The type and size of the rotor slot have a great impact on the performance of the motor, and sometimes directly determine the application characteristics of the motor. From a geometric perspective, the tooth width and yoke height of the rotor slot must match to ensure that the saturation level of each section of the magnetic circuit is basically consistent, while taking into account the processing technology and the mechanical strength of the core and the stiffness of each part. Specific actual parameter settings:

1) The shape or type of the slot is related to the application characteristics of the motor. For example, the change ratio of the width of the rotor slot along the slot height direction and the combination of the slot height will directly affect the overall performance level of the motor;

2) The size of the slot depends on the conductor current, and the slot size parameters ensure that the magnetic flux density in each part of the magnetic circuit is within a reasonable range.

Taking asynchronous motors as an example, a large effective area of rotor slots and low current density means low rotor resistance, high efficiency and low heat generation during stable operation of the motor, but low starting torque; when the rotor slot aspect ratio is small or a convex or knife-shaped slot with a sudden change in cross section is selected, the skin effect can be used to maximize the rotor resistance at start-up and improve the starting torque, while ensuring that the rotor resistance is small enough and the efficiency is high during stable operation. In fact, the reason why the rotor slot shapes of motors in different application conditions are very different is based on the above theory, which makes the application characteristics tend to be optimal.

The two extreme design schemes can clearly reflect the relationship between the rotor slot shape and the overall performance of the motor.

First, the double-cage motor generally has a small upper cage section and a large lower cage section. During starting, the skin effect is significant, mainly guided by the upper cage. The leakage flux of the lower cage turn chain is very large and the current flowing is very small, resulting in a very large rotor resistance, and thus a very high starting torque; during stable operation, because the frequency of the rotor current is very small, the skin effect can be ignored, and the double cages jointly bear the current-carrying role, so the rotor resistance is small, the loss and heat are small, and the motor efficiency is improved. Although the double-cage structure can make up for the defects in operating performance to a certain extent, the efficiency and power factor of this type of motor are still relatively low. Except for heavy-load starting equipment such as mine boring machines, this design is rarely used.

Second, among all types of rotor slots, the cage pear-shaped slot rotor has the best operating characteristics, but the worst starting performance. However, due to the development of power electronics technology, variable frequency motors powered by inverters are becoming more and more common, because the starting performance of single squirrel cage pear-shaped slot rotor motors can be compensated by inverter soft start, which can meet most application conditions.

In summary, the overall performance of the motor is closely related to the rotor slot shape, and how to select the rotor slot shape often requires appropriate adjustments depending on the actual application objectives.