# Objective Questions on Induction Motor | 5

1. In case of IM the torque is

T = kφsE2R2 / { ( R2)2 + (sX2)2 }
At normal speed, close to synchronism, the term ( sX2 ) is small hence negligible w.r.t R2
∴ T = kφsE2 / R2
So T ∝ s.

2. The torque developed in IM is nearly proportional to

T = kΦsE2R2 / { ( R2)2 + (sX2)2 }
E2 ∝ φ ∝ V.

3. In case of 3 - Φ IM, plugging means

An IM can be quickly stopped by simply inter changing any of its two stator leads. It reverses the direction of the rotating flux which produces a torque in the reverse direction, thus applying brake on the motor, this is called plugging.

4. When R2 is rotor resistance, X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running condition will be

T = kΦsE2R2 / { ( R2)2 + (sX2)2 }
The condition for maximum torque may be obtained by differentiating the above expression Y = { (R2)2 + ( sX2 )2 } / { kφsE2R2 } = { R2 / kφE2 } + { sX22 / kφsE2R2 }
⇒ dY / ds = ( - R2 / kφs2E2 ) + ( X22 / kφ s2E2 } = 0
⇒ R2 / kφs2E2 = X22 / kφs2E2
∴ (R2)2 = s2X22
⇒ R2 = sX2.

5. The rotor electromotive force of the IM is observed to make 100 complete alterations per minute, calculated slip

100 alterations per minute = 100 / 60 cycles / sec
1.6667 Hertz = sf
s = 1.6667 / 50 = 3.33 %

6. Nichrome is an alloy of

This group of materials on the nickel / chromium solid solution is known as nichrome, primarily used as a resistance heating wire.

7. In a star - delta starting of an IM

When the motor is star connected star, the applied voltage over each phase is reduced to 1 / √3 times the supply voltage.
∴ Line phase voltage is 57.7 % of line voltage.

8. A 3 - Φ induction motor can also be referred to as a

The rotor of a 3 - Φ IM can never run at synchronous speed. Because at synchronous speed the flux cutting between the stator and rotor is zero leading to no torque production, thus bring the rotor speed immediately below synchronism.

9. Which of the following methods of speed control is not affected through stator side?

The rotor resistance can only be added onto the rotor side and the other three modifications can be done from the stator side.

10. The wound rotor type IM is used for____________speed operation.

In this type of IM, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate, by this motor can obtain variable speed.

11. The rotor speed of an induction motor is given by

The rotor speed of an IM is given by Nr = ( 1 - s)Ns. And Ns = 120f / p
∴ Nr = 120f / p × ( 1 - s ). It is seen that the motor speed can be changed by change in frequency f, no. of poles P, or slip s.

12. Pole changing methods are suitable only for ___________ type induction motor.

The methods of speed control by changing the poles are suitable for cage type induction motor because of the cage rotor automatically develops number of poles equal to poles of stator winding.

13. In a 3 - Φ IM in order to eliminate cogging the number of stator slots must be

In order to reduce or eliminate cogging the number of stator slot are never made equal to or an integral multiple of the number of rotor slots. Cogging can also be reduced by using skewed rotor.

14. A slip ring Induction motor runs at 290 rpm at full load, when connected to 50 Hz supply. Determine the number of poles and slip.

Since N is 290 rpm; Ns has to be somewhere near it say 300 rpm. If Ns is assumed as 300 rpm then 300 = 120 × 50 / P
∴ pole = 20
Slip = ( 300 - 290 ) / 300 = 3.33 %.

15. _____________ types of induction motor are widely used.

Almost 90 % of induction motors are squirrel cage type IM, because in this type of motor the rotor is of squirrel cage type and this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible.

16. The rotor of a three phase induction motor has an impedance of 0.4 + j4 ohm per phase at standstill the maximum torque occurs at a slip of _____________.

Maximum torque occurs at R2 = sX2
% Silp = ( R2 / X2 ) × 100 = ( 0.4 / 4 ) × 100 = 10 %.

17. The speed of a 50 Hz three phase induction motor under full load condition is 725 rpm the number of poles of the motor are ______________.

Full load speed is 725 rpm, Ns near to this value can be taken as 75 rpm where Ns is synchronous speed.
Ns = 750 = 120 × 50 / p
⇒ p = 8.

18. What will be the maximum power factor of an induction motor for dispersion coefficient of 0.05?

Maximum power factor (p.f) = 1 - 0.05 / ( 1 + 0.0.5 ) = 0.905.

19. Value of specific electric loading of Induction motor is in the range of

Specific electric loading of induction motor is in the range of 5000 - 45000 ampere conductor / metre.

20. Larger values of air flux density is taken while designing IM for

Output equation and bore diameter of IM is proportional to the air flux density.