Comparative Study of 6/4 FSPM and SPM Machine for High-Speed Applications Mingda Liu, William Sixel, and Bulent Sarlioglu Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC) Dept. of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706 sarlioglu@wisc.edu Abstract – The surface permanent magnet (SPM) and flux- switching permanent magnet (FSPM) machines are good candidates for high power density and high-speed applications. The SPM and FSPM machines share several similarities such as negligible reluctance torque and high equivalent air gap length. This paper gives a comprehensive comparison on the design of an eight pole SPM and a dual-stator 6/4 FSPM at two different design speeds. The choice and characteristics of magnet retaining sleeves in the SPM machine such as material, thickness, and loss at different speeds are discussed. Depending on the choice of the sleeve, the SPM machine can have severe thermal issues at the high-speed conditions due to either eddy current losses in the sleeve and magnet or low thermal conductivity of the sleeve material. In contrast, the dual-stator 6/4 FSPM machine can operate at high speeds easily because there is no magnet or retaining sleeve in the rotor. I. INTRODUCTION High-speed permanent magnet machines have been investigated in a wide range of applications because of their higher efficiency, higher power density, and more compact size compared to induction machines [1]-[4]. The flux- switching permanent magnet (FSPM) machine is an attractive machine topology because of its unique features such as robust rotor structure and easy thermal management [5]-[7]. In reference [8], a 6-slot 4-pole (6/4) FSPM machine with dual-stator geometry is proposed. Compared to the most commonly seen 12/10 FSPM machine, the 6/4 FSPM machine reduces the fundamental frequency by 60%, which makes the 6/4 FSPM machine more suitable for high-speed applications. The FSPM machine and SPM machine share several similarities such as negligible machine saliency and large equivalent air gap in the machine magnetic circuit. By developing a quantitative comparison between FSPM and SPM machine, the differences in the machine performance can be used to guide their selection in various high-speed applications. Interior permanent magnet (IPM) machines are not widely used in very high-speed applications because of the structural sizing of the iron bridge in the rotor cavity between the permanent magnets [9]. Surface permanent magnet (SPM) machines use a retaining sleeve over the permanent magnets to protect the magnets from flying apart. With high strength sleeve material, the SPM machine is able to achieve more than 200 m/s tip speed [9]. However, in the SPM and IPM machines, the magnets are more thermally isolated from the stator cooling structure and the cooling of the magnets can be difficult [10]-[12]. Conventional cooling strategies such as stator water jacket and shaft-mounted fan are either thermally far away from the rotor or have limited heat rejection capability, respectively. In contrast, the FSPM machine has most of the loss components in the stator, which have short thermal paths to the heat sink. Several papers have presented comparisons between the SPM and FSPM machine [13]-[15] for different topologies and performance characteristics. It is shown that the FSPM machine has a higher air gap flux density due to flux focusing effect and less mechanical issues in the rotor compared to the SPM machine. However, the comparisons are limited to only electromagnetic performance. The structural and thermal issues are not typically addressed. In addition, the machine fundamental frequency is not always kept constant for a fair comparison. In references [16], [17], FSPM machine is compared with SPM machine at high- speed conditions. It is shown that the SPM machine flux linkage has a 33% reduction due to the thick retaining sleeve. The structural aspect is investigated in both papers. However, the rotor heating issue due to the retaining sleeve and magnet eddy current losses is not discussed. In addition, existing papers focus on the 12/10 FSPM machine or other high pole topologies, which is not preferable for high-speed applications due to the high fundamental frequency. This paper presents a comprehensive comparison between 6/4 FSPM machine and 8-pole SPM machine in terms of electromagnetic performance and thermal behavior at high- speed conditions. Section II presents the design of the FSPM and SPM machine. The characterizations of the magnet retaining sleeve at different rotation speeds are also presented. The electromagnetic performance of the FSPM machine and the SPM machine are shown and compared in Section III. Section IV compares the thermal behavior of the FSPM machine and SPM machine. A discussion on the comparison of the FSPM and SPM machine is given in Section V. The conclusions are drawn in Section VI. II. DESIGN PARAMETERS OF 6/4 FSPM MACHINE AND SPM MACHINE UNDER COMPARISON In reference [8], a 10 kW, 15 krpm 6/4 FSPM machine with NdFeB magnet is designed. With the dual-stator structure as shown in Fig. 1, there are negligible even harmonics in the flux linkage and back EMF which is a severe issue in the conventional 6/4 FSPM topology. The machine sizing and operating parameters are given in Table I. An SPM machine is designed with the same output power 978-1-5386-9310-0/19/$31.00 ©2019 IEEE