Important Short Questions and Answers: Induction Machines and Synchronous Machines

INDUCTION MACHINES AND SYNCHRONOUS MACHINES

1. Write the principle of operation of 3 phase induction motor.

Induction motor works on the principle of faraday’s laws of electromagnetic induction. It consists of stator and rotor windings. As these rotor windings or bars rotate within the magnetic field created by the stator magnetizing currents, voltages are induced in them. If the rotor were to stand still, then the induced voltages would be very similar to those induced in the stator windings.

2. Name the types of alternators.

i. Salient pole or projected type

ii. Cylindrical or non salient pole type

3. Compare slip ring and squirrel cage type rotor.

Slip ring rotor: It has windings

Squirrel cage rotor: It has copper bars and shorted through end rings

4.   Mention the characteristic features of synchronous motor.

1.     Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.

2.     The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

3.     This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

5. Mention some applications of synchronous motor.

1) used in machine tools

2) motor generator sets

3) synchronous clocks

4) belt driven reciprocating compressors

5) fans and blowers

6. Why synchronous motor is called so?

Since the motor is always run with the synchronous speed.

7. Define slip of an induction motor.

It is the ratio between slip speed to synchronous speed.

Slip = (N_s-N_r)/N_s

Where, N_s = Synchronous speed, N_r = Rotor speed

8. Name any four types of single phase induction motors based on method of starting.

1. Split phase induction motor.

2. Capacitor start induction motor.

3. Capacitor start and capacitor run motor.

4. Shaded pole induction motor.

9.Find the speed at which a 6 pole alternator is to be driven to obtain the frequency of emf induced to be 50 Hz.

Speed, Ns=120f/p= 120X50/6= 1000 rpm.

10. Write down the relation between speed and frequency.

Synchronous speed, Ns=120f/p

Where, f = Frequency of supply in Hz, p = Number of poles in the stator.

11. What are the two types of 3 phase induction motor?

There are two types of 3-phase induction motor based on the type of rotor used:

(i) Squirrel cage induction motor.

(ii) Slip ring induction motor.

12.            What are the losses present in the Induction motor?

1. Stator copper loss.

2. Stator iron loss.

3. Rotor copper loss.

4. Windage loss & friction loss.

13.  Why induction motors are called asynchronous?

Because their rotors can never run with the synchronous speed.

14. What is synchronous speed?

The speed at which the stator field rotates is called synchronous speed and it depends upon the frequency of supply and number of poles for which the stator is wound.

Ns = synchronous speed in rpm.

= 120 f / p.

f = frequency of supply.

p = Number of poles in the stator.

15. What are the advantages of the slip-ring induction motor over squirrel cage Induction motor?

Advantages:

Ø  It is possible to speed control by regulating rotor resistance.

Ø  High starting torque of 200 to 250% of full load voltage.

Ø  Low starting current of the order of 250 to 300% of the full load current.

Hence slip ring induction motors are used where one or more of the above requirements are to be met.

16. What is the function of slip-rings in 3-phase induction motor?

The function of slip rings in the case of slip ring induction motor is to add external resistance in the Rotor circuit. If External resistances are added in the rotor circuit, it can be used for speed control operation.

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

In shaded pole motors, the necessary phase-splitting is produced by induction. These motors have salient poles on stator and a squirrel cage type rotor. The poles are shaded i.e.; each pole carries a copper band one of its unequally divided part called shading band.

When single phase A.C. supply is given to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated.

18. What are the methods of starting of synchronous motor?

The various methods of starting of synchronous motor are:

i) Using pony motors

ii)  Using damper winding

iii) As a slip ring induction motor

iv)Using small D.C. machine coupled to it.

19. What is meant by synchronous motor?

A motor having a speed directly proportional to the frequency of the AC power that operates it.

20. Write the use of damper winding?

Damper windings help the synchronous motor to start on its own (self starting machine) by providing starting torque.

21.            Compare salient pole rotor and cylindrical  rotor. Salient Pole Synchronous Alternator:

Ø Salient pole Generators will have large diameter and short axial length

Ø Pole shoes cover 2/3 of the pitch

Ø Salient Poles are laminated in order to reduce eddy currents

Non-Salient pole Synchronous Alternator:

Ø Non-Salient pole generators will have smaller diameter and longer axial length

Ø They are used for High speed operation (typically speed will be 1500 and 3000 rpm)

Ø Better in dynamic balancing because of absence of salient poles.