How does the rotor magnetisation process affect NVH performance?

As a core component of new energy vehicles, the performance of electric motors is directly linked to the vehicle’s overall energy efficiency and driving experience, with noise, vibration and harshness (NVH) serving as key indicators of motor quality. Rotor magnetisation is the process of using an external strong pulsed magnetic field to align the magnetic domains within permanent magnet materials (such as neodymium-iron-boron) and maintain residual magnetism. It is a critical stage in the entire design and manufacturing process of Permanent Magnet Synchronous Motors (PMSMs) and Brushless DC Motors (BLDCs). It has a significant impact on motor performance, particularly in terms of NVH performance, which is primarily reflected in the following aspects:

1. Magnetic field uniformity determines noise levels

If the magnetisation process results in an uneven magnetic field distribution on the surface of the rotor poles (such as local fluctuations in magnetic flux density or abnormal harmonic components), it will cause distortion of the air-gap magnetic field. This distortion causes the electromagnetic force waves generated by the interaction between the stator and rotor magnetic fields to exhibit non-sinusoidal characteristics, increasing high-frequency electromagnetic noise. Particularly at specific rotational speeds, this is prone to resonance with the motor’s natural frequency, amplifying vibration and noise.

2. Magnetisation methods and parameters influence vibration amplitude

Comparison of pre-magnetisation and post-magnetisation: ‘Magnetisation before assembly’ is prone to polarity misalignment or magnetic field distortion due to repulsive forces between magnets; whereas ‘assembly followed by overall magnetisation’ (post-magnetisation technology) ensures a more uniform magnetic field distribution and accurate polarity, effectively reducing the rate of air-gap magnetic flux density distortion and thereby minimising electromagnetic noise.

The critical role of coil design: If the coil structure is unreasonable or axial coverage is insufficient, local fluctuations may occur in the surface magnetic field curves of each layer of the rotor core. This is particularly evident in a significant increase in the amplitude of higher harmonics (such as the 7th harmonic), which directly excites electromagnetic noise of the corresponding order (such as 72nd-order noise).

Matching requirements for magnetisation parameters: The magnetisation voltage determines the magnetic field strength. Failure to meet the material’s saturation magnetisation standard will result in insufficient magnetic flux density, exacerbating torque ripple and inducing vibration, which directly compromises NVH performance.

3. Advantages of suppressing electromagnetic noise at source

Optimisation of the magnetisation process differs from subsequent structural improvements (such as the addition of damping materials) or control strategy optimisation (such as harmonic current injection) in that it attenuates harmful harmonics at the source of the magnetic field, yielding a more direct and fundamental effect. By precisely controlling the distribution of the magnetising magnetic field, key harmonics that cause noise can be significantly reduced, thereby minimising the transfer of vibration energy to the vehicle body. This is also the core value of this process as a key influencing factor in NVH.

In summary, optimising the rotor magnetisation process is a key method for enhancing the NVH performance of electric motors. By improving the magnetic field distribution, reducing harmonic content, ensuring pole alignment and maintaining stable magnetic performance, it is possible to effectively reduce electromagnetic noise and vibration, thereby enhancing ride comfort.