Different types of motors have different requirements for magnetic steel due to their different use requirements and environments. The following is divided into three parts: focusing on the different requirements of DC motors and brushless motors for magnetic steel.
Both DC motors and brushless motors use magnetic tiles or magnetic rings, but the main difference between the two is the different requirements for their magnetization. In the magnetization waveform, the quality of magnetization can be judged mainly by observing the following parameters in the waveform: average extreme value, extreme difference and area (or duty cycle). The average extreme value indicates whether the performance of magnetization or magnetic steel meets the requirements of the product; the extreme difference indicates the uniformity of magnetization; the area (or duty cycle) indicates the size of the magnetization waveform. Under the same extreme value, its size determines the output of the motor. However, the larger it is, the greater the positioning torque of the motor , and the worse the feel of rotation. Generally, in DC motors, the output is required to be large, so the duty cycle is large; while brushless motors require smooth rotation. It has an indicator – torque fluctuation. Especially at low speeds, the smaller the torque fluctuation, the closer the magnetization waveform is required to be to a sine wave . This is why we require the rising edge of the magnetization waveform to rise smoothly and slowly.
Here we first talk about the types of magnetization:
a. External filling of magnetic ring – that is, the outer surface of the magnetic ring is filled with magnetic poles, generally used for the rotor of the motor;
b. Internal filling of magnetic ring – that is, the inner surface of the magnetic ring is filled with magnetic poles, generally used for the stator or outer rotor of the motor ;
c. Oblique charging of magnetic rings – that is, the magnetic poles charged on the surface of the rotor form an angle less than 90° with the two end surfaces of the magnetic rings;
d. Axial magnetization – that is, magnetization along the axis of the magnetic ring or magnetic sheet, which can be divided into:
⑴ Axial two-pole magnetization – that is, one end of the magnetic part is the N pole and the other end is the S pole, which is the simplest magnetization;
⑵Axial single-sided multi-pole magnetization – the main product is magnetic sheets, that is, there are more than two magnetic poles on one surface of the magnetic part;
⑶ Axial double-sided multi-pole magnetization – that is, there are more than two magnetic poles on both sides of the magnetic part, and the polarities are opposite.
For axial single-sided or double-sided multi-pole magnetization , the surface magnetism of the single side is higher than that of the double side, but the surface magnetism of the other side of the single-sided magnetization is very low. In fact, the sum of the surface magnetism of the two sides of the single side is the same as the sum of the two sides.
e. Radial magnetization – As the name implies, radial magnetization means that the magnetization magnetic field radiates from the center of the circle to the surroundings. For the magnetic ring, after magnetization, the inner circular surface is all of one polarity, and the outer circular surface is of one polarity; while for the magnetic tile, the effect of radial magnetization is better than that of ordinary magnetization, and it can make the surface magnetic field of each point on the inner arc surface of the magnetic tile more similar.
Generally speaking, the number of poles refers to the multi-pole magnetization of the motor. For magnetic rings, 2-pole magnetic rings are mostly used in small DC motors, and some have 4 poles; while the magnetic rings used in stepper motors, brushless motors, and synchronous motors have even-numbered poles such as 4, 6, 8, 10, etc.
As for the magnetic tile, it is generally used in DC motors and brushless motors. In DC motors, 2-pole and 4-pole are the most common, which can be judged by its center angle , as mentioned above. When a brushless motor uses a magnetic tile as a stator, it generally has more than 6 poles, so its center angle is much smaller than that of a magnetic tile used in a DC motor; but when the magnetic tile is used as a rotor of a brushless motor, it can have more than 4 or 6 poles. For 4 poles, the rotor is magnetized on the outer surface, and because it has to be assembled into a circle, its center angle is close to 90°, which can be distinguished from a DC motor. If it is a magnetic ring, it is mainly used to distinguish stepper motors, brushless motors and synchronous motors. The outer diameter of the magnetic ring used in stepper motors is small, most of which are around 20, and rarely exceed 30, and its wall thickness is thin, between 1.0-1.5; its number of poles is large, more than 10 poles, and some can reach 50 poles. The diameter of the magnetic ring of a brushless motor is generally greater than 20, the number of poles is between 4-12, and the wall thickness is mostly between 1.5-5.0. The magnetic ring of a small synchronous motor is between 20-40, the number of poles is 8-16, and the wall thickness is 1.0-3.0.
