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The properties, shapes and tolerances of permanent magnet materials directly affect the performance of permanent magnet motors

The performance of permanent magnet materials (such as remanence, coerce, etc.) and the geometric shape of permanent magnet materials (such as thickness of magnetic steel, width of magnetic steel, chamfer of magnetic steel, etc.) directly affect the performance of permanent magnet motors. The following are described separately.


Relationship between magnetic steel performance and motor performance

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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 higher the maximum torque, the higher the efficiency of the maximum efficiency point. In the actual test, the remanence standard of magnetic steel is generally determined by the no-load speed and the maximum torque.


For the same winding parameters and electrical parameters, the reason why the higher the remanence is, the lower the no-load speed is, and the lower the no-load current is, is that the motor in operation, at a relatively low speed, will produce enough reverse induced voltage, so that the electromotive force applied on the winding will be reduced.


2) The influence of coercivity


In the process of motor operation, there is always the influence of temperature and reverse demagnetic field. From the point of motor design, the higher the coerced force is, the smaller the thickness direction of the magnetic steel can be. The smaller the coerced force is, the larger the thickness direction of the magnetic steel can be. But magnetic steel is useless beyond a certain coerage, because the other components of the motor cannot work steadily at that temperature. Coercive force is sufficient to meet the requirements. It is standard to meet the requirements under the recommended experimental conditions. There is no need to waste resources.


3) Influence of square degree


The square degree only affects the flatness of the motor performance test efficiency curve. Although the flatness of the motor efficiency curve has not been listed as an important index standard at present, it is very important for the hub motor's running distance under natural road conditions. Because of different road conditions, the motor cannot always work at the maximum efficiency point, which is one of the reasons why the maximum efficiency of some motors is not high and the travel distance is longer. A good hub motor should not only have a high maximum efficiency, but also the efficiency curve should be as horizontal as possible. The lower the slope of the efficiency reduction, the better. This will gradually become an important standard as the hub motor market, technology, and standards mature.


4) The impact of consistency of performance


Inconsistency of remanence: even those with extremely high performance are not good, because the magnetic flux of each one-way magnetic field section is inconsistent, resulting in the vibration of asymmetric torque.


Unconformity of coerced force: especially the coerced force of individual products is too low, easy to produce reverse demagnetization, leading to different magnetic steel magnetic flux resulting in motor vibration. This effect is more significant for brushless motors.


Influence of geometrical shape and tolerance of magnetic steel on motor performance


1. Influence of magnetic steel thickness


When the inner or outer magnetic circuit is fixed, when the thickness increases, the air gap decreases and the effective magnetic flux increases. With the same magnetic remanence, the no-load speed decreases, the no-load current decreases and the maximum efficiency of the motor increases. However, there are also adverse aspects, such as the increase of commutating vibration of the motor, the motor efficiency curve is relatively steeper. Therefore, the thickness of magnetic steel should be as consistent as possible to reduce vibration.


2. Influence of magnetic steel width


For the magnetic steel of brushless motor with dense row distribution, the total cumulative clearance should not exceed 0.5 mm, too small can not be installed, too large will cause motor vibration and efficiency reduction, this is because the position of hall element measuring the position of magnetic steel does not correspond to the actual position of magnetic steel. In addition, the consistency of the width must be guaranteed, otherwise the efficiency of the motor is low and the vibration is large.


For brush motor, there is a certain gap between the magnetic steel, is reserved for the mechanical transition zone. Although there is a gap, but most manufacturers in order to ensure the motor magnetic steel installation position accurate, have a strict magnetic steel installation tools, to ensure the installation accuracy. If the width of the magnetic steel exceeds, it will not be installed. If the magnetic steel width is too small, it will cause the positioning misalignment of the magnetic steel, increase the vibration of the motor and reduce the efficiency.


3. Influence of chamfer size and chamfer of magnetic steel


If the Angle is not avoided, the change rate of magnetic field along the magnetic field edge of the motor is large, resulting in pulse vibration of the motor. The larger the chamfer, the smaller the vibration. However, general chamfering has a certain loss of magnetic flux. For some specifications, when chamfering reaches 0.8, the loss of magnetic flux is 0.5~1.5%. When the remanence of brush motor is too low, appropriately reducing the chamfer size is beneficial to compensate the remanence, but the pulse vibration of the motor increases. Generally speaking, when the remanence is too low, the tolerance of length direction can be appropriately enlarged, so as to improve the effective magnetic flux to a certain extent and keep the performance of the motor basically unchanged.


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