The double squirrel cage induction motor is characterized by two sets of squirrel cages on the rotor, namely, the upper squirrel cage and the lower squirrel cage.

The cross-sectional area of the upper squirrel cage guide bar is small, and it is usually made of materials with higher resistance coefficient, such as brass or aluminum bronze, so the resistance is larger; the lower squirrel cage guide strip has a larger cross-sectional area and is made of copper with lower resistance coefficient, so the resistance is smaller; usually, the two sets of squirrel cages have their own end rings, which can make the upper squirrel cage and the lower squirrel cage elongate independently due to heat.

Cast aluminum rotors are also commonly used in double squirrel cage motors, and the end rings of the upper and lower cages are common.

For the double squirrel cage rotor, the leakage flux of the lower cage is much more than that of the upper cage, so the leakage reactance of the lower cage is much larger than that of the upper cage.

During starting, the rotor current frequency is higher, the rotor leakage reactance is greater than the resistance, and the current distribution between the upper and lower cages depends on the leakage reactance.

As the leakage reactance of the lower cage is much larger than that of the upper cage, the current mainly flows from the upper cage.

Therefore, the cage plays a major role in starting, because of its large resistance, it can produce a larger starting torque, so it is often called the starting cage.

During the normal operation of the motor, the rotor current frequency is very low, and the rotor leakage reactance is much smaller than the resistance, so the current distribution between the upper and lower cages will depend on the resistance. most of the rotor current flows through the lower cage with low resistance, resulting in electromagnetic torque in normal operation, so the lower cage is called the running cage.

The double squirrel cage motor has a large starting torque and can generally start with rated load; at the same time, it also has a higher speed under rated load, that is, it has a smaller slip rate, so it has better performance.

In particular, the parameters of the upper and lower cages can be flexibly changed by changing the geometric dimensions of the upper and lower cages, the materials used and the gap size between the upper and lower cages, so as to obtain different torque characteristics to meet the needs of different loads. this is an outstanding advantage of double squirrel cage motor.

However, because the rotor leakage reactance of the double squirrel cage is larger than that of the ordinary squirrel cage, the power factor and maximum torque of the double squirrel cage motor are also slightly lower than that of the ordinary squirrel cage motor.

In order to improve the performance of the squirrel cage induction motor, in addition to the deep slot and double squirrel cage rotors mentioned above, other rotor grooves are often used, that is, the skin effect is used to increase the rotor resistance during starting, so as to improve the starting performance of the motor.