Induction Motor Operation

Principle of Operation of Induction Motor

Consider a portion of 3-phase induction motor as shown in Fig. (1). The operation of the motor can be explained as under:

(i) When 3-phase stator winding is energized from

a 3-phase supply, a rotating magnetic field is

set up which rotates round the stator at synchronous

speed Ns (= 120 f/P).

(ii) The rotating field passes through the air gap and

cuts the rotor conductors, which as yet, are

stationary. Due to the relative speed between the rotating flux and the  stationary rotor, e.m.f.s are induced in the rotor conductors. Since the

rotor circuit is short-circuited, currents start flowing in the rotor conductors.

(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor

conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field.

(iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary

rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.

fig.(1)

Slip

We have seen above that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor can never reach the speed of stator flux. If it did,

there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor. The

friction and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less than the suitor field speed (Ns). This difference in speed depends upon load on the motor.

The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of

synchronous speed i.e.,

% age slip, s=((Ns- N)\Ns) *100

(i) The quantity Ns - N is sometimes called slip speed.

(ii) When the rotor is stationary (i.e., N = 0), slip,

 s = 1 or 100 %.

(iii) In an induction motor, the change in slip from no-load to full-load is hardly 0.1% to 3% so that it is essentially a constant-speed motor.

Effect of Slip on The Rotor Circuit

When the rotor is stationary, s = 1. Under these conditions, the per phase rotor e.m.f. E2 has a frequency equal to that of supply frequency f. At any slip s, the relative speed between stator field and the rotor is decreased. Consequently, the

rotor e.m.f. and frequency are reduced proportionally to sEs and sf respectively. At the same time, per phase rotor reactance X2, being frequency dependent, is reduced to sX2.

Consider a 6-pole, 3-phase, 50 Hz induction motor. It has synchronous speed Ns = 120 f/P = 120 ´ 50/6 = 1000 r.p.m. At standsill, the relative speed between stator flux and rotor is 1000 r.p.m. and rotor e.m.f./phase = E2(say). If the fullload speed of the motor is 960 r.p.m., then,

s =( 1000 – 960)\1000 =0.04

(i) The relative speed between stator flux and the rotor is now only 40 r.p.m. Consequently, rotor e.m.f./phase is reduced to:

E *( 40\ 1000) =0.04E2            or sE2

(ii) The frequency is also reduced in the same ratio to:

50* 40\1000 = 50* 0.04         or sf

(iii) The per phase rotor reactance X2 is likewise reduced to:

X *( 40\ 1000)  =0.04X2     or sX2

Thus at any slip s,

Rotor e.m.f./phase = sE2

Rotor reactance/phase = sX2

Rotor frequency = sf

where E2,X2  and f : are the corresponding values at standstill.