At present, most mainstream permanent magnet brushless motors use surface-mounted or embedded methods to splice magnetic tiles into a ring-shaped magnetic circuit. However, the spliced magnetic ring has the disadvantages of high processing precision requirements for magnetic tiles, high assembly difficulty, poor magnetic pole transition smoothness, and severe motor noise. In addition, the structure requires a frame structure of soft magnetic materials to fix the magnetic tiles, which affects the assembly efficiency.
Radiating magnetic rings are a type of circular permanent magnet with a special orientation. They are magnetized radially along the circumference of the magnetic ring. They can replace spliced magnetic rings and can be widely used in servo motors, magnetic transmissions, magnetic bearings, and sensors.
Figure Common magnetization methods for radiating magnetic rings (picture from the Internet)
The main advantages of radiating magnetic rings include:
1. The radiating ring is an integral magnetic ring, with better control of dimensional accuracy and a simpler assembly process;
2. The surface field is sinusoidal, evenly distributed, and the transition zone between magnetic poles is small, so the motor runs stably and has low noise;
3. The magnetization methods and magnetic field distribution are diverse, and single-pole radiation, multi-pole direct charging, and multi-pole oblique charging can be magnetized, and the magnetic circuit design is more flexible.
Radiation magnetic rings can be divided into magnetic field oriented and pressure oriented magnetic rings according to the orientation method. Among them, sintered or bonded magnetic rings are mostly magnetic field oriented, and hot pressed/hot deformed magnetic rings are mostly pressure oriented. According to the material, they can be divided into: ferrite permanent magnetic rings, rare earth permanent magnetic rings and other permanent magnetic rings. Among them, rare earth permanent magnetic rings mainly include samarium cobalt permanent magnetic rings and neodymium iron boron permanent magnetic rings. The highest magnetic properties are sintered or hot pressed/hot deformed neodymium iron boron magnetic rings. According to the shape, the inner and outer diameter ratio is less than 0.7 for thick-walled rings, and the inner and outer diameter ratio is greater than 0.9 for thin-walled rings.
Due to the mature bonding process and low price, the bonded NdFeB radiation ring has the largest output. However, the density and performance of bonded magnetic rings are low, and high-end application scenarios are limited. However, high-performance sintered and hot-pressed/hot-deformed NdFeB radiation magnetic rings are very easy to break during the preparation, magnetization and assembly process due to the large difference in the shrinkage ratio and thermal expansion coefficient between the easy magnetization axis and the difficult magnetization axis of the NdFeB grains. The yield is low and the price is generally high. Japan started the development of radiation ring equipment and processes earlier, and its equipment accuracy and stability, as well as the brand of the product, have great advantages. Although the research on radiation rings in China started late, many companies and research institutes have been able to stably supply radiation ring products of various sizes and brands.
Regarding the size of radiation rings, generally speaking:
Bonded radiation rings are less restricted by size;
Hot-pressed radiation rings are mostly thin-walled magnetic rings, with diameters of less than 30mm and wall thicknesses of less than 3mm;
Sintered radiation rings can produce products with an outer diameter greater than 200mm, a wall thickness of more than 5mm, and a height less than 50mm. However, due to the restrictions on the pass rate and cost, most of the magnetic rings on the market are small-diameter magnetic rings with an outer diameter of less than 100mm.
