Two Marks Questions with Answers

Two Marks Questions with Answers

Q. 1 Why is single phase induction motor not self starting ?

Ans. :

According to double revolving field theory, an alternating stator flux can be resolved into two rotating components which rotate in opposite directions i.e. is forward component rotating in anticlockwise direction while ϕ_b is the backward component rotating in clockwise direction. Both components produce rotor flux. The rotor flux interacts with to produce torque in anticlockwise direction while it reacts with ϕ_f to produce torque in clockwise direction. At start these two torques are equal in magnitude but opposite in direction. Thus net torque experienced by the rotor is zero at start. And hence the single phase induction motors are not self starting.

 

Q. 2 Where are split-phase motors used ?

Ans. :

The split-phase motors have low starting current and moderate starting torque. These are used for easily started loads like fans, blowers, grinders, centrifugal pumps, washing machines, oil burners, office equipments etc.

 

Q. 3 List out four applications of single phase induction motor.

Ans. :

The single phase induction motors are used in fans, blowers, toy motors, record players, air circulators, advertising displays, hair dryers etc.

 

Q. 4 Name the starting methods for single phase induction motors. AU : Dec.-08, 09, 12, 15, May-11, 16, 17,18

Ans. :

Depending upon the methods of producing rotating stator magnetic flux, the single phase induction motors are classified as,

1. Split phase induction motor        

2. Capacitor start induction motor

3. Capacitor start capacitor rim induction motor

4. Shaded pole induction motor.

 

Q. 5 State the limitations of shaded pole single phase induction motor.

Ans. :

1. The starting torque is poor.

2. The power factor is very low.

3. Due to copper losses in the shading ring the efficiency is very low.

4. The speed reversal is very difficult.

5. The size and power rating is very small.

 

Q. 6 Draw the equivalent circuit of single phase induction motor without core losses.

 

Q. 7 Draw the equivalent circuit of single phase induction motor with core losses.

 

Q. 8 Draw the torque speed characteristics of split phase induction motor.

 

Q. 9 Draw the torque speed characteristics of capacitor start induction motor.

Q. 10 Draw the torque speed characteristics of shaded pole induction motor.

 

Q.11 Why are centrifugal switches provided on many 1-phase induction motors ? AU : May-12

Ans. :

In many 1-phase induction motors, the starting winding is provided only to make it self starting. Once it is started, there is no need of the starting winding. Hence centrifugal switches are provided on many 1-phase induction motors to cut out the starting winding when it achieves 70 to 80 % of its rated speed.

 

Q. 12 What type of single phase induction motor would you use for the following applications : i) Ceiling fan ii) Wet grinder ?

Ans. :

 i) For ceiling fan, capacitor induction motor is used,

 ii) For wet grinder split phase induction motor or universal motor is used.

 

Q. 13 What is the function of capacitor in a single phase induction motor ? AU : May-03

Ans. :

Due to the capacitor connected in series with the auxiliary winding, the capacitive circuit draws a leading current which increases the phase angle a between two currents Im and I_st. This produces high starting torque. The capacitor also improves the power factor.

 

Q. 14 What is the use of shading coil in the shaded pole motor ?

Ans. :

When a.c. supply is given to the stator winding, it produces alternating flux. Due to this flux, e.m.f. gets induced in the shading coil and current is circulated in the shading band. So shading band also produces its own flux. Depending on the nature of alternating stator flux at various instants, flux produced by shading coil interacts with the stator flux due to which overall magnetic axis keeps on shifting. This produces rotating magnetic field necessary to make the motor self starting.

 

Q. 15 Why capacitor-start induction motors are advantageous ?

Ans. :

The starting torque produced is much higher and there is overall improvement in the power factor.

 

Q. 16 List out four applications of shaded pole induction motor ? AU : May-05, 10, Dec.-ll, 16

Ans. :

Toy motors, advertising displays, record players, gramphones, hair dryers and small fans.

