Development of a Novel Yokeless and Segmented Armature Axial Flux Machine Based On Soft Magnetic Powder Composites (WP ) I.

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1 Manuscript refereed by Chairman Name (Company, Country) Development of a Novel Yokeless and Segmented Armature Axial Flux Machine Based On Soft Magnetic Powder Composites (WP ) Bo Zhang (Karlsruhe Institute of Technology, Rintheimer Querallee 2 Building 70.04, 76133, Karlsruhe, Germany) bo.zhang@kit.edu; Schoppa Andreas (PMG Füssen GmbH, Hiebelerstr. 4, Füssen, Germany) andreas.schoppa@pmgsinter.com; Martin Doppelbauer (Karlsruhe Institute of Technology, Rintheimer Querallee 2 Building 70.04, 76133, Karlsruhe, Germany) Martin.Doppelbauer@kit.edu; Abstract The magnetic and mechanical properties of soft magnetic powder composites (SMC) have been recently significantly improved. Within the project Design and analysis of an electric motor based on new material components (SMC) Daimler AG, the Institute of Electrical Engineering (ETI) at the Karlsruhe Institute of Technology (KIT) and PMG Füssen GmbH examined and evaluated the use of soft magnetic composites in the electrical machine with segmented armature torus structure, especially as a main drive in hybrid electric or pure electric vehicles. Because of the existing threedimensional magnetic flux in this machine topology, the complex soft magnetic components can only be manufactured with SMC rather than with conventional electrical steels. Based on 3D FEM electromagnetic analysis, the mechanical construction and analysis have been carried out and a prototype has been built at KIT. The measurements of the manufactured prototype on the test bench in ETI proved its simplicity, stability and functionality. I. Introduction Due to the fast development of electric mobility in recent years, a lot of researches have been carried out to develop the novel topologies of electrical machines to meet the special requirements such as high compactness, high efficiency, high power and torque density for the application as electrical traction machine in the hybrid electric vehicle and pure electrical vehicle. Owing to the rapid development of the material science, the performance of electrical machine has been continuously improved. Usually, the non-grain-oriented electrical steel (NGO) has been widely used to manufacture the soft magnetic components of traditional electrical machines with radial magnetic flux in the air gap. Nevertheless, the soft magnetic composites (SMC) have become a competitive alternative as the mechanical and magnetic properties of SMC have been significantly improved in the past years. Compared with the laminated electrical steel, the SMC, whose microstructure is illustrated in Fig. 1, consist of many fine iron particles coated by the insulation layers. a) SMC for electrical machine b) SMC for power electronics Fig. 1: Micro structure of SMC [KIT - Institute for Applied Materials] Due to the special structure, the SMC have many unique characteristics, which can be utilized to improve the performance of traditional electrical machines and to develop novel designs of electrical

2 machine with more complex magnetic circuit. First of all, the SMC are isotropic, which means the physical properties such as thermal conductivity and relative permeability are independent from the direction [1]-[2]. Subsequently, the electrical resistance of SMC is much higher than NGO because of the thin insulation layer around each iron particle, which results in lower eddy current losses. At last, the manufacturing process of SMC consists of only compaction and heat treatment. Therefore, complex components with high surface quality and tight tolerance can be net-shaped manufactured. However, the disadvantages of SMC should also be taken into account during the electromagnetic design and construction of electrical machine, which are mainly: Low magnetic permeability due to insulation layer and pores High values of magnetic field strength required to reach magnetic saturation High hysteresis losses and inferior mechanical properties Limited geometrical size of the available parts due to required high pressure during the compacting process The paper is organized as follows. In chapter II, as the basis of the electromagnetic design, the most important properties such as magnetization curve and iron losses are determined. Subsequently, the electromagnetic design and mechanical construction of an axial flux machine are described in the chapter III. In order to validate the results of electromagnetic design, a prototype has been built and measured, which is described in chapter IV. II. Properties of SMC Although the manufacturing process of SMC components consists of only compaction and heat treatment, there are still a lot of factors such as the compaction pressure and the temperature during the heat treatment, which can influence the ultimate properties of SMC significantly. The most important magnetic properties of SMC are measured based on the ring specimens, as illustrated in Fig. 2. Fig. 2: Geometry of SMC ring specimen and the measured commutation curves The measured commutation curves of the different SMC products of PMG Füssen GmbH are illustrated in Fig. 2. For a better comparison, the commutation curves of the conventional laminated electrical steels NO20 and M235-35A are added into Fig. 2 as well. The NGO has better permeability and higher flux density than SMC. However, the differences become smaller with increasing magnetic field strength. Furthermore, the SMC Somaloy700HR3P and SironS720 demonstrate better performance because of the absence of organic binder. However, the newly developed SironSTestb with organic binder shows obviously better permeability and higher saturation flux density than the other machinable SMC, even than that of the SironS720 at high magnetic field strength. Besides the permeability, the iron losses are another important magnetic characteristic to evaluate SMC. With help of the accurate power analyser WT3000, the iron losses within the SMC ring are measured for electrical excitation with different combinations of frequency and flux density. For

