1. Introduction of high-speed motor

High-speed motors usually refer to motors with a speed of more than 10,000 r/min. The high-speed motor is small in size and can be directly connected with high-speed loads, eliminating the need for traditional mechanical speed-increasing devices, reducing system noise and improving system transmission efficiency. At present, the main ones that have successfully achieved high speed are induction motors, permanent magnet motors, and switched reluctance motors.

The main characteristics of high-speed motors are high rotor speed, high frequency of stator winding current and magnetic flux in the iron core, and high power density and loss density. These characteristics determine that high-speed motors have key technologies and design methods that are different from those of constant-speed motors, and the design and manufacturing difficulty is often double that of ordinary-speed motors.

Application areas of high-speed motors:

(1) High-speed motors are used in various applications such as centrifugal compressors in air conditioners or refrigerators.

(2) With the development of hybrid vehicles in the automotive industry, high-speed generators with small size and light weight will be fully valued, and have good application prospects in hybrid vehicles, aviation, ships and other fields.

(3) The high-speed generator driven by the gas turbine is small in size and has high mobility. It can be used as a backup power source for some important facilities, and can also be used as an independent power source or a small power station to make up for the lack of centralized power supply and has important practical value. .

The key technology of high-speed motor

High-speed and ultra-high-speed applications have broad prospects but at the same time bring extremely high challenges to the motor. After combining these problems with similar items, we found that there are six categories. They are heat dissipation, type selection, rotor structure, vibration and noise, high-efficiency design, and bearings.

Second, the development status of high-speed motors at home and abroad

1. High-speed induction motor

Induction motors have a simple rotor structure, low rotational inertia, and can run for a long time under high temperature and high speed conditions, so induction motors are widely used in high-speed fields.

At present, among the high-speed induction motors at home and abroad, the highest power is 15MW, and its speed is 20,000 r/min. It was developed by ABB in 2002 and adopts a solid rotor structure. The highest speed induction motor is developed by Westwind Air Bearings, with a speed of 300000 r/min and a power of 200 W, and is used for the spindle of a PCB drilling machine. Similarly, a high-speed induction motor with a power of 10kW and a speed of 180000r/min has also been realized abroad as a test motor.

The picture below shows a high-speed induction motor developed by Westwind Air Bearings, with a power of 200W and a speed of 300,000 r/min.

Domestic research is relatively backward. Among them, Shenyang University of Technology, Chongqing Damon Electric Machinery, Naval Engineering University, Zhejiang University and other research institutes have carried out a lot of research work on high-speed induction motors.

Chongqing Damon Electric has developed a 100kW, 25000r/min high-speed induction motor. Shenyang University of Technology has carried out relevant research on high-speed induction motors with a power of 280kW, a rotational speed of 12000r/min, a linear speed of 132m/s, and a common lamination structure. The Naval Engineering University has carried out related research on the 2.5 MW high-speed induction motor and so on.

Table 1 Research on high-speed induction motors at home and abroad

2. High-speed permanent magnet motor

Permanent magnet motors are favored in high-speed applications due to their high efficiency, high power factor, and wide speed range. Compared with the outer rotor permanent magnet motor, the inner rotor permanent magnet motor has the advantages of small rotor radius and strong reliability, and has become the first choice for high-speed motors.

At present, among the high-speed permanent magnet motors at home and abroad, the high-speed permanent magnet motor with the highest power is researched in the United States. The power is 8MW and the speed is 15000r/min. It is a surface-mounted permanent magnet rotor. The protective cover is made of carbon fiber, and the cooling system adopts The combination of air and water cooling is used for high-speed motors matched with gas turbines.

The Swiss Federal Institute of Technology Zurich designed a high-speed permanent magnet motor with the highest speed. The parameters are 500000 r/min, the power is 1kW, the line speed is 261m/s, and the alloy protective sleeve is used.

Domestic research on high-speed permanent magnet motors is mainly concentrated in Zhejiang University, Shenyang University of Technology, Harbin University of Technology, Harbin Institute of Technology, Xi'an Jiaotong University, Nanjing Aerospace Motor, Southeast University, Beihang University, Jiangsu University, Beijing Jiaotong University, Guangdong University of Technology, CSR Zhuzhou Electric Co., Ltd., etc.

They carried out related research work on the design characteristics, loss characteristics, rotor strength and stiffness calculation, cooling system design and temperature rise calculation of high-speed motors, and produced high-speed prototypes with different power levels and speeds.

Shenyang University of Technology has cooperated with Jiangsu Aerospace Power Electromechanical Co., Ltd. and has developed a high-speed permanent magnet motor of 1120 kW and 18000 r/min. As shown in the figure, the motor adopts a surface-mounted permanent magnet rotor structure, and the surface linear speed of the rotor is 180m / s, with carbon fiber protection measures.

In addition, Zhejiang University has carried out in-depth research on the protection measures and eddy current loss of 2.3 kW, 150000 r/min high-speed permanent magnet brushless DC motor; Southeast University has carried out research on high-speed permanent magnet motor with power of 600 W and speed of 20000 r/min ; Guangdong University of Technology conducted a theoretical analysis of a 0.6 kW, 200,000 r/min high-speed permanent magnet brushless DC motor, etc.

However, the development of high-speed permanent magnet motors in China is mostly concentrated in the stage of small and medium power and low-to-medium speed below 500kW, and there is less research on high-power, especially megawatt-level and ultra-high-speed permanent magnet motors. High-speed permanent magnet motors have two rotor structures, surface-mounted (SPM) and built-in (IPM). Except for a few with built-in rotor structure, the rest mostly use surface-mounted permanent magnet rotor structure.

