Bottleneck issues in the application of permanent magnet motors in electric vehicles

1 Introduction

At present, my country's industrial energy consumption accounts for about 70% of the total energy consumption, of which motor energy consumption accounts for about 60% to 70% of industrial energy consumption. According to the survey, my country's IEI-level motor market share is close to 90% of the market share, while the proportion of enterprises with high-efficiency energy-saving motors above IE2 is only 8%. At present, the installed capacity of motors in my country is about 1.2 billion kilowatts. It is estimated that by 2020, the installed capacity of motors in my country will reach about 4.5 billion kilowatts. The newly added more than 3 billion kilowatts of motors will bring huge market space for high-efficiency motors. In theory, the operating efficiency of amorphous motors with stator cores made of amorphous materials can reach more than 95%, and up to 98%. Especially in some medium and high frequency applications, the efficiency of traditional silicon steel sheet motors is very low, while the operating efficiency of amorphous motors is also above 90%, and the energy-saving potential contained is very considerable, which provides a broad stage for the application of amorphous materials in high-efficiency motors. High-efficiency permanent magnet synchronous motors have been widely used in many fields. Because of their high efficiency and high-precision controllability, they have been widely used in automated transportation operating systems and electric vehicle power system design. However, in many fields, such as electric vehicles, permanent magnet motors need to be designed with flat structures. Therefore, disc permanent magnet motors must be designed with multiple stages, and the motor speed range needs to reach 2000~6000r/min. The high smoothness of the motor will cause a sharp increase in the loss of ferromagnetic materials, which will reduce the motor efficiency and bring great difficulties to the design of the cooling system. Disc permanent magnet motors made of amorphous materials can greatly reduce the high-frequency iron loss of the motor, improve the motor efficiency, and optimize the cooling system design. The application of amorphous materials has better solved the bottleneck problem of the application of permanent magnet motors in the field of electric vehicles.

2 Introduction to amorphous materials

2.1 Formation of amorphous materials

After the metal is melted, the internal atoms are in an active state. Once the metal begins to cool, the atoms will slowly arrange themselves in order according to certain crystalline laws as the temperature drops, forming crystals. But if the cooling process is very fast, the atoms will solidify before they have time to rearrange themselves, and amorphous materials will be produced. Amorphous alloys are metal alloys with amorphous atomic structures. A rapid solidification process is used to prepare amorphous alloys. The high-temperature copper water in a molten state is sprayed onto a high-speed rotating cooling roller. The molten steel cools rapidly at a rate of one million degrees per second, and it only takes one thousandth of a second to reduce the 1300°C pot water to below 200°C, forming an amorphous strip. The thickness of amorphous strips is generally controlled between 0.02 and 0.03 mm, and online winding can be achieved. The process is shown in Figure 1:

2.2 Characteristics of amorphous materials

Compared with crystalline alloys, amorphous alloys have undergone significant changes in physical, chemical and mechanical properties. Taking iron-based amorphous alloys as an example, it has the characteristics of high saturation magnetic induction intensity and low loss. However, amorphous alloy materials also have disadvantages that cannot be ignored; the hardness is too high and it is extremely brittle, which is not easy to process and cut; the thickness is only 0.03mm, the filling factor is low; it is also very sensitive to mechanical stress, which makes its application difficult.

2.3 Application of amorphous materials

Because of the excellent properties of amorphous materials, systematic theoretical and applied research has been carried out both at home and abroad, and it has a broad application space in many fields such as electronics, aviation, aerospace, machinery, microelectronics, etc. At present, it has been widely used in various transformers and inductor products, but its application in the motor field is still in its infancy. It is precisely because of the characteristics of amorphous materials that its application in motor products has become a major issue at home and abroad.

3 Advantages of amorphous motors

Compared with traditional material motors, amorphous motors have excellent performance and advantages. The following table lists the different characteristics of silicon steel sheets and amorphous material stator cores using DW47 and Metglus2605SAl as examples.