3 instance, the measured iron losses of SMC under alternating field at 50Hz and 2000Hz are compared with that of three laminated electrical steels, as illustrated in Fig. 3. a) P fe at 50Hz Fig. 3: Iron losses of SMC ring under alternative magnetic field b) P fe at 2000Hz It can be noted that the electrical steels have obviously lower iron losses than that of SMC at low frequency region. However, the differences become smaller with increasing frequency. The transition between M235-35A and SMC occurs at the frequency 2000Hz and induction equaling to 1.5T. In this case, the iron losses of M330-35A is already higher than that of SMC. It should be noted that for the NO20 with thickness equals to 0.20mm, the iron losses remain very low, even at a very high frequency. This tendency depends on the specific density of the SMC-material. With increasing density the magnetizing behavior of SMC becomes better and the eddy current losses lower, shifting the transition point between NGO and SMC to lower values of operating frequency [3]. Recent investigations show that the iron losses of SMC can already be lower than these of electrical steels (for thickness 0.35mm) at frequency values of approx. 500 Hz [4]. In order to calculate the iron losses of SMC with the finite element method (FEM), the following iron losses model based on Bertotti formula has been utilized in this work, where f and B are the frequency and induction of the excitation magnetic field, the a, b, c, khy, kw and ka are coefficients calculated through surface fitting of the measured data, the j and i are the number of the SMC elements in FEM model and the order of the harmonic. P fe = 2 ˆ ˆ a B1 + b B1 + c k + + hy f Bˆ kw( i f ) Bˆ i ka( i f ) Bˆ i j j i In order to validate the developed iron losses model, a test system has been built, which consists of a yoke and a tooth made from the same SMC material SironSTestb, as illustrated in Fig.4. (1) Fig. 4: Test system and the corresponding 3DFEM model to validate the iron losses model During the measurement, the primary coil is excited with a sinusoidal voltage, while the iron losses of the complete system are measured with the same power analyser. On the other hand, the iron losses of the system calculated with 3D FEM using (1) for different voltage of the primary coil. The results are listed in Table 1. It can be noted that the deviation is lower than 8% for the frequency lower

4 than 500Hz, which becomes larger with increasing frequency, especially for the low voltage of primary coil. The most important reasons are the measuring error of current in the primary coil, the machining influence, the small air gap between the yoke and tooth, the existence of rotational field and some other undefined reasons. In spite of the errors, the measurement results match relatively well with the calculated iron losses based on FEM, which implies the developed model is valid and can be utilized for the iron losses calculation of the electrical machine. TABLE I: COMPARISON OF THE MEASURED AND CALCULATED IRON LOSSES OF THE TEST SYSTEM f [Hz] P fe (u 1 = 24 V) P fe (u 1 = 32 V) P fe (u 1 = 36 V) Mes. FEM Mes. FEM Mes. FEM W 16.3 W 24.3 W 24.6 W 27.3 W 29.3 W W 11.6 W 19.3 W 18.4 W 23.8 W 22.2 W W 7.44 W 12.3 W 11.5 W 14.7 W 14.3 W W 6.08 W 8.49 W 9.40 W 10.2 W 11.5 W III. Electromagnetic design and mechanical construction of an AFM There are many novel electrical machines suitable for the application of SMC, among which the axial flux machine (AFM) and the transverse flux machine (TFM) illustrated in Fig. 5 are two promising candidates considering the 3D magnetic flux and the complex soft magnetic components [5]-[6]. The AFM with internal segmented armature (AFM-ISA) consists of an internal stator with double layer concentrated winding and two external rotors with surface mounted permanent magnet (PM). The other topology is the TFM with claw pole structure (TFM-CP). In comparison, the PMs with opposite circumferential magnetization directions are embedded in the rotor. All the stator segments with claw pole structure are placed on one side of the rotor and a ring winding is located within the stator segments for each phase. Fig. 5: AFM with internal segmented armature and one phase TFM with claw pole structure Considering the geometric design constrains, the available inverter, the required PM mass, an AFM-ISA and a TFM-CP have been designed. Through a detailed comparison of the two topologies with a radial flux machine, the AFM-ISA has been determined as the most suitable topology for the application of SMC, whose parameters are listed in Table II and efficiency map is illustrated in Fig. 6. TABLE II: MAIN PARAMETERS OF AFM-ISA Symbol Description Value Symbol Description Value D o active outer diameter 300 mm N rated rated rotational speed 2600 rpm D i active inner diameter 200 mm T rated rated torque 115 Nm L motor active axial length 75 mm I rms Maximum phase current 300 A p number of pole pairs 12 U rms Maximum phase to phase voltage 170 V N s number of stator segments 36 mpm mass of PM 0.83 kg L g axial length of air gap 1 mm w number of turns per segment 26

