Research breakthrough｜Research results of integral supported ultra-high speed miniature permanent magnet motor

Gao Qixing, Wang Xiaolin, Gu Cong, Liu Sihao, and Li Dinghua, researchers from Nanjing University of Aeronautics and Astronautics Key Laboratory of Multi-Electric Aircraft and Electrical Systems and NZSB, wrote an article in the Journal of Electrotechnology, Vol. 14, 2021: based on the problem of ultra-high-speed miniature permanent magnet motor (UHSMPMM) governed by multi-physical field characteristics, they’ve conducted a comprehensive design study on the support system, electromagnetic (thermal) design, structural strength, and dynamics of the UHSMPMM.

Firstly, they designed an integral support system and motor architecture by combining the operating characteristics of the ultra-high-speed miniature permanent magnet motor with the structural characteristics of the miniature rotor. Secondly, they studied the electromagnetic-loss-temperature rise characteristics under high frequency conditions, especially focusing on the influence of the temperature rise characteristics on the structural strength of the rotor and offering an optimal method of the key parameters of the structural strength of the ultra-high speed rotor based on the temperature field coupling. Thirdly, they investigated the factors affecting the critical rotor speed in the overall support system and the variation law. Finally, a 550000 rpm/110W principle prototype was developed and experimentally tested on the basis of the multi-coupling characteristics analysis and optimization results. The results displayed that the prototype achieved stable operation, thus proving the effectiveness of the proposed design.

With the astounding rise in demand for portable, high power density energy conversion devices in the fields of aerospace, national defense and security, production and life, ultra-high speed micro motors have become a indispensable research content and been viewed as certain development direction today. The power of ultra-high-speed micro motors is generally from a few tens of watts to several kilowatts, and its speed is normally more than 100,000 rpm. The characteristics of high speed, small size and high energy density make it more suitable for the special requirements of modern high-end equipment.

Figure 1 shows the current research status and application areas of today’s ultra-high speed micro-miniature permanent magnet motors. Among them, a 100W-(150,000 to 300,000) rpm external rotor flywheel energy storage device was designed for aerospace applications by the Pennsylvania State University, USA. The Swiss Federal Institute of Technology in Zurich developed a 100W-500000 rpm ultra-high-speed permanent magnet motor, which was utilized as the generator part of the gas turbine unit. The British company Dyson has equipped its latest V11COMPLETE vacuum cleaner with an ultra-high speed motor of 125,000 rpm, featuring small size, light weight and strong suction power; it won a wide recognition from the market.

In addition, ultra-high-speed micro-motors in the flywheel energy storage, medicine, high-precision grinding machines and other fields still have greater development space and prospects. For instance, the speed of high-speed dental handpieces in the medical field ranges from 300,000 to 450,000 rpm on account that it is still mainly driven by air turbines, which makes it difficult to accurately control the speed and torque. Provided that the ultra-high-speed electric drive is put into use instead of air-driven equipment to achieve precise regulation of its speed and torque, the efficiency of clinical treatment can be significantly improved.

Nowadays, there exists relatively few domestic studies on ultra-high-speed microminiature motors, among which the 1kW-130000 rpm ultra-high-speed switched reluctance motor developed by Nanjing University of Aeronautics and Astronautics and the 2.3kW-150000 rpm permanent magnet motor designed by Zhejiang University have both managed building the experimental platform and given the experimental waveforms at 130,000 rpm and 100,000 rpm respectively. The experimental waveforms are portrayed under the circumstances of no-load operation. Guangdong University of Technology designed the stator-rotor structure of 980W-200000 rpm ultra-high-speed permanent magnet brushless DC motor and analyzed theoretically the motor characteristics such as electromagnetic, loss, rotor strength and so on.

Whether in theoretical research or industrial applications, the development of ultra-high-speed motors still gets very limited. Main factor lies in that the extreme speed and miniature size equip ultra-high-speed motors with ultra-high energy density, but also make them encounter a good many technical problems electromagnetic design, rotor strength, rotor dynamics, loss suppression, cooling methods, bearing support, etc.

For this reason, domestic and foreign scholars have been conducting comprehensive research in the field of ultra-high-speed motors. The relationship between mechanical stress and rotor vibration in ultra-high-speed permanent magnet motors was studied, and the effects of sheath thickness, interference and speed on rotor stress were analyzed. The linear support stiffness of the electromagnetic bearing was calculated based on magnetic field analysis, demonstrating that the support of the magnetic bearing on the rotor was isotropic, and a magnetic bearing high-speed motor was designed in view of the result. An in-depth analysis of the winding copper loss of a 1.5kW - 150,000 rpm permanent magnet motor was conducted, and the winding copper loss has been effectively reduced by means of magnetic shielding and conductor splitting. Some scholars took 1kW-280000 rpm motor model as an example to compare the cooling effect of different cooling methods on ultra-high-speed motors, and finally assessed that the motor power density could more than double by choosing a suitable cooling scheme.

Considering the main technical difficulties currently faced by ultra-high-speed micro-motors, researchers at Nanjing University of Aeronautics and Astronautics have studied a comprehensive and optimized design method for ultra-high-speed motors that supports system stability and multi-physics field coupling characteristics.

Figure 2 Multi-physical field characteristics coupling relationship and design flow

They put forward a model of an ultra-high-speed micro-motor with an integral support system, and designed a rotor structure with a 2-pole surface-mounted Nd2Fe14B alloy sheath and a stator structure with “no slot - 6 virtual slots”. In the multi-physics analysis, the electromagnetic, loss and temperature rise coupling characteristics of the designed prototype were studied, verifying that the characteristics met the design requirements. Then, based on the temperature field variation, the rotor strength under 0 rpm-22°C, 300000 rpm-35°C, 550000 rpm-50°C and 550000 rpm-80°C operating conditions were calibrated, and the overfill amount was optimally designed to range from 8 to 12μm. The influence of support position and support stiffness on the critical speed was investigated for the overall support structure, the support position was reasonably selected and the design range of support stiffness was judged. After several iterations of designing, an all-round design solution to meet the requirements of multiple physical fields was obtained.

                                                                                                   Figure 3 Schematic diagram of integrated rotor support system

Figure 4 Experimental prototype and platform

The researchers finally realized the processing of the prototype based on the theoretical design and conducted a comprehensive test and evaluation of the prototype. The experimental results showed that the prototype successfully achieved a stable operation of 550,000 rpm, which proved the rationality and feasibility of the design.

The above research results were published in Journal of Electrotechnology, Vol. 14, No. 2021, entitled “Multi-coupling-based design of an integrally supported ultra-high-speed miniature permanent magnet motor” by Qixing Gao and Xiaolin Wang.

Reproduced from: Scientists from China Southern Airlines and other units released the research results of integrally supported ultra-high-speed micro permanent magnet motors 2021-12-19, Electric New Technology.