The differences and common applications of injection molded magnets, bonded NdFeB and sintered NdFeB
Injection molded magnets, bonded NdFeB, and sintered NdFeB are three materials commonly used in small motors, especially the first two. These three materials each have their own characteristics, which are only briefly summarized here.
Their performance is increasing, and so is their price, which determines their respective applications. Injection molded magnets are divided into injection molded ferrites and injection molded NdFeB. Their bonding bodies are nylon 6, 12 and PPS. Nylon is slightly cheaper than PPS, but the surface finish, strength and temperature resistance of the magnetic parts it forms are worse than those of PPS. Their common feature is that they can be injection molded with various parts or shafts to ensure the quality of the product. Injection molded ferrite magnets are divided into isotropic (isotropic) and anisotropic (heterotropic). The magnetic energy product of isotropic is lower, at about 1.5MGOe, and the magnetic energy product of anisotropic is about 2.1 MGOe. It is mainly used for large-scale and wide-ranging products, such as toy motors, stepper motors for air conditioners, fans, etc. The highest magnetic energy product of injection molded NdFeB is about 6.0MGOe. If imported raw materials are used, it can reach 6.5MGOe, but the price is higher. At present, the application of injection molded NdFeB is not widespread, mainly because it is not well known. In fact, it can replace a large number of low-energy-product bonded NdFeB magnetic parts, especially in stepper motors and rotors with shaft injection molding.
Bonded NdFeB is the most widely used in high-performance products. It can be classified as sintered NdFeB products on the upper side and ferrite products on the lower side. This is mainly because its performance is between the two, and it is isotropic and suitable for various magnetization methods. Its disadvantage is that it has poor temperature resistance, which is up to 150℃, which determines that it is suitable for small motors and control motors. For drive motors, it depends on the specific situation.
Sintered NdFeB is widely used due to its high performance, but it is currently mainly used in drive motors. Among the motors we are talking about, they are mainly brushless motors and AC servo motors, and they are mainly tile-shaped, because the current sintered NdFeB is mainly unidirectionally oriented, that is, the magnetic parts can only be magnetized in one direction, so it cannot be made into magnetic rings for magnetization of more than 2 poles. The radially oriented sintered NdFeB currently developed can do this. The difference between the two is that the orientation direction is different during pressing, but the mold of the radial product is more complicated, and there is a mold fee for the magnetic parts. The radially oriented sintered NdFeB magnetic ring will first be used in brushless motors and AC servo motors, which is determined by the prices of the two. The use of radial orientation is very beneficial even for magnetic tiles, and the waveform after magnetization is close to a rectangular wave, not a saddle shape.
Generally speaking, it is easier to replace high-performance and low-performance materials in the same material, while different materials, although they also have different performances, are very different, and the structure of the motor must be changed. Otherwise, even if high-performance materials are used to replace low-performance ones, good performance cannot be obtained. Generally, high-performance materials replace low-performance materials, and the structure becomes smaller, and low-performance materials replace high-performance materials, and vice versa.
The torque of the motor
Generally speaking, the larger the motor torque, the higher the magnetic steel performance, and the smaller the motor, the lower the performance. In small motors, the detent torque is expected to be small, that is, the torque at the moment when the motor goes from static to rotation, which is mainly determined by the magnetization waveform and the core slot shape.
The speed of the motor
The motor speed has no-load speed and load speed. When discussing the relationship between speed and magnetic steel performance, the influence of other resistance torques of the motor should be excluded. The no-load speed is high, the magnetic performance is low, and vice versa, the magnetic performance is high; the load speed is high, the magnetic performance is high, and vice versa, the magnetic performance is low.
The current of the motor
The current of the motor also has no-load current and load current. When discussing the relationship between current and magnetic steel performance, the influence of other resistance torques of the motor should also be excluded. The larger the no-load current, the lower the magnetic performance, and vice versa, the higher the magnetic performance. However, this is not obvious in the motor. The size of the no-load current is more related to the resistance of the winding; the larger the load current, the lower the magnetic performance, and vice versa, the higher the magnetic performance.
The above relationship can be illustrated by the following table:
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In fact, the three are interrelated, and the changes in magnetic properties must be considered comprehensively.
Post time: Sep-19-2024