The induction motor rotates due to the rotating magnetic field in 3 phase induction motor, which is produced by the stator winding in the air gap between in 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 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.
Rotating magnetic field in three phase induction motor image
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.

Induction Motors — 1 | Objective Type Question Answers



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#1 As the load on an induction motor increases

its power factor goes on increasing up to full load then it falls again

#2 The slip in actual induction motor is generally

3% to 5%

#3 The maximum speed of an induction motor cannot be

3000 RPM

#4 In a squirrel cage induction motor, the starting current is

5 to 7 times the rated current

#5 When N1 is the synchronous speed and N2 is the actual speed of the rotor, the slip is given by

(1 – N2/N1)

#6 When ‘f’ is the supply frequency and ‘s’ is the slip, the frequency of the rotor current is given by


#7 When a motor is switched on, the rotor frequency is

same as the supply frequency

#8 In an induction motor, the rotor reactance per phase is proportional to


#9 A 7.5 HP, 3 phase, 400 V motor will draw a full load current of

11 A

#10 Type of bearing used for a 25 HP motor is

roller bearing

#11 Motors for fan and pumps are selected in the RPM range

1440 to 2880

#12 The word TEFC stands for

Totally Enclosed Fan Cooled

#13 The class of insulation generally not provided on electric motors

class F

#14 A 3 phase 420 V, 60 HZ induction motor is to run on 50 HZ supply. Which of the following will be the satisfactory supply voltage for the machine.

350 V

#15 The efficiency of induction motor is expected to be in the range of

80 to 90 percent

#16 The speed-load characteristics of an induction motor resembles which of the following motor:

DC shunt motor

#17 Negative sequence currents are set up in an induction motor when

unbalanced 3 phase supply is given

#18 Direct on line starting current as compared with star delta starting current is approximately

3 times

#19 In an induction motor the rotor resistance and reactance are 0.2 and 5 ohms respectively. In order that torque of the motor may be maximum, the value of slip should be

4 %

#20 The stator of a 3 phase, 50 Hz, 1440 rpm squirrel cage induction motor is replaced by a stator of six pole motor, without any other alteration. The motor will

run at less than 1000 RPM


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