 

Q. 17 Draw the vector diagram for (a) capacitor start and (b) split phase induction motor.

 

Q. 18 What are the drawbacks of the presence of the backward rotating field in a single phase induction motor ?

Ans. :

1. It produces a backward torque which is opposite to the forward torque, producing breaking effect.

2. The motors are not self starting.

3. It results into pulsating torque making the motors noisy.

 

Q. 19 Explain how to start the single phase induction motor ? or Why single phase induction motor has two windings on its stator ?

Ans. :

The single phase induction motor is not self starting due to absence of rotating magnetic field. Hence in addition to the main winding, an additional winding called auxiliary winding is provided which is highly capacitive or resistive. The main winding is reactive. Due to this there exists a phase difference between the main winding current and auxiliary winding current. The resultant of the fluxes produced due to these currents is rotating magnetic field. Due to this, e.m.f. gets induced in the rotor which intum produces rotor current and rotor flux which interacts with the main flux to produce the unidirectional torque and motor starts.

 

Q. 20 How do you size the capacitor rating required for an induction motor

Ans. :

The size of the capacitor for an induction motor is decided from the no load reactive kilovoltampers (kVAR), drawn by the motor. This no load kVAR value is obtained only from the no load test on the motor. The capacitor is then selected not to exceed 90 % of the no load kVAR.

For capacitor start induction motor, the typical capacitor value is 300 µF for 0.5H.P. while for capacitor start capacitor ran, it ranges between 20 to 50 µF.

 

Q. 21 How to reverse the direction of rotation of a split-phase motor ?

Ans. :

The direction of rotation of a split-phase motor can be reversed by reversing the terminals of either main winding or auxiliary winding.

 

Q. 22 Define double revolving field theory as applied to single phase induction motor.

Ans. :

According to double revolving field theory, an alternating stator flux can be resolved into two rotating components which rotate in opposite directions i.e. is forward component rotating in anticlockwise direction while ϕb is the backward component rotating in clockwise direction. Both components produce rotor flux. The rotor flux interacts with to produce torque in anticlockwise direction while it reacts with ϕf to produce torque in clockwise direction. At start these two torques are equal in magnitude but opposite in direction. Thus net torque experienced by the rotor is zero at start. And hence the single phase induction motors are not self starting.

 

Q. 23 State any two applications of capacitor start induction motor.

Ans. :

Compressors, fans, blowers, air conditioners, grinders.

 

Q. 24 Which parameters are obtained from the blocked rotor test of a single phase induction motor ?

Ans. :

Stator and rotor reactances are obtained from the blocked rotor test of a single phase induction motor.

 

Q. 25 Why do induction motor runs at low power factor when lightly loaded ? 

Ans. :

At no load an induction motor draws large magnetizing current to create flux in the air gap. This current has very low power factor of the order 0.2 lag. The power is mainly drawn to provide for core loss. As the motor is loaded, it draws a load component of current with much higher power factor. Therefore the power factor of the net current drawn from main increases with load. It rises to 0.8 - 0.85 at full load. At light load the power factor will be slightly more than that at no load.

 

Q. 26 Draw the torque slip characteristics of single phase induction motor.^^^^^^

OR

What are inherent characteristics of plain l-phase induction motor ?

Ans. :

The torque-slip characteristics of single phase induction motor is shown in the Fig. 8.1.

 

Q. 27 What will be the direction of rotation of a shaded pole single phase induction motor ?

Ans. :

The direction of shaded pole single phase induction motor is in the direction specified by the nonshaded part of the pole face to the shaded part of the pole face.

 

Q. 28 How can the direction of a capacitor run motor be reversed ? AU : May-14,16, Dec.-15

Ans. :

The direction of a capacitor run induction motor can be reversed by interchanging the connections of main winding or auxiliary winding.

 

Q. 29 How is the direction of rotation of a single phase induction motor reversed ? AU : Dec.-17

Ans. :

The direction of rotation of a single phase induction motor can be reversed by reversing the direction of rotation of rotating magnetic field produced by main and auxiliary winding. This is possible by interchanging the connections of main winding or auxiliary winding, any one of the two windings.