DOUBLE CAGE INDUCTION MOTOR

Welcome to ELECTRICAL ENCYCLOPEDIA. In this article, we gonna learn about "Double cage induction motor".

CONTENTS

* Need of Double Cage Induction Motor
* Construction of Double Cage Induction Motor
* Working of Double Cage Induction Motor
* Torque-slip characteristics of Double Cage Induction Motor
* Comparison between Single Cage & Double Cage

WHAT IS THE NEED OF DOUBLE CAGE INDUCTION MOTOR?

Why do we require double cage induction motor? Is there any problem with conventional squirrel cage motor?

So answer to the latter question is YES. Conventional squirrel cage motors suffers from the disadvantage of low starting torque.
So, low starting torque is the problem. But this could be achieved by increasing the rotor resistance. It will give higher starting torque as well as low starting current. So why do we need "DOUBLE -CAGE INDUCTION MOTOR".
This is because higher rotor resistance  on one hand increases the starting torque but on the other hand it reduces the full load speed , increases the losses and decreases the efficiency. Thus low rotor resistance is required for running period.

"Summing up, we get conclusion that for good overall performance of Induction Motor, it should have high rotor resistance during starting period and low rotor resistance during running period."

But how can we achieve this, answer to this question is double cage induction motor.

CONSTRUCTION OF DOUBLE-CAGE INDUCTION MOTOR

Stator of double cage induction motor is similar to the ordinary induction motor. But the rotor consists of two windings or cages namely outer cage and inner cage.

Bars of outer cage have small cross sectional area than the inner cage bars and it is made up of high-resistivity materials.

Bars of inner cage are made up of material having low resistance.

Since outer cage is more closer to the stator hence it will have lesser leakage reactance and inner cage is more away from the stator and that's why most of its flux does not links with stator and hence inner cage has higher leakage reactance.

So,
OUTER CAGE - high resistance, low reactance.

INNER CAGE - low resistance, high reactance.

WORKING OF DOUBLE-CAGE INDUCTION MOTOR.

During starting conditions, frequency of induced emf in rotor is same as the supply frequency and leakage reactance of inner cage is very much larger than that of outer cage. Current division of rotor current between two cages depends upon their impedance (i.e. vector sum of resistance and leakage reactance). Therefore most of the starting current flows through outer cage which offers low impedance to the flow of current. The higher resistance outer cage  therefore develops a high starting torque.

As the speed of the rotor increases, the frequency of  rotor emf decreases due to slip. This decrease in frequency makes the leakage reactance of cages negligibly small.

So this time , current division between cages depends upon their resistances. Outer cage is having resistance 5-6 times that of inner cage. So most of the rotor current flows through the inner cage of lower resistance. Hence under normal operating conditions, current flows through low resistance path and therefore it has lower ohmic losses and higher efficiency.

Keep in mind that during starting, current flows through outer cage.

Under normal running conditions , current flows through inner cage.

Thus, problem of higher starting torque is solved by double cage induction motor.

TORQUE-SLIP CHARACTERISTICS OF DOUBLE-CAGE INDUCTION MOTOR

Given below is the fig. showing torque-slip characteristics of double cage induction motor.

TORQUE-SLIP CHARACTERISTICS OF DOUBLE-CAGE INDUCTION MOTOR

It is assumed that two cages develop separate torques. The total torque of the motor is equal to the sum of the torque of each cage.

As we studied above that, at starting (i.e, when slip is 1) current flows through the outer cage and produces higher torque which we can see in the above figure. But as the slip reduces, majority of the current starts to flow through the inner cage.

COMPARISON BETWEEN SINGLE CAGE & DOUBLE CAGE

1. A Double Cage Induction Motor has low starting current and high starting torque. That's why it is suitable for direct-on-line starting.

2. A Double Cage Induction motor has higher overall effective reactance because of inner cage also. Therefore, full-load power factor of the motor is reduced.

3. Higher resistance of Outer Cage increases the effective resistance of the motor which increases the full-load copper losses and thereby decreases the efficiency.

4. By a suitable choice of resistances and reactances of the inner and outer cage, we can obtain a wide range of torque-slip characteristics with double-cage motor which is not possible with single-cage motor.

5. Cost of double cage motor is 20% to 30% higher than the single cage motor of the same rating.

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