0322Halbach Array,Halbach permanent magnet

0322The Halbach array is a magnet arrangement structure. Before we understand this structure, let's take a look at the magnetic field line distribution of some common permanent magnets.

From this picture, it is not difficult to find that the placement direction and arrangement of the magnet will directly affect the distribution of magnetic field lines, that is, it will affect the form of the magnetic field distribution around the magnet.

The concept of Halbach array

Halbach Array (Halbach permanent magnet) is a kind of magnet structure. In 1979, the American scholar Klaus Halbach discovered this special permanent magnet structure during electron acceleration experiment and gradually improved it, and finally formed the so-called "Halbach" magnet. 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 use the least amount of magnets to generate the strongest magnetic field.

This kind of array is entirely composed of rare earth permanent magnet materials. By arranging permanent magnets with different magnetizing directions according to a certain rule, the magnetic lines of force can be concentrated on one side of the magnets, and the lines of force can be weakened on the other side, thereby obtaining an ideal unilateral magnetic field. This is of great significance in engineering. With its excellent magnetic field distribution characteristics, Haierbek arrays are widely used in industrial fields such as nuclear magnetic resonance, magnetic levitation, and permanent magnet special motors.

On the left is a single magnet with all north poles facing upwards. From the color, it can be seen that the strength of the magnetic field is located at the bottom and top of the magnet. On the right is a Halbach array. The magnetic field at the top of the magnet is relatively high, while the bottom is relatively weak. (Under the same volume, the magnetic field intensity of the strong side surface of the Halbach array magnet is about √2 times (1.4 times) that of a traditional single magnet, especially when the thickness of the magnet is 4-16mm in the magnetizing direction)

The most common example of the Halbach array may be the flexible refrigerator sticker. These thin, soft magnets are usually printed on the refrigerator or on the back of the car. Although their magnetism is very weak compared to NdFeB (only 2%-3% strength), they are Its low price and practicality make it widely used.

The form and application of Halbach array

Linear array

The linear type is the most basic Halbach array composition. This array magnet can be regarded as a combination of a radial array and a tangential array, as shown in the figure below.

Linear Halbach arrays are currently mainly used in linear motors. The levitation principle of the maglev train is that the moving magnet interacts with the magnetic field generated by the induced current in the conductor to generate levitation force, and at the same time, it is accompanied by magnetic resistance. Improving the buoyancy and drag ratio is the key to improving the performance of the levitation system, which requires the weight of the on-board magnet Light weight, strong magnetic field, uniform magnetic field and high reliability. The Halbach array is installed horizontally in the center of the car body and interacts with the winding in the center of the track to generate propulsive force, which maximizes the magnetic field with a small amount of magnets, and the other side has less magnetic fields, which can prevent passengers from being exposed to strong magnetic fields.

Circular array

The circular Halbach array can be regarded as a combination of linear Halbach arrays end to end to form a circular ring shape.

In the permanent magnet motor, the permanent magnet motor using the Halbach array structure has an air gap magnetic field closer to the sinusoidal distribution than the traditional permanent magnet motor. In the case of the same amount of permanent magnet material, the Halbach permanent magnet motor has a larger air gap magnetic density. The iron loss is small. In addition, Halbach ring arrays are also widely used in permanent magnetic bearings, magnetic refrigeration equipment and magnetic resonance equipment.

Fabrication and production method of Halbach array

Method 1: According to the topology of the array, use magnet glue to bond the magnetized magnet segments together. Because the mutual repulsion between the magnet segments is very strong, the mold should be used for clamping during adhesion. This method has low manufacturing efficiency, but is easier to implement, and is more suitable for use in the laboratory research stage.

Method 2: First use the method of filling mold or pressing mold to manufacture a complete magnet, and then magnetize in a special fixture. The array structure processed by this method is similar to the figure below. This method has high processing efficiency and comparison It is easy to realize mass production. However, it is necessary to specially design magnetizing fixture and formulate magnetizing process.

Method 3: Use a special-shaped winding array to realize the Halbach-type magnetic field distribution, as shown in the figure below.

The process and effect of the self-made Halbach array in the laboratory