Important Two Marks & 16 Marks Question and Answers

TWO MARKS QUESTION AND ANSWERS

1. Explain why single-phase induction motor is not self-starting one.

When the motor is fed from a single phase supply its stator winding produces an alternating or pulsating flux, which develops no torque which is explained in double revolving field theory.

2. What type of motor is used for ceiling fan?

Capacitor start and capacitor run single-phase motor is used for ceiling fans.

3. What is the type of induction motor used in wet grinders?

Capacitor start capacitor run single-phase induction motor.

4. What kind of motor is used in mixie?

Single-phase ac series motor is used in mixie.

5. What is the application of shaded pole induction motor?

Because of its small starting torque, it is generally used for small fans, toys, instruments, hair driers, ventilators, electric clock etc.

6. In which direction does a shaded pole motor run?

The rotor starts rotation in the direction from unshaded part to the shaded part.

7. Why single-phase induction motor has low power factor?

The current through the running winding lags behind the supply voltage by a very large angle. Therefore power factor is very low.

8. Differentiate between "capacitor start" and "capacitor start capacitor run" induction motor?

In capacitor start motor, capacitor is connected in series with the starting winding. But it will be disconnected from the supply, when the motor picks up its speed. But in capacitor start capacitor run motor the above starting winding and capacitor are not disconnected, but always connected in the supply. So it has high starting and running torque.

9. State the application of an induction generator?

i. Used in windmill for generating electric power.

ii. Used in regenerative breaking places like traction.                

10. What do you mean by residual EMF in a generator.

The EMF induced in the armature conductor only due to the residual flux in the field In poles is known as residual EMF.

11. State the effect of rotor resistance on starting torque?

Starting torque increases with increase in value of rotor resistance.

12. How can varying supply frequency control speed?

We know that

N_S = 120 f / P

From the equation it is clear that by varying frequency speed can be varied it is vary rarely.

13. How is speed control achieved by changing the number of stator poles?

Here change in stator poles is achieved by having two or more independent stator windings in the same slot. Each winding gives different number of poles an different speeds. At a time only one winding is used and other is closed.

14. What are the main disadvantages of rotor rheostatic control?

i. The speed can be decreased by increasing the rotor resistance, but increases I'R loss and hence decreases efficiency.

ii. Speed depends on load also and so used for small periods only.

15. What are the methods of speed control preferred for large motors?

i. Kramer system

ii. Scherbius system.

16. What is an induction regulator?

An induction regulator is used to obtain the constant voltage at the feeder end. Varying the range between the magnetic axes of the primary and secondary windings controls the voltage; it may be a single phase. Rotor is moved usually by a maximum of 180 de degree.

17. Define-Slip frequency.

The relation motion of the stator flux and the rotor conductors induces the voltage of frequency Sf called slip frequency.

18. Define-Asynchronous torque.

When stator and rotor fields are stationary with respect to each other, a steady torque is produced and rotation is maintained. Such a torque existing at any mechanical speed other than synchronous speed is called as an asynchronous torque.

19. What is the main use of squirrel cage winding in synchronous motor starting?

When a squirrel cage winding called the amortissuer or damper winding is inserted in the rotor pole faces, the rotor comes up to the synchronous speed by induction motor action with the field winding unexcited.

20. What is breakdown torque?

From the torque verses slip characteristics, we can infer that as the torque increases, slip increases upto a maximum torque developed is called a breakdown torque.

21. Why a 3-phase synchronous motor will always run at synchronous speed?

Because of the magnetic coupling between the stator poles and rotor poles the motor runs exactly at synchronous speed.

22. What are the two classification synchronous machines?

The classification synchronous machines are:

(i) Cylindrical rotor type,

(ii) Salient pole rotor type.

23. What are the essential features of synchronous machine?

(i) The rotor speed is synchronous with stator rotating field.

(ii) Varying its field current can easily vary the speed.

