The stator and rotor cores of an electric motor are formed by stacking laminations. Theoretically, the laminations should be stacked neatly to ensure a smooth and uniform core slot shape. However, in actual production, the slot shape of any core will inevitably have varying degrees of irregularity. For the stator core and the rotor core of a wound-rotor motor, the windings are embedded in the core slots. If the slots are not smooth, the cross-sectional area of the core slots will be reduced, making it difficult to embed the windings during processing, and in severe cases, damaging the slot insulation. For cast aluminum rotor cores, the irregularity in the slots, visually speaking, is equivalent to reducing the cross-sectional area of the rotor aluminum bars, which has a significant impact on the motor's performance.
If the rotor core is not neat, for open-slot rotors, obvious serrations or even twisting deformation will be seen at the slot openings after the rotor is turned. For closed-slot rotors, although no obvious defects can be seen from the rotor surface, it actually has a significant impact on the performance of the motor.
Compared to other processes, the aluminum casting process is a relatively hidden project, and the quality of the rotor must be guaranteed by standardized operating procedures and perfect molds.
Why do high-efficiency motors use closed slots?
The squirrel-cage rotor of a conventional squirrel-cage induction motor typically uses semi-closed slots. High-efficiency motors utilize closed slots for two reasons: first, it reduces air gap reluctance, resulting in a smaller excitation current and lower copper losses; second, it effectively reduces additional core losses caused by magnetic harmonics pulsating on the core surface. Clearly, the efficiency improvement effect of these two factors is quite significant, hence manufacturers are widely adopting closed-slot rotors.
Detailed Explanation of Rotor Closed Slots
For motors using the same aluminum casting process, semi-closed slot and closed slot designs represent drastically different design philosophies. So, how exactly do closed slots on the rotor affect various aspects of motor performance? How can we maximize the benefits and minimize the drawbacks to improve motor efficiency?
●Although using closed slots in the rotor can reduce stator copper losses and stray losses in the motor, the reduction is positively correlated with the height of the closed slot arch or the thickness of the magnetic bridge.
● If the rotor closed slot magnetic bridge is too thick, the rotor leakage reactance will increase significantly, the power factor will tend to deteriorate, the load current will increase, the copper and aluminum losses of the stator and rotor will increase, offsetting the efficiency improvement or even causing the efficiency to decrease.
● The rotor uses closed slots, which increases leakage reactance and reduces starting current, but also reduces starting torque and maximum torque. Therefore, when using closed slots, it is necessary to ensure that changes in starting and overload capacity data do not fall below the standard allowable limits.
● The use of closed slots in the rotor reduces noise to a certain extent. It has a particularly significant effect on suppressing electromagnetic noise and vibration.
●If the slip of a closed-slot motor can be maintained at the level of a semi-closed-slot motor, the efficiency improvement will be even greater. Based on specific test data accumulated by various motor manufacturers, closed-slot motors with a bridge arch height or magnetic bridge thickness of approximately 0.2mm show better performance.
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