Double Cage Induction Motor – Introduction

Understanding Double Cage Induction Motor:

In this small post we will discuss about the Double cage Induction motor in brief.

What is meant by Double Cage Induction Motor?

As the name indicates, The Double Cage Induction Motors have two cages in the rotor and they have the advantages of both squirrel-cage induction motors and wound rotor induction motors.

Let us refresh the advantages of  squirrel-cage induction motor and wound rotor induction motors.
The advantages of squirrel-cage motors compared with the wound rotor type are that they:
(i) are cheaper and more robust
(ii) have slightly higher efficiency and power factor
(iii) are explosion-proof, since the risk of sparking is eliminated by the absence of slip rings and brushes.

The advantages of the wound rotor motor compared with the cage type are that they:
(i) have a much higher starting torque
(ii) have a much lower starting current
(iii) have a means of varying speed by use of external rotor resistance.

  • This type of induction motor is specially constructed with the rotor having two cages, one inside the other.
  • The outer cage has high resistance conductors so that maximum torque is achieved at or near starting.
  • The inner cage has normal low resistance copper conductors but high reactance since it is embedded deep in the iron core.
  • The torque-speed characteristic of the inner cage is that of a normal induction motor, as shown in below Figure.
  • At the time of starting, the outer cage produces the torque, but the running condition, the inner cage produces the torque.
  • The combined characteristic of inner and outer cages is shown in below figure
  • The double cage induction motor is highly efficient when running.

Torque Speed characteristic of Double Cage IM

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