Characteristics Of Double Squirrel-Cage Motors

In motor design, manufacturers shall strive for simplified production while ensuring all motor performance indicators meet specified requirements. Nevertheless, necessary structural measures are often adopted in the design phase to satisfy special operating conditions.

For squirrel-cage motors, starting performance is always a critical technical indicator, and it is essential to properly balance starting performance with other operational parameters. We have previously introduced deep-bar rotors as a method to optimize the starting performance of squirrel-cage induction motors. This article focuses on double squirrel-cage rotor motors.

A double squirrel-cage rotor consists of two independent cages: an inner cage known as the running cage and an outer cage serving as the starting cage, with the outer cage adopting conductor bars of a smaller cross-sectional area. Two mainstream types of double squirrel-cage rotors are widely applied. The first is the copper-bar double cage, in which the starting cage bars are made of brass or bronze with a high resistivity, while the running cage bars use pure copper. The second type is the die-cast aluminum double squirrel-cage rotor.

The double squirrel-cage motors leverage the skin effect to improve starting performance. During motor startup, the majority of current flows through the outer cage. Benefiting from the specific cross-sectional size and material properties of outer cage bars, the starting current is effectively suppressed, and a high starting torque is guaranteed.

Double squirrel-cage motors deliver favorable operating characteristics with a high starting torque, enabling direct startup under rated load while maintaining relatively high rotational speed at rated operating conditions.

Compared with deep-bar rotors, the radial height between the inner and outer cages, as well as the cross-sectional area and geometric shape of the two sets of rotor bars in double squirrel-cage structures, can be flexibly adjusted to optimize motor performance parameters and fulfill design requirements for various load characteristics, which constitutes the core advantage of this motor type.

However, the double squirrel-cage motors rotor features relatively large leakage reactance, which inevitably leads to a certain reduction in the motor's power factor and maximum torque.

In addition to the typical deep-bar and double squirrel-cage rotors, other rotor slot configurations are also adopted to enhance the starting performance of squirrel-cage motors, including bottle-shaped slots with narrow inlets and wide bottoms, trapezoidal slots and convex-shaped slots.

The above content mainly compares different rotor slot design schemes. From the perspective of manufacturing processes, theoretical performance advantages cannot always be fully realized in actual production. In the past, motors were produced according to unified design standards, and any design revision was treated with extreme caution. Even under such standardized specifications, finished motors still exhibited inconsistent quality levels. Similarly, any slot structure optimization intended to improve motor starting performance must be compatible with the production process capability and equipment conditions of the manufacturer.