The type and size of the rotor slots have a significant impact on motor performance, sometimes directly determining the motor's application characteristics. From a geometric perspective, the tooth width and yoke height of the rotor slots must be matched to ensure that the magnetic circuit saturation level is basically consistent across all sections, while also considering the requirements of manufacturing processes, core mechanical strength, and the rigidity of various parts. Regarding the setting of specific practical parameters:
1) The shape or type of the slots is related to the application characteristics of the motor. For example, the ratio of the variation in the width of the rotor slots along the slot height and the matching of slot heights will directly affect the overall performance level of the motor;
2) The size of the slot depends on the conductor current, and the slot shape 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 a low current density mean low rotor resistance, high efficiency and low heat generation during stable operation, but low starting torque. When the rotor slot width-to-height ratio is small, or when convex or knife-shaped slots with abrupt changes in cross-section are selected, the skin effect can be utilized to maximize rotor resistance and increase starting torque during startup, while ensuring sufficiently low rotor resistance and high efficiency during stable operation. In fact, the reason why the rotor slot shapes of motors differ so greatly for different application conditions is based on the above theory, aiming to optimize application characteristics.
Comparative Analysis of Two Extreme Design Schemes
The two extreme design schemes can clearly reflect the relationship between rotor slot shape and overall motor performance.
First, double-cage motors typically have a smaller upper cage cross-section and a larger lower cage cross-section. During startup, the skin effect is significant, with current primarily carried by the upper cage. The lower cage has a very large leakage flux and a very small current, resulting in very high rotor resistance and consequently, high starting torque. During stable operation, the skin effect is negligible due to the low frequency of the rotor current. Both cages share the current-carrying capacity, leading to low rotor resistance, low losses and heat generation, and improved motor efficiency. Although the double-cage structure can compensate for performance deficiencies to some extent, the efficiency and power factor of this type of motor are still relatively low. Except for heavy-duty starting equipment such as mining tunneling machines, this design is rarely used.
Secondly, among all types of rotor slots, the single-cage pear-shaped slot rotor has the best operating characteristics, but also the worst starting performance. However, due to the development of power electronics technology, variable frequency motors powered by frequency converters are becoming increasingly common. This is because the starting performance of the single-cage pear-shaped slot rotor motor can be compensated for by the soft start of the frequency converter, which can meet most application conditions.
In summary, the overall performance of a motor is closely related to the rotor slot shape, and the selection of the rotor slot shape often needs to be adjusted appropriately depending on the actual application objectives.
