1. Stator and rotor slot matchingForce waves with low vibration order and large amplitude play a major role in the vibration and noise of the motor. When the force wave order is large, it can be ignored; when the force wave order is low, the magnetic field harmonics that generate the force wave are large, and the magnetic field amplitude is small, it can also be ignored. Therefore, from the perspective of reducing force waves and reducing motor noise, it is always hoped that the force wave order is higher.Here we mainly consider the case where the force wave order of the stator-rotor harmonic magnetic field interaction is less than 4, and pay special attention to the effect of the tooth harmonic magnetic field.In a permanent magnet brushless DC motor, the magnetic poles are surface-attached and the rotor has no teeth, so the motor's tooth harmonics are only stator tooth harmonics.In a DC motor, the waveform of the no-load air gap magnetic field is approximately a flat-top wave, which is quite different from the waveform of the air gap magnetic flux density of an induction motor. Using finite element software, the no-load air gap magnetic fields of three motors of the same specifications, 6 poles and 36 slots, 6 poles and 32 slots, and 6 poles and 15 slots, were calculated.Since the stator slots corresponding to each pole of the integer slot motor are exactly the same, while the slot positions corresponding to each pole of the fractional slot motor are different. For the fractional slot motor: when each pole and each phase is an even number, the main pole magnetic field has both odd and even harmonics; when each pole and each phase is an odd number, the main pole magnetic field has only odd harmonics. Since the fractional slot motor has more harmonics than the integer slot motor, the vibration and noise of the motor are increased. In order to reduce the electromagnetic noise of the motor, it is important to choose a reasonable pole slot combination.2. Stator skew slot (skewed pole) structureThe stator skew slots or rotor skew poles will cause the radial force wave to shift in phase along the motor axis, reducing the average radial force along the axial direction, thereby reducing the vibration and noise of the motor.3. Winding structureSelecting appropriate winding pitch and short-pitch coils can reduce the harmonic content and force wave amplitude in the magnetomotive force waveform. There is no third harmonic in the three-phase winding, and the main harmonics are the fifth and seventh harmonics. To eliminate the fifth and seventh harmonics, the pitch is generally selected to be 5/6 times the full pitch, and the pitch of a two-pole motor is 2/3 times the full pitch.4. Air gap sizeProperly increasing the amplitude of the harmonic magnetic field can reduce the electromagnetic noise. At the same time, improving the manufacturing accuracy can reduce the uneven air gap caused by the misalignment of the stator and rotor, reduce the amplitude of the radial force wave, and thus reduce the electromagnetic noise.
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