- Introduction of high speed motor
High-speed motor usually refers to the motor whose speed exceeds 10000 r / min. High-speed motors are small in size, can be directly connected to high-speed loads, eliminating the need for traditional mechanical speed increase devices, reducing system noise and improving system transmission efficiency. Currently, induction motors, permanent magnet motors and switched reluctance motors are the main ones that have successfully achieved high speed.
The main characteristics of high-speed motors are high rotor speed, high stator winding current and flux frequency in the core, high power density and loss density. These characteristics determine the high-speed motor has different key technology and design methods unique to normal speed motors, and the design and manufacturing difficulties are often exponentially greater than those of ordinary speed motors.
Application areas of high-speed motors.
(1) High-speed motors are used in various applications such as centrifugal compressors for air conditioners or refrigerators.
(2) With the development of hybrid vehicles in the automotive industry, small size and light weight of high-speed generators will be given full attention and have good prospects for application in hybrid vehicles, aviation, ships and other fields.
(3) High-speed generators driven by gas turbines are small in size and have high mobility, and can be used as backup power for some important facilities, or as independent power supply or small power station to make up for the shortage of centralized power supply, which has important practical value.
Key technologies of high-speed motors
High-speed, ultra-high-speed applications are promising but at the same time bring extremely high challenges to the motor, we will combine these problems after similar items found that there are six major categories. They are heat dissipation, selection, rotor structure, vibration and noise, high efficiency design, bearings.
Second, high-speed motor development status at home and abroad
1、High-speed induction motor
Induction motor rotor structure is simple, low rotational inertia, and can operate for a long time under high temperature and high speed conditions, so induction motors are widely used in the high-speed field.
At present, domestic and foreign high-speed induction motor, the power of the largest 15MW, its speed of 20000 r / min, developed for ABB in 2002, using a solid rotor structure. High-speed induction motor speed is the largest developed by Westwind Air Bearings, speed of 300000 r/min, its power is 200 W, used for PCB drilling machine spindle. Similarly, a high-speed induction motor with a power of 10 kW and a speed of 180,000 r/min has been realized abroad for use as a test motor.
The picture below shows a high-speed induction motor developed by Westwind Air Bearings with a power of 200 W and a speed of 300,000 r/min.
Domestic research is relatively backward, among which Shenyang University of Technology, Chongqing Dema Electric, Naval Engineering University, Zhejiang University and other research units have carried out a lot of research work on high-speed induction motors.
Chongqing Dema Motor has developed 100kW, 25000r/min high-speed induction motor. Shenyang University of Technology has conducted research on high-speed induction motors with 280kW power, 12000r/min speed, 132m/s linear speed and common laminated structure. Naval Engineering University has conducted research on high-speed induction motors of 2.5 MW, etc.
Table 1 Research on high-speed induction motors at home and abroad
2、High-speed permanent magnet motor
Permanent magnet motors are popular in high-speed applications because of their high efficiency, high power factor and large speed range. Compared with the outer rotor permanent magnet motor, the inner rotor permanent magnet motor has the advantages of small rotor radius and high reliability, which makes it the first choice for high-speed motor.
At present, among the high-speed permanent magnet motors at home and abroad, the most powerful high-speed permanent magnet motor is researched in the United States, with a power of 8MW and a speed of 15000r/min. It is a face-mounted permanent magnet rotor with a carbon fiber protective sleeve and a cooling system that uses a combination of air-cooled and water-cooled for high-speed motors matched with gas turbines.
The Swiss ETH Zurich designed the highest speed high speed permanent magnet motor. The parameters are 500000 r/min, power is 1kW, linear speed is 261m/s, and alloy protective jacket is used.
Domestic research on high-speed permanent magnet motor is mainly concentrated in Zhejiang University, Shenyang University of Technology, Harbin Institute of Technology, Harbin Institute of Technology, Xi’an Jiaotong University, Nanjing Aerospace Motor, Southeast University, Beijing University of Aeronautics and Astronautics, Jiangsu University, Beijing Jiaotong University, Guangdong University of Technology, CSR Zhuzhou Motor Co.
