What is a arc magnet?

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What is a arc magnet?

Arc magnet is a kind of tile magnet which is mainly used in permanent magnet motor.

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Classification of arc magnet

According to the different raw materials, arc magnets can be divided into three categories:

Purpose of arc magnet

The magnetic pad is mainly used in permanent magnet DC motor. Different from the electromagnetic motor which generates the magnetic potential source through the excitation coil, the permanent magnet motor uses the permanent magnet material to generate the constant magnetic potential source. It has many advantages, such as simple structure, convenient maintenance, light weight, small volume, reliable use, less copper consumption, low copper consumption and energy consumption.

Influence of arc magnet performance on motor:

  • (1) High residual magnetic induction BR. Because of the high br to ensure that the motor has a high speed, large output torque and high power. The motor will have higher efficiency.
  • (2) High HCB. Because of the high HCB, the electromotive force required by the motor output can be ensured, the working point of the motor is close to the maximum magnetic energy product, and the ability of the magnet can be fully utilized.
  • (3) High Hcj. Hcj high can ensure that the motor has strong resistance to overload, demagnetization, anti aging and low temperature resistance.
  • (4) High (BH) max. The higher the (BH) max is, the better the actual operating coefficient of the permanent ferrite in the motor.
  • (5) The larger the magnetic energy Φ, the better, which will greatly improve the working efficiency of the motor.
  • (6) The better the rectangularity of demagnetization curve is, the smaller the dynamic loss is.
  • (7) The higher the resistivity of permanent ferrite is, the smaller the eddy current loss is.
  • (8) The temperature coefficient of permanent ferrite is small, so it has good temperature stability at high temperature.

Development History 

The development of arc magnet is based on the demand of permanent magnet motor itself.

  • 1) Permanent magnet motor is divided into permanent magnet DC motor and permanent magnet AC motor.
  • Permanent magnet AC motor refers to multi-phase synchronous motor with permanent magnet rotor. In the past, ferrite and Al Ni Co magnetic pad were mainly used. With the increasing power requirements, ferrite and Al Ni Co can not meet the requirements due to low magnetic energy product. Now NdFeB arc magnet is widely used in the field of high-power synchronous motor and generator. Of course, as rare earth resources are precious, the price of raw materials has doubled, and its manufacturing cost and production price will continue to rise in the future, which will affect its extensive use in some low-end fields. The negative temperature coefficient and low Curie temperature of NdFeB permanent magnet also limit its high temperature application.
  • Permanent magnet DC motor is divided into brushless motor and ordinary DC motor with commutator
  • In the ordinary DC motor, 2-pole and 4-pole are the majority, and the arc magnet is basically used as the motor stator. Most of the micro DC motor use ferrite arc magnet, which is mainly used in toys, household appliances and automobile fields. When brushless motor uses magnetic pad as stator, it usually exceeds 6 poles, so its center angle is far less than that of common DC motor. However, when the arc magnet is used as the rotor of 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 is to be assembled into a circle, its center angle is close to 90 °, which is different from ordinary DC motor.
  • 2) According to the different application fields, the performance and magnetic field waveform requirements of the arc magnet are different. As the rotor field, the surface field requirements of the outer arc surface are high, and the basic requirement of the waveform is sine wave. As the stator, it is selected according to the output requirements of the motor, the spark and the noise requirements. The arc magnet of the stator has high surface field of the inner arc, which is determined by the shape and production characteristics of the arc magnet Its waveform is mainly saddle shaped, which has large output, but spark noise and large no-load current. In order to improve the vibration noise, reverse spark and reduce the no-load current, the shape and size of magnetic steel and mold design should be carefully designed. Among them, the arc magnet with different thickness has been widely used. The principle is to make the magnetic field waveform of the inner arc air gap of the arc magnet tend to flat top wave.
  • 3. Special use: the earliest domestic ferrite tile enterprises, such as Sichuan 899 (Yibin Jinchuan Electronics Co., Ltd.), used the arc magnet as the magnetic source of the magnetizing cup in the 1980s and 1990s. At that time, the domestic demand for ferrite arc magnet was very small. 

Production technology

According to different materials and types, the technology of arc magnet is also very different.
The ferrite arc magnet is mainly sintered ferrite, and NdFeB arc magnet is divided into sintering and bonding:

  • 1. The production process of sintered ferrite arc magnet is mainly divided into wet pressing anisotropy, dry pressing isotropy and dry pressing anisotropy. The difference between the anisotropy and isotropy lies in whether there is a oriented magnetic field during press forming. This paper mainly introduces the wet compaction process. The technological process of wet pressing is as follows: raw material – pre – sintering – coarse grinding (primary ball milling) – batching – secondary ball milling (wet grinding) – magnetic field shaping – sintering – grinding – Cleaning – magnetizing. Because of the moisture content in the slurry, the particles formed in the magnetic field are easy to turn, so the orientation degree is higher than that of the dry compaction performance, and the performance is higher.
  • 2. Sintered NdFeB arc magnet: proportioning – smelting – crushing – pulverizing – magnetic shaping – isostatic pressing – vacuum sintering and tempering – wire cutting and other processing – electroplating – magnetizing
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There are three commonly used magnetization methods:

  • 1. The arc magnet is separately magnetized, then installed into the shell, and then assembled as a whole;
  • 2. The arc magnet is installed into the shell (glued) and then magnetized, and then assembled as a whole;
  • 3. Install the arc magnet into the shell (glued), then assemble the finished product, and finally conduct the whole magnetization.

The influence of different ways on the magnetization of arc magnet is as follows:

  • 1. The surface remanence of the first magnetization method is about 10% lower than that of the second and third methods. Generally speaking, the second method is more reasonable.
  • 2. The first method is very convenient. Some factories use solenoid tunnel magnetization, which has high production efficiency.
  • 3. In the second way, some factories use external punching, while others use internal punching. The arc magnet or ring needs to be glued and dried first, so the production is very convenient.
  • 4. The third way of magnetization, there are many small motors (mostly three slot) use this method, when installing the rotor, because there is no magnetic force, it is very easy to assemble, but it is not convenient to check whether the magnetic is well charged.
  • 5. The first type of magnetization belongs to open circuit magnetization. Since there is no external magnetic circuit and no closed circuit, it is not easy to be filled and saturated during magnetization, which has an impact on the magnetic flux density of arc magnet, which is generally lower than that of closed-circuit magnetization.
  • 6. The second magnetization method is the most commonly used, which belongs to closed-circuit magnetization, and the magnetic flux density after magnetization is the highest. Moreover, the waveform of air gap flux density can be adjusted by the shape of magnetic head to meet the requirements of different motor performance. However, special tooling is required for assembly during motor assembly, otherwise rotor may be damaged or arc magnet may be produced due to magnetic suction Magnetic fragmentation.
  • 7. The third magnetization method also belongs to closed-circuit magnetization (because the rotor sheet is a good magnetic material). The magnetic flux density of the arc magnet after magnetization is generally between the first and the second. Note: when using the whole machine for magnetization, it is better to keep the carbon brush and rotor winding in an open circuit state, otherwise there may be ignition on the contact surface between the carbon brush and the commutator.
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