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The development of permanent magnet motors is closely related to the development of permanent magnet materials

The first motor in the world that appeared in the 1820s was a permanent magnet motor that generates an excitation magnetic field from a permanent magnet. However, the permanent magnet material used at that time was natural magnetite (Fe3O4), which had a very low magnetic energy density. The motor made from it was bulky and was soon replaced by an electric excitation motor.

With the rapid development of various motors and the invention of current magnetizers, people have conducted in-depth research on the mechanism, composition and manufacturing technology of permanent magnet materials, and successively discovered carbon steel and tungsten steel (the maximum magnetic energy product is about 2.7 kJ/m3 ), cobalt steel (maximum magnetic energy product is about 7.2kJ/m3) and many other permanent magnet materials. Especially the AlNiCo permanent magnets (maximum magnetic energy product up to 85kJ/m3) that appeared in the 1930s and ferrite permanent magnets (maximum energy product up to 40kJ/m3) that appeared in the 1950s have great magnetic properties To improve, various micro and small motors have used permanent magnets for excitation. However, the coercivity of AlNiCo permanent magnets is low (36～160 kA/m), and the remanence density of ferrite permanent magnets is not high (0.2～0.44 T), which limits their application range in motors. Until the 1960s and 1980s, samarium cobalt permanent magnets and neodymium iron boron permanent magnet materials came out one after another. Their high remanence, high coercivity, high energy product and excellent magnetic properties of linear demagnetization curve are particularly suitable for Manufacturing motors, so that the development of permanent magnet motors has entered a new historical period.

The relationship between magnetic steel performance and motor performance

1) The influence of remanence

For DC motors, 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 at the highest efficiency point. In the actual test, the no-load speed and the maximum torque are generally used to judge the remanence standard of the magnet.

For the same winding parameters and electrical parameters, the reason why the higher the remanence, the lower the no-load speed and the lower the no-load current, is that the running motor produces sufficient reverse inductance at a relatively low speed The voltage is generated so that the algebraic sum of the electromotive force applied to the winding is reduced.

2) The influence of coercivity

In the process of motor operation, there is always the influence of temperature and reverse demagnetization. From the perspective of motor design, the higher the coercive force, the smaller the thickness direction of the magnet can be. The smaller the coercive force, the larger the thickness direction of the magnet. But after the magnetic steel exceeds a certain coercive force, it is useless, because other components of the motor cannot work stably at that temperature. The coercivity is sufficient to meet the demand, and the standard is to meet the demand under the recommended experimental conditions, and there is no need to waste resources.

3) The influence of squareness

The squareness only affects the straightness of the efficiency curve of the motor performance test. Although the straightness of the motor efficiency curve has not been listed as an important index standard, it is very important for the continuous travel distance of the in-wheel motor under natural road conditions. important. Because of the different road conditions, the motor cannot always work at the maximum efficiency point. This is one of the reasons why some motors have a low maximum efficiency and a long running distance. A good in-wheel motor should not only have high maximum efficiency, but also the efficiency curve should be as level as possible, and the slope of efficiency reduction should be as small as possible. As the market, technology and standards of in-wheel motors mature, this will gradually become an important standard.

4) The impact of performance consistency

Inconsistent remanence: even some of the ones with particularly high performance are not good, because the magnetic flux of each unidirectional magnetic field is inconsistent, resulting in asymmetry of torque and vibration.

Inconsistent coercivity: In particular, if the coercivity of individual products is too low, it is prone to reverse demagnetization, resulting in inconsistent magnetic fluxes of each magnet and causing the motor to vibrate. This effect is more significant for brushless motors.

The influence of magnet geometry and tolerance on motor performance

1. The influence of magnet thickness

When the inner or outer magnetic circuit ring is fixed, when the thickness increases, the air gap decreases and the effective magnetic flux increases. With 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 increased commutation vibration of the motor, the efficiency curve of the motor is relatively steep. Therefore, the thickness of the motor magnet should be as consistent as possible to reduce vibration.

2. The influence of magnetic steel width

For close-packed brushless motor magnets, the total cumulative gap cannot exceed 0.5 mm. If it is too small, it will not be installed. If it is too small, it will cause the motor to vibrate and reduce the efficiency. This is due to the position and magnetic The actual position of the steel does not correspond. In addition, the width must be consistent, otherwise the efficiency of the motor will be low and the vibration will be large.

For brushed motors, there is a certain gap between the magnets, which is left to the transition zone of mechanical commutation. Although there is a gap, most manufacturers have strict magnetic steel installation tooling to ensure the accuracy of the installation of the motor magnets to ensure the installation accuracy. If the width of the magnet is exceeded, it will not be able to be installed; if the width of the magnet is too small, it will cause the magnet to be misaligned, increase the vibration of the motor and reduce the efficiency.

3. The influence of magnetic steel chamfer size and non-chamfer

If the angle is not chamfered, the rate of change of the magnetic field at the edge of the magnetic field of the motor is large, causing the motor to pulse. The larger the chamfer, the smaller the vibration. But chamfering generally has a certain loss of magnetic flux. For some specifications, when the chamfering reaches 0.8, the magnetic flux loss is 0.5~1.5%. When the residual magnetism of the brushed motor is low, appropriately reducing the chamfer size is beneficial to compensate for the residual magnetism, but the pulsation of the motor increases. Generally speaking, when the remanence is low, the tolerance in the length direction can be appropriately enlarged, so that the effective magnetic flux can be increased to a certain extent, so that the performance of the motor is basically unchanged.

Thank you for reading our article and we hope it can help you to have a better understanding of the most commonly used neodymium motor magnets. If you want to learn more about rare earth motor magnets, we would like to advise you to visit BEARHEART Magnets for more information. 

We can provide high-quality permanent magnets like neodymium magnets, ferrite magnets and magnetic assembly at a very competitive price. Any inquiries and orders are welcomed.