Halbach Array

Halbach array is a kind of magnet arrangement structure. Before learning this structure, let's take a look at the distribution of magnetic lines of force of some common permanent magnets.

From this picture, it is not difficult to find that the orientation and arrangement of magnets will directly affect the distribution of magnetic lines of force, that is, the distribution of magnetic fields around magnets.

The concept of halbeck array

Halbach array is a kind of magnet structure. In 1979, Klaus Halbach, an American scholar, discovered this special permanent magnet structure and gradually improved it. Finally, the so-called "Halbach" magnet was formed. It is an approximate ideal structure in engineering. It uses a special arrangement of magnet units to enhance the field strength in the unit direction, The goal is to produce the strongest magnetic field with the smallest number of magnets.

The array is made up of rare earth permanent magnets. By arranging the permanent magnets with different magnetization directions according to a certain rule, the magnetic lines of force can be converged on one side of the magnet and weakened on the other side, so as to obtain an ideal unilateral magnetic field. This is of great significance in engineering, because of its excellent magnetic field distribution characteristics, Halbach array is widely used in industrial fields such as nuclear magnetic resonance, magnetic levitation, permanent magnet special motors and so on.

On the left is a single magnet with the north pole all up. From the color, we can see that the strength of the magnetic field is at the bottom and top of the magnet. On the right is a halbeck array with a high magnetic field at the top and a relatively weak field at the bottom（Under the same volume, the magnetic field intensity on the strong side of the halebeck array magnet group is about √2 times (1.4 times) of that of the traditional single magnet, especially when the thickness of the magnet in the magnetizing direction is 4-16MM

The most common example of the Halbach array is probably the flexible refrigerator. These thin, soft magnets are usually printed and pasted on refrigerators or the back of cars. Although they are very weak (only 2% - 3% strength) compared with NdFeB magnets, they are widely used due to their low price and practicability.

The form and application of Halbach array

• Linear array

Linear type is the most basic form of Halbach array, which can be regarded as a combination of radial array and tangential array, as shown in the following pic..

The linear Halbach array is mainly used in linear motor.The levitation principle of maglev train is that the moving magnet interacts with the magnetic field generated by the induced current in the conductor to produce the levitation force, and also has magnetic resistance. The key to improve the performance of the suspension system is to improve the buoyancy and resistance ratio. This requires light weight, strong magnetic field, even magnetic field and high reliability of the on-board magnet. The Halbach array is installed horizontally in the center of the vehicle body, which produces propulsion force by the winding in the center of the track. The magnetic field is maximized with less magnet consumption, while the other side is less magnetic field, so passengers can avoid exposure to strong magnetic field.

• Ring array

The annular Halbach array can be regarded as a ring shape formed by connecting the ends of the linear Halbach array.

In permanent magnet motors, the air gap magnetic field of Halbach array permanent magnet motor is closer to sinusoidal distribution than that of traditional permanent magnet motor. Under the same amount of permanent magnet material, the air gap magnetic density of Halbach permanent magnet motor is higher and the iron loss is smaller. In addition, Halbach ring array is widely used in permanent magnet bearings, magnetic refrigeration equipment and magnetic resonance equipment.

Fabrication and production of Halbach array

Method 1: according to the topological structure of the array, the pre magnetized magnet segments are glued together. Because of the strong mutual repulsive force between the magnet segments, the mold should be used to clamp them when they are glued. The manufacturing efficiency of this method is low, but it is easy to realize, so it is more suitable for the laboratory research stage.

Method 2: Firstly, a complete magnet is made by filling or pressing the mold, and then magnetized in a special fixture.The structure of the array processed by this method is similar to the figure below. This method has high processing efficiency and is easy to realize mass production. However, it is necessary to design magnetization fixture and formulate magnetization process.

Method 3: Halbach type magnetic field distribution is realized by using winding array with specific shape, as shown below pic..

Reference: zhaocicai.com