(iii) It is used for constant speed operation.

24. Mention the methods of starting of 3-phase synchronous motor.

(a) A D.C motor coupled to the synchronous motor shaft.

(b) A small induction motor coupled to its shaft. (pony method)

(c) Using damper windings-started as a squirrel cage induction motor.

25. What are the principal advantages of rotating field system type of construction of synchronous machines?

i. Form Stationary connection between external circuit and system of conditions enable the machine to handle large amount of volt-ampere as high as 500 MVA.

ii. The relatively small amount of power required for field system can be easily supplied to the rotating field system via slip rings and brushes.

iii. More space is available in the stator part of the machine for providing more insulation to the system of conductor.

iv. Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.

v. Stationary system of conductors can easily be braced to prevent deformation.

vi. It is easy to provide cooling arrangement.

26. Write down the equation for frequency of emf induced in an alternator.

F = PN/120 Hertz

Where

P = No. of Poles

N = Speed in rpm.

27. What are the advantages of salient pole type of construction used for synchronous machines?

i. They allow better ventilation.

ii. The pole faces are so shaped radial air gap length increases from the pole center to the pole tips so that flux distribution in the air gap is sinusoidal in shape which will help to generate sinusoidal emf.

iii. Due the variable reluctance, the machine develops additional reluctance power, which is independent of excitation.

28. Why do cylindrical rotor alternators operate with steam turbines?

Steam turbines are found to operate at family good efficiency only at high speeds. The high-speed operation of rotor tends to increase mechanical losses, so the rotors should have smooth external surface. Hence smooth cylindrical type rotors with less diameter and large axial length are used for synchronous generators driven by steam turbines with either 2 or 4 poles.

29. Which type of synchronous generators are used in Hydroelectric plants and why?

As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type synchronous generator are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

28. Why a single phase induction motor is not self-starting?

When single phase supply is given to the single phase stator winding, it produces a magnetic field that pulsates 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 single phase induction motor is not self-starting.

29. How a single phase induction motor can be made to self-start?

To make a single phase IM self-starting, a revolving stator magnetic field should be produced. This may be achieved by converting a single phase supply into two phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting winding may be removed depending the type of the Motor.

30. List the difference types of single phase 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.

31. Give the expression for rotor frequency.

f ' = sf

where

f ' = rotor current frequency toilibbs agolevabam

s = Slip

f= frequency of supply.

32. Where are split phase motors used?

1. Fans and flowers

2. Centrifugal pumps

3. Washing machines

4. Small machine tools

5. Domestic refrigerators.

33. What is the functions 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 two currents drawn by two stator windings. It is obvious that bn the increase in the angle also increases the starting torque. This necessary phase angular difference is produced by connecting a capacitor in the motor.

34. An 8 pole, lap wound armature rotated at 350 rpm's required to generate 260 V. The useful flux, pole is 0.05 wb. If the armature has 120 slots. Calculate the number of conductors per slot. (April/May 2015)

 

PART B-QUESTIONS

1. Develop the equivalent circuit for 3-phase induction motor?

2. Explain the different speed control methods of squirrel cage induction motor.

3. Describe the principle of operation of synchronous induction motor.

4. Draw the slip-torque characteristics for a three-phase induction motor and explain.

5. Draw and explain the equivalent circuit of a three-phase induction motor.

6. Explain why a synchronous motor does not have starting torque.

7. Explain one method of starting a synchronous motor.

8. What is meant by hunting in a synchronous motor? Why is it undesirable? What is done to minimize it?

9. Draw the power angle diagram of a synchronous machine.

10. Explain the working of 10 induction motor.

11. Describe the construction and working of a capacitor start 10 induction motor.

12. Explain the types of 10 induction motor with neat diagrams.

13. Explain the operation of 30 induction motor and write the condition for maximum torque.

14. Explain the principle operation of a synchronous motor.

15. Explain the different types of torque in synchronous motor.