ωt = 0

ΦA= Φm sin(0) = 0

ΦB = Φm sin(0 - 120) = -0.866 Φm

ΦC = Φm sin(0 - 240) = +0.866 Φm

Case 2 :- At ωt = 60

ΦA = Φm sin(60) = +0.866 Φm

ΦB= Φm sin(- 60) = -0.866 Φm

ΦC = Φm sin(- 180) = 0

Case 3 :- At ωt = 120

ΦA= Φm sin(120) = +0.866 Φm

ΦB = Φm sin(0) = 0

ΦC= Φm sin(- 120) = -0.866 Φm

Case 4 :- At ωt = 180

ΦA = Φm sin(180) = 0

ΦB = Φm sin(60) = +.866 Φm

ΦC = Φm sin(- 60) = -0.866 Φm

Standard relation between speed, poles, frequency

#cycles per rotation =P/2

# Rotation per second = N/60

Cycle/second = PN/120


(or) N=  120f/P

Flux always moves from leading phase 2 lagging phase. That's why  Rotating magnetic field is important for us.

Speed is decrease with increase in the number of poles in machine. i.e. 2 poles machine rotates with high RPM as compared to the 4 pole machine.


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