Induction Motors: 2 Marks Important Questions with Answers

Basic Electronics and Electrical Engineering

CHAPTER‒ 5: INDUCTION MOTORS

2 Marks Important Questions with Answers

1. Why a 1ϕ induction motor is not self‒starting?

When 1ϕ supply is given to the 1ϕ stator winding, it produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel‒cage rotor. So a 1ϕ induction motor is not self‒starting.

2. How a 1ϕ induction motor can be made to self‒star?

To make a 1ϕ IM self‒starting, a revolving stator magnetic field should be produced. This may be achieved by converting a 1ϕ supply into 2ϕ supply through the use of an additional (starting) winding. when the motor attains sufficient speed, the starting winding may be removed depending the type of the motor.

3. List the different types of 1ϕ induction motor based on the methods of making them self starting

(a) Split ‒ phase motors.

(b) Capacitor motors.

(i) Capacitor‒start motors.

(ii) Capacitor‒start capacitor‒run motors.

(c) Shaded pole motors.

4. Give the applications of various types of 1ϕ induction motors

Split‒phase induction motor

(i) Fans

(ii) Washing machines

(iii) Oil burners

(iv) Small machine tools

Capacitor‒start induction motor

(i) Compressors

(ii) Large fans

(iii) Pumps

(iv) High inertia loads.

Capacitor‒start capacitor‒run induction motor

(i) Hospitals

(ii) Studios

(iii) Other place where silence is important.

Shaded‒pole induction motor:

(i) Small fans

(ii) Toys

(iii) Hair dryers

(iv) Desk fans (Table fans).

5. Give the expression for rotor frequency.

 f ' = sf

where

f ' = rotor current frequency

s = slip

 f = frequency of supply

6. Why the slots on the induction motor are usually skewed?

The rotor bars are slightly inclined to the shaft axis due to the skew provided while stacking the rotor stampings. The reason for this skewing are:

(i) To help the motor run quietly by reducing magnetic hum.

(ii) To help reducing the locking tendency of the rotor teeth with the stator teeth due to direct magnetic attraction between the two.

7. Where are split phase motors used?

Applications where split‒phase motors are used are

(i) Fans and blowers

(ii) Centrifugal pumps

(iii) Washing machines

(iv) Small machine tools

(v) Domestic refrigerators

8. What is the function of capacitor in a single phase induction motor?

The torque developed by single‒phase induction motor is proportional to the sine of the angle between the 2 currents drawn by 2 stator windings. It is obvious that the increase in the angle also increases the starting torque. This necessary phase angular difference is produced by connecting a capacitor in the motor.

9. What is the use of shading coil in the shaded pole motor?

When ac supply is fed to the exciting winding surrounding the whole pole, the magnetic  axis of the pole shifts from the unshaded part to the shaded part (part of pole with shading coil), the rotor starts rotating in the direction of this shift. Thus the direction of rotation of motor is fixed by the position of shading coil.

10. Define synchronous speed (N_s).

The speed at which the rotating magnetic field (RMF) rotates in the stator, is called as "synchronous speed", N_S.

N_S = 120f / P

where, f ‒ Frequency of supply (in Hz).

P‒ Number of stator poles.

11. What are the types of induction motors.

3 Phase induction motor

(i) Squirrel cage induction motor.

(ii) Slip ring induction motor.

1‒ Phase induction motor

• Split phase induction motor

(i) Induction start ‒ induction ran motor

(ii) Capacitor start ‒ induction ran motor

(iii) Capacitor start ‒ induction ran motor

• AC series motor (or) universal motor.

• Shaded pole. motor.

• Reluctance motor.

• Repulsion motor.

12. In a single phase induction motor what are the two windings and what is their purpose?

(i) Stator winding - To produce an alternating magnetic field

(ii) Rotor winding - To produce the rotating torque.

13. Mention the various types of single phase induction motor.

• Split phase induction motor

(i) Induction start ‒ induction ran motor

(ii) Capacitor start ‒ induction ran motor

(iii) Capacitor start ‒ capacitor ran motor

• AC series motor (or) universal motor.

• Shaded pole. motor.

• Reluctance motor.

• Repulsion motor.

14. What are the different types of stepper motor?

(i) Variable reluctance stepper motor

(ii) Permanent magnet stepper motor

(iii) Hybrid stepper motor