Torque Speed Characteristics Of Double Cage Induction Motor

Torque speed characteristics of Double cage induction motor

The main disadvantage of squirrel cage motor is poor starting torque  because of low rotor resistance .The starting torque could be increased by having a cage of  high resistance but then the motor will have poor efficiency under normal running conditions.The difficulty with squirrel cage induction  motor is that its cage is permanently short circuited ,so no external resistance can be introduced . Many efforts have been made to build a squirrel motor with high starting torque without scarifying its electrical efficiency under normal running conditions ,which cannot be possible easily .Two  independent cages on the rotor was tried and a motor called as double cage motor is invented .The outer cage consist of bare of high resistance where as the inner cage has low resistance .Hence outer cage has high resistance and low ratio reactance to reactance where as the inner cage has low resistance but being situated at deep in the rotor .has large ratio of reactance to reactance .Hence the outer cage develops maximum torque at starting while the inner cage about 15%  slip .At starting and at large slip values ,frequency of induced emf in the rotor is high ,so the reactance of inner cage and therefore its impedance are both high .Hence very little current flows in it . Most of starting current is confused to outer cage ,despite its high resistance .Hence the motor develops high starting torque due to high resistance outer cage .
                                               As the speed increase the frequency of rotor emf decreases ,so that the reactance and hence the impedance of inner cage decreases and becomes very small under normal running conditions . Most of the current then flows through it and hence it develops the grater part of the motor torque .When speed is normal ,frequency of the rotor emf is so small that the reactance of both cages is practically negligible .The current is carried by two cages in parallel giving a low combined resistance .Hence it has been made possible to construct a  single machine which has a good starting torque with reasonable starting current and which maintain high efficiency with good speed regulation under operating conditions.