The objective existence of teeth and slots in the stator and rotor cores of the motor, the non-orthodox waves of the winding magnetic potential in the space distribution, and the uneven saturation of the magnetic circuit system all lead to the existence of high-order harmonic magnetic fields in the air gap magnetic field of the motor to varying degrees. High-order harmonic magnetic fields will produce additional torques that are not conducive to the performance of the motor. One type of additional torque is asynchronous additional torque and the other is synchronous additional torque. Among the asynchronous additional torques, the fifth and seventh harmonic magnetic fields have a greater impact; asynchronous additional torque will cause the torque characteristic curve of the motor to be distorted and the starting performance of the motor to deteriorate. If the pole pair number of a harmonic magnetic field established by a certain harmonic magnetic field of the stator inducing the rotor current is equivalent to the pole pair number of another harmonic magnetic field, then at a certain specific speed, the two stator and rotor magnetic fields with equal pole pairs rotate synchronously in space and are in a relatively static state, for which a synchronous additional torque equivalent to a synchronous motor is generated; if the synchronous additional torque happens to occur in a specific state of zero speed, the motor may not be able to start.

In order to effectively weaken the influence of additional torque on motor performance, necessary adjustments and controls are generally carried out based on the basic starting point of reducing and eliminating high-order harmonics. Stator or rotor skew slots are one of the more commonly used measures. From the perspective of the convenience of the manufacturing process, most rotors use skew slots; especially in the control of motor electromagnetic noise. However, due to the use of skew slots, the power factor and maximum torque of the motor will deteriorate due to the increase in leakage reactance. Adjusting the air gap between the stator and the rotor is also a relatively easy measure to take. By adjusting the unevenness of the stator and rotor air gaps, or directly increasing the air gaps between the two, it will also have a better effect; but when this measure is taken, it will lead to the adverse effects of increased motor excitation current and deterioration of power factor. In the motor design scheme, fractional slot windings should be avoided as much as possible; the use of short-distance and integer slot windings can effectively avoid fifth and seventh high-order harmonics; the reasonable selection of stator and rotor slots can eliminate the influence of synchronous additional torque.

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