Through the comparison in Table 1, it is easy to infer the advantages of amorphous motors: (1) High efficiency and energy saving Due to the low loss characteristics of amorphous alloy materials, the iron loss of motors made of this material as the core is much smaller than that of traditional silicon steel sheet core motors. As can be seen from Figure 2, the loss of amorphous alloy materials at medium and high frequencies is only 1/8 to 1/10 of that of ordinary silicon steel sheets, and the higher the frequency, the more obvious the trend. Therefore, the efficiency of amorphous motors is higher than that of traditional motors, especially when used in medium and high frequency applications. The motor operating efficiency can be increased by more than 5%, and the energy saving effect is very obvious.

Figure 2

(2) High power density and torque density

The power P converted by the motor is proportional to the following three parameters:

a) Magnetic flux of the magnetic field; b) Ampere-turns A of the coil; c) Induction electromagnetic frequency f of the first two

PαφAf

Magnetic flux φ = BS, where S is the effective cross-sectional area of ​​the magnetic circuit and B is the designed magnetic flux density. After the magnetic circuit size and magnetic circuit material are determined, the value of φ is determined. The ampere-turns A of the coil depends on the actual space and cooling conditions for installing the coil, that is, it depends on the actual size of the motor. It can be seen that the higher the electromagnetic frequency f, the higher the power of the motor of the same size (that is, φA remains unchanged). In other words, the smaller the size and weight of the motor at the same power and speed, the higher the frequency can increase the power density or torque density of the motor.

4 Application of amorphous materials in high-efficiency motors

The research and development of amorphous motors began in the early 1990s. In 1996, Light Engineering, Inc. (LE) in the United States successfully applied amorphous materials to motors and realized industrialization. After more than ten years of research and development, Xiangdian Light Electric Co., Ltd. (hereinafter referred to as Xiangdian Light) has developed a series of amorphous permanent magnet synchronous motors and generators, which are sold in batches in North America, Europe and Asia. It has also established the first amorphous motor production line in China, becoming a model for the application of amorphous materials in high-efficiency motors in China. This article will mainly introduce the application of amorphous materials in high-efficiency motors based on Xiangdian Light's axial magnetic field disc motor products.

4.1 Difficulties in the application of amorphous materials in the motor field

1) Amorphous metals are hard and difficult to process. It is easier to make a C-shaped or ring-shaped core, but it is difficult to make a multi-slot core used in motors;

2) Restricted by the maximum frequency of the motor driver, the amorphous motor base must generally be designed to be above 500Hz, and the maximum frequency can even reach above 1500Hz, which is a severe test for the motor driver;

3) The design method of high-frequency motors cannot fully utilize the potential advantages of amorphous metals in improving power density; 4) The saturation magnetization value of amorphous materials is slightly lower than that of ordinary silicon steel materials.

4.2 Amorphous Motor Design

Xiangdian Lai Te has built a disc-type amorphous motor R&D platform based on ANSOFT, and uses the 3D modeling and simulation capabilities of axial magnetic field motors introduced in the latest version of the software V15 to perform electromagnetic calculations and finite element analysis on disc-type amorphous motors. Xiangdian Lai Te has developed a series of amorphous disc-type permanent magnet synchronous motor products using this platform. Figures 3 and 4 are schematic diagrams of the company's motor modeling and finite element analysis.

Figure 3 Full model of amorphous disc motor Figure 4 Magnetic flux density of amorphous disc motor

Figure 5 is a typical structure of the axial magnetic field disc permanent magnet motor designed by Xiangdian Lai Te. The structure fully considers the processing requirements of amorphous materials, and the core adopts an open slot structure. The core is relatively easy to make. The few-slot structure is adopted, and the winding forming coil is concentrated to reduce the coil end and the loss caused by the effective material resistance. This structure is easy to realize automated production in the winding production and its offline production, and meets the production requirements of high quality and large batches. Improve the mechanization degree of product production, thereby improving the quality of the product.

Features of this structure:

1) The axial air gap structure makes it easy for the motor to realize a multi-stage structure, thereby achieving high frequency of the motor;