They have carried out research on the design characteristics, loss characteristics, rotor strength and stiffness calculation, and cooling system design and temperature rise calculation of high-speed motors, and have produced high-speed prototypes of different power levels and speeds.
Ltd. has developed a 1120 kW, 18000r/min high speed permanent magnet motor, as shown in the figure, which adopts a surface-mounted permanent magnet rotor structure with a rotor surface linear speed of 180m /s and carbon fiber protection measures.
In addition, Zhejiang University conducted an in-depth study on the protection measures and eddy current loss of the high-speed permanent magnet brushless DC motor with 2.3 kW and 150,000 r/min; Southeast University conducted a study on the high-speed permanent magnet motor with 600 W power and 20,000 r/min; Guangdong University of Technology conducted a 0.6 kW, 200,000 r/min high-speed permanent magnet brushless DC motor Theoretical analysis, etc.
However, the development of high-speed permanent magnet motor in China is mostly concentrated in the stage of small and medium power and low speed below 500kW, and the research on high power, especially megawatt and ultra-high speed permanent magnet motor is still relatively small. High-speed permanent magnet motors have two types of rotor structures: surface mount type (SPM) and built-in type (IPM). Except for a few with built-in rotor structure, most of them adopt surface-mounted permanent magnet rotor structure.
Table 2 Research on high-speed permanent magnet motors at home and abroad
3、Switching reluctance motor
Switched reluctance motors are attracting attention for their simple structure, robustness, low cost and high temperature resistance, and are increasingly used in high-speed applications.
The maximum power of high-speed switched reluctance motor is currently 250kW at 22,000r/min, and the maximum speed is 200,000r/min with 1kW power.
Nanjing University of Aeronautics and Astronautics, Beijing Jiaotong University and Huazhong University of Science and Technology have carried out research on high-speed switched reluctance motors, among which Nanjing University of Aeronautics and Astronautics has developed a 1 kW, 130,000 r/min switched reluctance motor.
Table 3 Comparison of three types of high-speed motors
Third, the high-speed motor protective sleeve design and rotor strength analysis
The rotor centrifugal force is very high when the motor rotates at high speed, and the rotor strength analysis and protective sleeve design are the keys to the design of high-speed motors. At present, the rotor strength analysis of high-speed permanent magnet motor mainly focuses on the steady-state stress analysis of the rotor when rotating 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 of the high-speed permanent magnet motors are made of NdFeB material, the compressive strength of this material is large, while the tensile strength is small, so for the permanent magnet of the inner rotor motor structure, protection measures must be taken.
At present, the most common protection measures are two kinds: one is to use carbon fiber to tie the permanent magnet, and the other is to add a high strength non-conductive alloy sheath outside the permanent magnet. However, the conductivity of the alloy sheath is larger, and the space and time harmonics will produce larger eddy current loss in the alloy sheath. The conductivity of the carbon fiber sheath is much smaller than that of the alloy sheath, which can effectively suppress the eddy current loss in the sheath, but the thermal conductor of the carbon fiber sheath is very poor, and it is difficult to dissipate rotor heat, and the processing process of the carbon fiber sheath is complicated and requires high processing accuracy. However, for high-speed external rotor permanent magnet motors, no protective measures are required, so there are fewer studies on rotor stress analysis.
Jason M. Yon, University of Bristol, UK, proposed a semi-permeable alloy sheath with a relative permeability of 7.2, and analyzed the electromagnetic characteristics of non-permeable and semi-permeable alloy sheaths.
Professor Wang Fengxiang of Shenyang University of Technology designed an alloy protective sleeve for a 60,000 r/min telltale high-speed permanent magnet motor, and performed theoretical analysis and two-dimensional finite element calculations on the mechanical strength of the rotor under different operating conditions such as rotation and standstill.