5 Fig. 6: Efficiency map of the AFM-ISA In order to validate the FEM results of electromagnetic design, a prototype has been built and measured. For this purpose, a mechanical construction of AFM-ISA is necessary to fix the electromagnetic relevant components. It is a great challenge due to the limited mechanical properties of SMC and the large axial force between the stator and the rotors, especially when the two air gaps are asymmetrical resulted from the inevitable manufacturing and assembly tolerances. The final developed mechanical construction is illustrated in Fig 7. Fig. 7: Mechanical construction of the AFM-ISA and the adhesive test system The developed mechanical construction should be validated with great attention. First of all, the mechanical stress and deformation of the critical components should be calculated with 3D FEM and compared with the allowable ultimate stress of the brittle materials and the tensile stress of the ductile material. On the other hand, the maximal allowable stress of adhesive at different temperature should be investigated with the test system in Fig. 7, as the PMs are glued on the rotor yoke and suffer from great axial and centrifugal force. Furthermore, the design of bearing system is also critical considering the large axial force. In the ultimate design, a fixed-floating bearing arrangement has been utilized for the prototype. Two accurate spindle bearings in O-arrangement are used as fixed bearings, while a conventional deep groove ball bearing works as the floating bearing. The lift time of the bearing system is examined for the extremely asymmetric air gaps, which is enough for the measurement on the test bench. At last, a thermal analysis coupled with the fluid dynamic analysis has been carried out to investigate the air flow and the temperature field in the AFM-ISA considering its complex structure and lack of calculation experience [7]-[8]. IV. Prototyping and measurement Based on the above electromagnetic, mechanical and thermal analyses and a series of preliminary tests, the prototype of IAFM-ISA has been built and measured at the self-developed test bench, as illustrated in Fig. 8.

6 Fig. 8: Prototype and test bench For instance, the no-load losses and the torque of AFM-ISA at 200rpm with only current along the quer axis are illustrated in Fig. 9. It can be noted that the FEM results match well with the measurement. For the maximal current iq = 440 A, there is a torque deviation equalling to 6.39 Nm. Fig. 9: No-load losses and torque of AFM-ISA at 200 rpm However, the deviation becomes higher with increasing speed. For instance, the torque of the prototype and the efficiency map are illustrated in Fig.10. The most important reason is the low level of current control quality, which leads to high harmonic contents of the current. The harmonics contained in the current cause higher copper losses, higher eddy current losses in PM and higher SMC iron losses. Fig. 10: Measured torque of AFM-ISA at 2000 rpm and the efficiency map V. Conclusion In this work, the electromagnetic design of an AFM-ISA has been carried out based on the electromagnetic properties measurement of many different SMC products of the company PMG Füssen GmbH. Subsequently, a prototype has been built and measured on the self-developed test bench. Based on the above work, it can be concluded that the novel electrical machines with complex SMC parts can be competitive alternative to the traditional radial flux machine, especially when there are some special demands such as short axial length, high torque density or high power density.

7 VI. References [1] H. Shokrollahi and K. Janghorban, "Soft magnetic composite materials (SMCs)," Journal of Materials Processing Technology, vol. 189, pp. 1-12, Feb [2] E. Bayramlıa, Ö. Gölgelioğlua and H. Bülent Ertanb, "Powder metal development for electrical motor applications," Journal of Materials Processing Technology, vol. 161, Apr. 2005, pp [3] A. Schoppa, P. Delarbre, Fertigungstechnische Herausforderungen bei weichmagnetischen Pulverwerkstoffen, 32. Hagener Symposium Pulvermetallurgie 2013, Hagen, , Band 29, Seite Pulvermetallurgie in Wissenschaft und Praxis [4] A. Schoppa, P. Delarbre, E. Holzmann, M. Sigl, Magnetic properties of soft magnetic powder composites at higher frequencies in comparison with electrical steels, 3rd Electric Drives Production Conference (EDPC), Nuremberg, , ISBN , Seite 1-5, Published: IEEE Nr /EDPC [5] B. Zhang, T. Epskamp, M. Doppelbauer and M. Gregor, "Development of a Yokeless and Segmented Armature Axial Flux Machine," IEEE Trans. Ind. Electron., vol. 63, no. 4, pp , Apr [6] B. Zhang, T. Seidler, R. Dierken and M. Doppelbauer, "Mechanical construction and analysis of an axial flux segmented armature torus machine," International Conference on Electrical Machines (ICEM), pp , Sept [7] F. Marignetti and V. D. Colli, "Thermal analysis of an axial flux permanent-magnet synchronous machine," IEEE Trans. Magn., vol. 45, no. 7, pp , Jul [8] F. Marignetti, V. Delli Colli and Y. Coia, "Design of Axial Flux PM Synchronous Machines Through 3-D Coupled Electromagnetic Thermal and Fluid-Dynamical Finite-Element Analysis," IEEE Trans. Ind. Electron., vol. 55, no. 10, pp , Oct