The performance, shape and tolerance of permanent magnet material directly affect the performance of permanent magnet motor

The performance of permanent magnet materials (such as remanence, coercivity, etc.) and the geometry of permanent magnet materials (such as thickness, width, chamfering, etc.) directly affect the performance of permanent magnet motor. The following are described separately.

Relationship between magnetic steel performance and motor performance

1) The influence of remanence
For DC motor, under the same winding parameters and test conditions, the higher the remanence, the lower the no-load speed and the lower the no-load current; the greater the maximum torque, the higher the efficiency of the highest efficiency point. In the actual test, the remanence standard of magnetic steel is generally judged by the no-load speed and the maximum torque.
For the same winding parameters and electrical parameters, the reason why the higher the remanence, the lower the no-load speed and the smaller the no-load current is that the motor in operation generates enough reverse induced voltage at a relatively low speed, so that the algebraic sum of the electromotive force applied on the winding decreases.
2) Influence of coercivity
In the process of motor operation, there are always the influence of temperature and reverse demagnetization. From the point of view of motor design, the higher the coercivity is, the smaller the thickness direction of the magnetic steel can be. The smaller the coercivity is, the larger the thickness direction of the magnetic steel is. However, if the coercivity of the magnet steel exceeds a certain value, it is useless, because other components of the motor can not work stably at that temperature. The coercivity can meet the demand, and the standard is to meet the demand under the recommended experimental conditions, so there is no need to waste resources.
3) The influence of squareness
Squareness only affects the straightness of motor performance test efficiency curve. Although the straightness of motor efficiency curve has not been listed as an important index standard at present, it is very important for the driving distance of hub motor under natural road conditions. Because of the different road conditions, the motor can not always work at the maximum efficiency point, which is one of the reasons why the maximum efficiency of some motors is not high, but the driving distance is far. A good hub motor, not only the maximum efficiency should be high, but also the efficiency curve should be as level as possible, the smaller the slope of efficiency reduction, the better. With the maturity of the market, technology and standard of hub motor, it will gradually become an important standard.
4) Impact of performance consistency
Inconsistent remanence: even some of them with high performance are not good, because the magnetic flux of each unidirectional magnetic field section is inconsistent, resulting in asymmetric torque and vibration.
The coercivity is not consistent: especially the coercivity of individual products is too low, it is easy to produce reverse demagnetization, resulting in the magnetic flux of the magnetic steel is not consistent, so that the motor vibrates. This effect is more significant for brushless motor.

Influence of magnetic steel geometry and tolerance on motor performance

1. Influence of magnetic steel thickness
When the inner or outer magnetic circuit is fixed, with the increase of thickness, the air gap decreases, the effective flux increases, the same remanence, the no-load speed decreases, the no-load current decreases, and the maximum efficiency of the motor increases; however, there are also disadvantages, such as the increase of commutation vibration and the relative steepness of the efficiency curve. Therefore, the thickness of motor magnetic steel should be as consistent as possible to reduce vibration.
2. Influence of magnetic steel width
For the densely distributed brushless motor magnets, the total cumulative gap should not exceed 0.5mm. If the gap is too small, it can not be installed. If the gap is too small, the motor vibration and efficiency will be reduced. This is because the position of the hall element measuring the position of the magnet does not correspond to the actual position of the magnet. In addition, it is necessary to ensure the consistency of the width, otherwise the efficiency of the motor is low and the vibration is large.
For the brush motor, there is a certain gap between the magnets, which is left for the mechanical commutation transition zone. Although there is a gap, most manufacturers have strict magnetic steel installation tooling to ensure the accuracy of installation in order to ensure the accurate installation position of motor magnetic steel. If the width of the magnetic steel is too wide, it will not be installed; if the width of the magnetic steel is too small, it will lead to inaccurate positioning of the magnetic steel, increase the vibration of the motor and reduce the efficiency.
3. Influence of chamfering size and non chamfering on magnetic steel

If there is no chamfering, the change rate of the magnetic field at the edge of the magnetic field of the motor is large, which causes the pulse vibration of the motor. The larger the chamfering is, the smaller the vibration is. However, chamfering generally has a certain loss of magnetic flux. For some specifications, when chamfering to 0.8, the loss of magnetic flux is 0.5 ~ 1.5%. When the remanence of the brush motor is low, reducing the chamfering size is helpful to compensate the remanence, but the pulsation of the motor increases. Generally speaking, when the remanence is low, the tolerance in the length direction can be appropriately enlarged, which can improve the effective flux to a certain extent and keep the performance of the motor unchanged.

Source: China Permanent Magnet Manufacturer www.ymagnet.com

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