Zhejiang University has proposed a circumferential and axial slotted alloy protective sleeve, and the results of its in-depth study 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 methods of iron and copper consumption of high-speed motors are quite different from those of general motors. For ordinary motors, the harmonic frequency of the magnetic field is relatively low, and the rotor eddy current loss is generally negligible, while for high-speed permanent magnet motors, the rotor eddy current loss is large, which will cause serious difficulties in motor heat dissipation, and the permanent magnets will undergo irreversible demagnetization at too high a temperature. Therefore, the reasonable electromagnetic design and accurate calculation of losses for high-speed motors have become one of the key issues for high-speed motors.
In the high-speed motor, due to the high operating frequency, the stator core loss becomes the main loss of the motor and plays a dominant role in the efficiency and heat generation performance of the motor. To calculate the stator iron consumption, the classical calculation method is to establish the Bertotti iron consumption discrete calculation model, which means that the iron consumption is divided into three parts, namely, hysteresis loss, classical eddy current loss and abnormal eddy current loss.
Professor Hu Piansheng of Southeast University used two mutually orthogonal alternating magnetizations to approximate the equivalent rotating magnetization, which makes this method consider both harmonic and rotating magnetic fields, and the loss coefficients can be obtained directly from the loss curve fitting under alternating magnetization provided by the silicon steel sheet manufacturer, so it is relatively simple to implement and has high accuracy.
Since the iron consumption of a high-speed motor is related to the amplitude of the flux density component, the electromagnetic field inside the motor needs to be analyzed in order to calculate the losses accurately. Both the University of Helsinki, Finland, and the Royal Institute of Technology, Stockholm, Sweden, have done a relatively detailed analysis of the electromagnetic field in high-speed motors.
The rotor losses mainly consist of rotor air friction losses and rotor eddy current losses. Rotor eddy current losses are mainly generated by the time and space harmonics of the stator current and the change in air gap permeability caused by the stator slot opening.
As the rotating speed of high-speed motor is up to tens of thousands of revolutions per minute or even hundreds of thousands of revolutions, the air friction friction loss on its rotor surface is much larger than that of ordinary motor, and occupies a larger proportion in the total loss of the motor, so the study of air friction loss is of practical significance. At present, there are few reports on the study of air friction loss in China, especially the calculation of air friction loss of high-speed motor rotor based on fluid field. The preliminary analysis and experimental verification of the calculation method of air friction loss based on fluid field were carried out by Shenyang University of Technology through the study of 60,000 r/min high-speed permanent magnet motor.
However, there are still some urgent problems in the design and analysis of high-speed motors, both in China and abroad. Specifically, there are several problems in the design and analysis of high-speed motors.
Firstly, the analysis technology of high-speed motors based on the coupling methods of electromagnetic field, stress field, rotor dynamics, flow field and temperature field is not mature.
Secondly, high-speed bearings face great problems, such as ball bearings cannot withstand excessive speed, air bearings have limited load capacity, magnetic bearings are complicated and expensive to control.
Third, high-power high-speed motor rotor dynamics design technology is not perfect, transformation system, control system and real-time monitoring system research and development is weak.
Fourth, the cooling structure of high-power high-speed permanent magnet motor is complex, and the cooling effect is limited due to the combination of air cooling and water cooling.
Fifth, the development of high-speed permanent magnet motor to ultra-high speed and high power is constrained by the low tensile strength and poor temperature resistance of permanent magnets.
Sixth, the surface-mounted permanent magnet motor alloy protective sleeve eddy current loss, carbon fiber protective sleeve poor thermal conductivity, is not conducive to its rotor heat dissipation.
Seventh, the conventional laminated rotor can not withstand the large centrifugal force, solid rotor eddy current loss.
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; coupled design based on multi-physics field and multi-disciplinary; theoretical research and experimental verification of stator-rotor losses; development and application of new materials such as permanent magnet materials with high strength and high temperature resistance, fiber materials with high thermal conductivity; research on high-strength rotor laminated materials and structures; application of high-speed bearings under different power and speed levels; good heat dissipation Design of high speed motor control system; Development of new rotor processing and assembly technology to meet the industrialization requirements, etc. Translated with www.DeepL.com/Translator (free versio