Impact of Magnet Shape and Tolerances on Permanent Magnet Motor Performance

1. Effect of Magnet Thickness: When the internal or external magnetic circuit is fixed, increasing the thickness reduces the air gap, thereby increasing the effective magnetic flux. This results in a lower no-load speed and reduced no-load current under the same residual magnetism, leading to higher maximum efficiency of the permanent magnet motor. However, there are downsides, such as increased commutation vibration and a steeper efficiency curve. Therefore, the thickness of the magnets in permanent magnet motors should be as consistent as possible to reduce vibration.
2. Effect of Magnet Width: For densely arranged brushless motor magnets, the total cumulative gap should not exceed 0.5 mm. If the gap is too small, installation becomes difficult; if too large, it can cause motor vibration and reduced efficiency. This is because the position of the Hall elements measuring the magnet’s location may not correspond to the actual position of the magnets. It is essential to ensure width consistency; otherwise, the motor's efficiency will decrease, and vibration will increase. For brushed motors, there is usually a gap between the magnets to allow for the mechanical commutation transition zone. Even with the gap, most manufacturers have strict installation procedures to ensure accurate placement of the magnets. If the width of the magnets is too large, installation will be impossible; if too small, the positioning will be inaccurate, leading to increased vibration and decreased efficiency.
3. Effect of Chamfer Size and Lack of Chamfering: If the magnet is not chamfered, the magnetic field variation at the edge of the permanent magnet motor's magnetic field will be significant, causing torque ripple. The larger the chamfer, the less vibration. However, chamfering generally results in some loss of magnetic flux. For some specifications, when the chamfer reaches 0.8 mm, magnetic flux loss can range from 0.5% to 1.5%. For brushed motors with low residual magnetism, reducing the chamfer size can help compensate for the residual magnetism, though it may increase torque ripple. Generally, when residual magnetism is low, increasing the tolerance in the length direction can improve effective magnetic flux and maintain motor performance.
4. Effect of Residual Magnetism: For DC motors, under the same winding parameters and test conditions, higher residual magnetism leads to a lower no-load speed and reduced no-load current. This results in a higher maximum torque and efficiency at the peak efficiency point. In practical testing, the standard for residual magnetism is usually judged by the no-load speed and the maximum torque. Under the same winding and electrical parameters, higher residual magnetism causes the motor to generate sufficient back EMF at a lower speed, reducing the algebraic sum of the EMF applied to the windings.
5. Effect of Coercivity: During motor operation, there is always the issue of temperature and reverse demagnetization. From a motor design perspective, higher coercivity allows for a smaller thickness direction in the magnet, while lower coercivity requires a greater thickness. However, beyond a certain point, increasing coercivity is pointless, as other motor components may not operate stably at those temperatures. If the coercivity meets the requirements, it is recommended to satisfy the needs under experimental conditions without wasting resources.
6. Effect of Squareness: Squareness mainly affects the flatness of the motor performance efficiency curve. Although flatness is not yet a critical standard, it is crucial for the cruising range of hub motors under natural road conditions. Due to varying road conditions, the motor cannot always operate at the point of maximum efficiency. This explains why some motors with lower maximum efficiency may have a longer cruising range. A good hub motor should have high maximum efficiency and as flat an efficiency curve as possible, with minimal slope. As the market, technology, and standards for hub motors mature, this will gradually become an important standard.
7. Effect of Performance Consistency:Inconsistent residual magnetism: Even if some magnets have particularly high performance, it is undesirable, as the inconsistency in magnetic flux across different sections can cause torque asymmetry and vibration.Inconsistent coercivity: Especially when individual magnets have low coercivity, reverse demagnetization may occur, leading to inconsistent magnetic flux across the magnets and causing motor vibration.

This impact is more significant for brushless motors. For brushless motor magnets arranged in dense rows, the total cumulative gap should not exceed 0.5 mm. If the gap is too small, installation becomes difficult; if too large, motor vibration and efficiency reduction occur due to the misalignment between the Hall elements measuring the magnet position and the actual magnet position. Additionally, width consistency must be ensured; otherwise, motor efficiency decreases, and vibration increases.