In case of 3Ø squirrel cage motor has low starting torque. This is due to the low Rotor Resistance. Then, the starting torque with increase but at the same time their will be more copper loss and %n of motor will decreases.
The double cage motor is so develop that it overcomes the above difficulties and given good starting torque with good efficiency for that purpose. The Rotor slot and Rotor conductor has modified shape or modified peculiar shape can be draw or can be develop.
It has two cage, one is upper cage near the surface of the Rotor periphery is called as starting cage. The conductor in this starting cage is made up of high Resistance Iron or brass material. The lower cage is located deeply. This cage is called as running cage. It has the conductors made up of low resistance copper.
At starting the sleep speed and hence the frequency of the educed current is high. The lower cage of copper forms lower inductive than the upper brass cage. This is completely surrounded by iron structure of Rotor thus the reactance due to higher Rotor frequency at start is more. At start the impedance of the lower cage is more compare with the upper cage. Hence the lower cage passes small current and more part passes through high resistance outer cage. Hence the starting torque is improved.
But under running condition as motor gain speed the frequency of rotor current become very small so that impedance of lower cage is very less in comparison with the lower cage and lower cage copper conductors take a great share of current and give good running Torque.
Double cage induction motor
Application of double cage induction motor
This type of motor is particularly useful were frequent starting under heavy load is required such as rolling mills, compressors, cranes, convertors, reciprocating pumps etc.