The Selection Process of Motor Rotor Slots: Four Performance-Oriented Trade-offs
The shape and size of the rotor slots have a direct impact on rotor resistance and magnetic leakage, which in turn affect various key performance indicators of the motor, such as efficiency, power factor, maximum torque, and starting torque. These performance aspects are critical to the overall quality of motor products. In practical applications, often one performance aspect has to be prioritized at the expense of others. The old adage, “you can’t have your cake and eat it too,” fits this situation quite well. Of course, revolutionary breakthroughs in new materials and processes can temporarily break this rule. For example, the initial application of the new insulation system in high-voltage motors using "mica tape with minimal resin powder" as the main material and the "vacuum pressure impregnation" technique achieved both reduced insulation thickness and improved voltage and corona resistance. However, the rule still holds in most cases, and these trade-offs or dilemmas are inevitable.
Trade-off between Starting Performance and Overload Capacity To increase a motor's overload capacity, the maximum torque must be increased, which requires reducing rotor leakage reactance. However, to achieve a low starting current and high starting torque, it is necessary to maximize the rotor's skin effect, which inevitably increases rotor slot magnetic leakage and leakage reactance.
Trade-off between Efficiency and Starting Performance Increasing rotor resistance can improve motor starting performance, for instance, by reducing rotor slot size or using a double-cage rotor. However, the increase in rotor resistance and leakage reactance leads to a significant increase in copper loss in both the stator and rotor, thereby reducing efficiency.
Trade-off between Power Factor and Starting Performance To improve the motor’s starting performance, methods such as utilizing the skin effect, deep narrow slots, convex slots, blade-shaped slots, deep slots, or double-squirrel cage slots are used to increase rotor resistance during starting. However, these measures directly increase rotor slot magnetic leakage and leakage inductance, resulting in an increase in reactive current, which in most cases leads to a decrease in the power factor.
Trade-off between Efficiency and Power Factor If the rotor slot area is increased, resistance decreases, reducing rotor copper loss and thus improving efficiency. However, as the cross-sectional area of the rotor yoke for magnetic conduction decreases, magnetic reluctance increases, the magnetic flux density rises, and core losses increase, leading to a drop in power factor. For many motors optimized for efficiency, this phenomenon is often observed: while efficiency improves significantly, rated current increases, power factor drops, and customers may complain that high-efficiency motors are inferior to standard motors.
Motor design inherently involves many trade-offs. This article addresses only those trade-offs reflected in the external characteristics. To balance these performance relations requires deeper exploration of intrinsic characteristics and the skilled application of iterative thinking to balance the trade-offs, thereby resolving so-called dilemmas or contradictions.