The two most commonly used magnet shapes in motors are arc magnets and multi-pole ring magnets, what are their respective advantages and disadvantages? How to choose? Today I come to the multi-pole magnetic ring and tile combination rotor for comparison and analysis.

1. Structural form and characteristics

Multi-pole magnetic ring and arc magnet combination ring, which can be magnetized in the outer circle or inner circle, can be used as a rotor or stator, and the following is an example of the magnetic ring used as a rotor of a motor with the magnetization of the outer circle to be analyzed. A motor rotor composed of a multi-pole magnetic ring and an arc magnet consists of a magnet, a shaft, and a material for fixing the magnet and the shaft, as shown in following pictures. Comparing the two structures, the multi-pole magnetic ring rotor has the advantages of high mechanical accuracy and good concentricity. Multi-pole magnetic ring can be easily processed by the coreless grinder, the outer circle accuracy can be guaranteed within 0.05mm, which is critical to the efficiency and stability of the motor, while the magnetic tile needs to be a single piece of grinding, and then assembled by a number of pieces, and thus the dimensional accuracy is much lower, and tolerances are often more than double. In addition, multi-pole magnetic ring rotor ring core part, do not have to use the magnetic material, can choose plastic, because in the magnetic ring wall has formed a closed magnetic circuit. The arc magnet combination rotor this part must be selected silicon steel sheet class of magnetically conductive materials, otherwise it will produce serious magnetic leakage, flux loss is obvious, resulting in a reduction in the flux density of the rotor surface.

The following pictures show a multi-pole toroidal rotor and a curved combination rotor.

2. Magnetization orientation and magnetic properties

To manufacture high-performance permanent magnet ferrite multi-pole rings, they must be oriented with a radial multi-pole magnetic field at the press molding stage. The multipole oriented magnetic field can be generated by a pulsed current or by a strong magnet, NdFeB. After rigorous design of the multipole orientation of the magnetic field, can make the magnetization of the ring wall to form a Halbach array structure, this structure of the ring and its surface magnetic flux density as shown in the figure below. Halbach array structure has a strong unilateral coalescence effect, that is, the application of the ring side of the magnetic flux density is very strong, the other side of the flux density is weak.

Magnetic pole distribution and surface flux density of multipole magnetic rings

Another significant feature of a ring with this structure is that the surface flux density of the ring has a sinusoidal waveform, which is a desirable waveform for some permanent magnet motors.

Traditional arc magnet, in the pressing of the DC field orientation, the magnetic moment of a single piece of tile for the parallel arrangement), by the arc magnet combination of multi-pole ring surface flux density distribution for the square wave, if you want to artificially magnetize the waveform into a sinusoidal wave, that is, the top of the square wave on both sides of the flux "cut off" part of the flux can be achieved by grinding the tile into two thin sides in the middle of the thick form to achieve. Grind the tile into two thin middle thick form to achieve. But no matter what way, the internal magnetic orientation of the tile has not changed, the surface flux density distribution of the square wave into a sinusoidal wave is also at the expense of magnetic flux.

Combined with the above analysis and comparison, the advantages and disadvantages of the two types of magnetic ring rotor can be summarized as follows: First, the same category of materials, the same size of the two types of rotor comparison, due to the multi-pole magnetic ring rotor magnetic density waveform amplitude is high, i.e., the surface flux density is higher than that of the magnetic tile combination rotor, and then due to the high dimensional accuracy, concentricity and good, the stator and rotor gap can be smaller, and these factors can make the motor power and efficiency is higher. Secondly, due to the advantages of sinusoidal flux density distribution on the surface of the multi-pole magnetic ring rotor and high mechanical precision, the motor operates stably and with low noise. Third, due to the whole multi-pole magnetic ring assembly synthesis rotor than with a number of pieces of tiles combined into a rotor is convenient and easy to do, easy to install; if the direct injection molding method is more convenient and quick, so it is conducive to saving time and reduce costs. Fourth, because the ring core material can be optional, do not have to be limited to the use of magnetically conductive materials, which provides a convenient and flexible design for motor designers.

In recent years, we have developed a series of multi-pole magnetic ring products, including 74mm diameter (8-pole), 68 diameter (8-pole), 54mm (8-pole), 54mm (6-pole), 45mm (8-pole), 36.5mm (4-pole), to replace the corresponding specifications of the magnet magnet rotor. The results are satisfactory to the users, in terms of output power, efficiency, operation stability, noise and so on. The results are satisfactory to the users and are better than the tile rotor motors in terms of output power, efficiency, operation stability, noise and so on.

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