Table 2 Research on high-speed permanent magnet motors at home and abroad

3. Switched reluctance motor

Switched reluctance motors have attracted much attention due to their simple structure, robustness, low cost, and high temperature resistance, and are increasingly used in high-speed fields.

The current maximum power of high-speed switched reluctance motor is 250kW, the speed is 22000r/min, the maximum speed is 200000r/min, and the power is 1kW.

Nanjing University of Aeronautics and Astronautics, Beijing Jiaotong University, Huazhong University of Science and Technology, etc. have carried out related research work on high-speed switched reluctance motors. Among them, Nanjing University of Aeronautics and Astronautics has developed a 1 kW, 130,000 r/min switched reluctance motor.

Table 3 Comparison of three high-speed motors

3. Design of protective sleeve for high-speed motor and analysis of rotor strength

When the motor rotates at high speed, the centrifugal force of the rotor is very large. The analysis of the rotor strength and the design of the protective sleeve are the keys to the design of the high-speed motor. At present, the rotor strength analysis of high-speed permanent magnet motors is mainly aimed at the steady-state stress analysis when the rotor rotates at high speed to check whether the stress on the permanent magnet exceeds the allowable stress to ensure the stable operation of the rotor. Since most high-speed permanent magnet motors use NdFeB permanent magnet materials, which have high compressive strength and low tensile strength, protective measures must be taken for the permanent magnets of the inner rotor motor structure.

At present, there are two main protection measures : one is to use carbon fiber to bind the permanent magnet, and the other is to add a high-strength non-magnetic alloy protective sleeve outside the permanent magnet. However, the electrical conductivity of the alloy sheath is relatively large, and the space and time harmonics will generate a large eddy current loss in the alloy sheath. The electrical conductivity of the carbon fiber sheath is much smaller than that of the alloy sheath, which can effectively suppress the eddy current in the sheath. However, the thermal wire of the carbon fiber sheath is very poor, the heat of the rotor is difficult to dissipate, and the processing technology of the carbon fiber sheath is complex, which requires high processing accuracy. However, for high-speed external rotor permanent magnet motors, protection measures are not required, so there are few studies on rotor stress analysis.

Jason M. Yon of the University of Bristol, UK, proposed a semi-permeable alloy protective sleeve with a relative magnetic permeability of 7.2, and analyzed the electromagnetic properties of the non-magnetic and semi-conductive magnetic alloy protective sleeves.

Professor Wang Fengxiang of Shenyang University of Technology designed an alloy protective sleeve for a 60000 r/min high-speed permanent magnet motor, and carried out theoretical analysis and two-dimensional finite element calculation of the mechanical strength of the rotor under different working conditions such as rotation and static.

Zhejiang University has proposed a circumferentially and axially slotted alloy protective sleeve, and conducted in-depth research on it. The results show that this structure can effectively reduce the eddy current loss in the alloy protective sleeve while meeting the requirements of rotor strength.

The calculation method of iron loss and copper loss of high-speed motor is quite different from that of general motor. For ordinary motors, the harmonic frequency of the magnetic field is relatively low, and the rotor eddy current loss can generally be ignored. For high-speed permanent magnet motors, the rotor eddy current loss is relatively large, which will bring serious difficulties to the heat dissipation of the motor. irreversible demagnetization occurs. Therefore, the reasonable electromagnetic design of high-speed motors and the accurate calculation of losses have become one of the key issues of high-speed motors.

In a high-speed motor, due to the high operating frequency, the stator core loss becomes the main loss of the motor, which plays a leading role in the efficiency and heating performance of the motor. To calculate the stator iron loss, the current classic calculation method is to establish the Bertotti iron loss discrete The calculation model, that is, the iron loss is divided into three parts, namely hysteresis loss, classical eddy current loss and abnormal eddy current loss.

Professor Hu Qiansheng of Southeast University and others used two mutually orthogonal alternating magnetizations to approximate the equivalent rotating magnetization, so that this method considers both the harmonic magnetic field and the rotating magnetic field, and the loss coefficient can be directly provided from the silicon steel sheet manufacturer The loss curve fitting under the alternating magnetization mode is obtained, so it is relatively simple to implement and has high precision.

Since the iron loss of a high-speed motor is related to the magnitude of the magnetic flux density component, in order to accurately calculate the loss, it is also necessary to analyze the electromagnetic field in the motor. Both the University of Helsinki in Finland and the Royal Institute of Technology in Stockholm, Sweden have made a detailed analysis of the electromagnetic fields in high-speed motors.

The rotor loss mainly includes rotor air friction loss and rotor eddy current loss. The rotor eddy current loss is mainly caused by the time and space harmonics of the stator current and the air gap permeability change caused by the opening of the stator slot.

Since the rotation speed of the high-speed motor is as high as tens of thousands or even hundreds of thousands of revolutions per minute, the air friction wear on the rotor surface is much larger than that of the ordinary motor, which accounts for a large proportion of the total motor loss. Therefore, the research on the air friction loss have practical significance. At present, there are few reports on the research on air friction loss in China, especially the calculation of air friction loss of high-speed motor rotor based on the fluid field. Shenyang University of Technology conducted preliminary analysis and experimental verification on the calculation method of air friction loss based on fluid field through the research on 60000 r/min high-speed permanent magnet motor.

The main research and development directions of high-speed motors are :

Research on key issues of high-power high-speed motors and ultra-high-speed high-speed motors; coupling design based on multi-physics and multi-disciplines; theoretical research and experimental verification of stator and rotor losses; permanent magnet materials with high strength and high temperature resistance, high thermal conductivity Development and application of new materials such as fiber materials; research on high-strength rotor lamination materials and structures; application of high-speed bearings under different power and speed levels; design of good heat dissipation systems; development of high-speed motor control systems; meeting industrialization requirements Rotor processing and assembly new technology.