# Objective Questions on Induction Motor | 1

1. Type of single phase motor having highest p.f at full load is

Capacitor run has better running p.f and efficiency and smoother operation. Modern application is ceiling fan.

2. Induction motors is having advantage of

As compared with D.C motors the IM doesn’t have any commutators and brushes. Hence it requires less maintenance. Poly-phase induction motors are used because of:
1. Less cost
2. Simple and robust construction.
3. Less maintenance required.

3. For a single phase capacitor start induction motor which of the following is valid

For changing direction of rotation, we have to change main winding terminal and direction of magnetic flux will be reversed. The single-phase capacitor start induction motor is an A.C. single phase motor having two windings as in split phase motor. One heavy duty electrolyte capacitor is connected in series with the starting winding in order to produce the phase difference between the currents in the two windings and thereby producing starting torque. The direction of rotation of capacitor-start, induction run motor is reversed by reversing either the starting or running winding leads, not both. So, the valid statement is-direction of rotation can be changed by reversing main winding terminals.

4. Which motors are preferred for refrigeration and air conditioning in smaller units?

Under construction.

5. Types of rotating machines

Rotating machines are of 3 types – induction machine, synchronous machine and DC machine.

6. A 230 V, 50 Hz, 4 pole single phase induction motor is rotating in the clockwise (forward) direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ω. Then the effective rotor resistance in the backward branch of the equivalent circuit will be

Synchronous speed = Ns = 120f / P = 120 × 50 / 4 = 1500 rpm, Slip s = 1500 − 1425 / 1500 = 0.05, Resistance of backward branch rb = r2 / (2 − s) = 7.8 ⁄ (2 − 0.05) = 4 Ω.

7. Which of the following motor used in hair dryers?

Shaded pole induction motor used in hair dryers, small fans etc.

8. Which of the following motor is used in grinders?

The capacitor start capacitor run IM is applied for blowers, refrigerators, drilling machines, grinders etc.

9. Capacitor start capacitor run provides

Two capacitors are used in this type of motor. One capacitor is used at the starting and another capacitor is used at the time of normal running for providing high staring torque and high power factor.

10. Slip speed is the

Slip speed is defined as the difference of synchronous speed and rotor speed. The difference between the speed of the stator field, known as synchronous speed ( Ns) and the actual speed speed of rotor (N) is known as the slip and denoted by s.
Fraction slip (s ) = ( Synchronous speed - Rotor speed ) / Synchronous speed = (Ns - N) / Ns and Percentage slip = [( Ns - N ) / Ns] × 100 %

11. At slip s = 1 torque developed in machine is

The torque developed at unity slip is starting torque or stalling torque.

12. Capacitor fitted with ceiling / table fans is of the value of

Capacitor used in ceiling or table fans are of small values and is of the order of 2 - 3.15 microfarads.

13. Which 1 - Φ motor does operate at high p.f?

Capacitor run type 1 phase motor operate at high power factor p.f

14. Why are capacitors used with large a.c. induction motor?

Three phase capacitors banks are used with large A.C induction motor to improve the power factor of the circuit and to get the resultant effect of electric current reduction.

15. Which 1 Φ motor is used for refrigerator?

Split phase / capacitor start 1 - Φ motor is used for refrigerator. Capacitor run is used for room cooler, universal is used in drill machine, hooter, mixing machine, repulsion is used for compressor.

16. What type of dielectric material is used in capacitors used for fans and for p.f correction?

Oil impregnated paper is used as dielectric medium for capacitors used in fans / capacitors used for pf correction and also small size large value capacitors. Air / vacuum is used for precision small value capacitor. Mica / glass is used for standard capacitors used in labs.

17. A fan blade of length 2R rotates in a time period T in a magnetic field B. The induced voltage in the blade in

E = -d(φ) / dt = d(BA) / dt = R2B / T.

18. A three phase induction motor has a synchronous speed of 1500 rpm the machine runs at 1460 rpm at a particular load. The slip at this load is ___________.

% Silp = (Ns- N / % Silp = (Ns) × 100
∴ % s = (1500 - 1460 / 1500) × 100 = 2.67 %.

19. A 3φ, 12 pole, 600 V, 50 Hz star connected IM has rotor resistance and stand still resistance of 0.03Ω and 0.5 Ω respectively. The speed of maximum torque will be

For 12 pole, 50 Hz motor, Synchronous speed = 120 × 50 / 12 = 500 rpm.
For r = 0.03 Ω and x = 0.5 Ω.
The slip for maximum torque is related as : smT = a = r / x = 0.03 / 0.05 = 0.06
∴ Corresponding speed = 500( 1 - smT ) = 470 r.p.m.

20. A 3 - Φ, 400 / 200 V, Y - Y connected wound rotor induction motor has 0.06 Ω rotor resistance and 0.3 Ω standstill reactance per phase. To make the starting torque equal to maximum torque of the motor the additional resistance required

We know,Tst / Tmax = 2a / 1 + a2;
Since Tst = Tmax
1 = 2a / 1+a2 ⇒ a=1
Now a = R2 + r / X2
Where r = external resistance / phase added to rotor circuit
∴ 1 = 0.06 + r / 0.3 i.e r = 0.24 Ω.