The ordinary squirrel cage induction motor has excellent running characteristics, but it suffers from the drawback that due to its very low rotor resistance, it’s starting torque is poor.
 
The starting torque can be improved by increasing the rotor resistance but in case of squirrel cage induction motor external resistance can’t be added in the rotor circuit during starting since its rotor is permanently short circuited.
 
If the rotor circuit is made of high resistance to obtain high starting torque, then the motor will have poor efficiency under normal running condition because of more rotor copper losses.
 
However, the starting torque of this motor can be improved by employing another cage on the rotor, and the motor is called double squirrel cage induction motor.
 

Construction of Double Squirrel Cage Induction Motor

 
double squirrel cage induction motor working
 
A double squirrel cage induction motor consists of a rotor which has two independent cages one above the other in the same slot. A double squirrel cage rotor is shown in the figure.

 
The upper slot conductors from the outer cage and the lower slot conductors form the inner cage. The outer cage consists of bars of high resistance, generally bars of brass alloy of a smaller area of cross-section, are employed.
 
The inner cage consists of bars of low resistance generally bars of copper metal of a larger area of cross section, are employed. Since the inner cage is embedded deep in the iron, it has relatively more leakage flux and has high reactance.
 
Thus the outer cage has high resistance and low reactance whereas the inner cage has low resistance and high reactance.
 
A slit is provided between upper and lower slot so that it offers high reluctance for the stator field. Thus the stator field instead of linking with only outer cage, links both the cages simultaneously.
 

Double Squirrel Cage Induction Motor Working

 
The working of double squirrel cage induction motor is as under:

    double squirrel cage induction motor

  • Since the maximum torque produced by each cage is independent of their resistance but inversely proportional to their standstill reactances. Therefore, the value of the maximum torque produced by the inner cage is more than the outer cage.
  •  

  • At the start, the rotor current frequency is equal to the supply frequency; therefore, the reactances (2πfL) of the two cages are high as compared to their resistances.
     
    The division of the rotor current between the cages is determined by their relative impedance (reactance).
     
    Therefore, at the start, comparatively only small current flows through inner cage bars while large current flows through the outer cage in spite of its high resistance.
     
    Thus, at the start the torque is mainly produced by the outer cage.
  •  

  • As the motor picks up speed, the rotor current frequency decreases and is very low at the rated speed. Under this condition, the reactance of the two cages is very, low as compared to their resistances.
     
    Therefore, the division of the rotor current between the cages is determined by their relative resistance and most of the current flows through the outer cage.
     
    Thus, under normal running condition, torque is mainly produced by the inner cage.

 

Induction Motors – 6 | Objective Type Question Answers

 

#1 Which of the following features of induction motor helps in preventing cogging of motor?

Skewed slots

#2 Synchronous wattage of induction motor means

rotor input in watts

#3 If stator voltage of a squirrel cage induction motor is reduced to one-half its normal value, its starting current is reduced to…………. percent of its full voltage value.

50

#4 In a 3-phase induction motor which of the following statements regarding frequency of induced rotor e.m.f. is incorrect ?

Its speed varies inversely as slip.

#5 The stator frame in an induction motor is used

as a return path for the flux

#6 The noise and tooth pulsation losses can be minimized by using

large number of narrow slots in stator

#7 The value of average flux density in air gap in an induction motor should be small

to get good power factor

#8 An induction motor with large number of slots has

high over-load capacity

#9 In an induction motor the pulsation losses and noise can be reduced by using

large number of narrow slots

#10 Whenever any polyphase induction motor is loaded

induced e.m.f  in the rotor increases and its frequency falls

#11 Which of the following motors requires the most complicated speed control arrangement?

Three-phase squirrel cage induction motor

#12 A 3-phase induction motor is running at a load of the rated torque. What happens when one of the outer mains is interrupted while the motor is running?

Motor keeps on running but draws more current.

#13 A 3-phase induction motor is running at a load of the rated torque. What happens when one of the outer mains is interrupted while the motor is running?

Motor keeps on running but draws more current.

#14 What Is the function of putting resistance in parallel to one phase of 3-phase induction motor ?

To achieve a smooth starting torque.

#15 What is the disadvantage of the speed control of slip-ring induction motor with the help of resistances in the rotor circuit ?

This method is associated with high losses.

#16 The reactance per phase as compared to the resistance per phase of an induction motor is

quite high

#17 What is the disadvantage of starting an induction motor with a star-delta starter?

The starting torque is one third of the torque in case of the delta connection.

#18 The induction motors which are provided with open slots have

more break down torque

#19 For a 50 H.P. motor, which type of rotor will be preferred ?

Wound rotor.

#20 If a three phase squirrel cage induction motor runs slow, which of the following could not be the reason for the same ?

High frequency

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