The Study of Demagnetization of the Magnetic Orientation of Permanent Magnets for IPMSM with Field-Weakening Control under Hot Temperature

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1 Journal of Electrical Engineering 6 (2018) doi: / / D DAVID PUBLISHING The Study of Demagnetization of Magnetic Orientation of Permanent Magnets for Noriyoshi Nishiyama 1, Hiroki Uemura 2 and Yukio Honda 2 1. Panasonic Corporation, Moriguchi City, Osaka , Japan 2. Osaka Institute of Technology, Osaka City, Osaka , Japan Abstract: In this study, we investigate demagnetization resistance of a concentrated winding IPMSM (interior permanent magnet synchronous motor) accounting for field weakening control by changing magnetization direction of permanent magnet under a high-temperature environment. IPMSMs are investigated by FEA (finite element analysis) using same volume of permanent magnet while changing magnet s width, thickness and magnetic field orientation angle. FEA found that a V-shaped angle Va = 100 and a changed magnet length of 97% using an oblique magnetic-field-oriented magnet strike a good balance between demagnetization resistance and torque at 180 C. Comparison between demagnetization of negative d-axis current (current phase β = 90 ) and demagnetization of field weakening control (β = 80 ) using concentrated winding V-shaped angle Va = 100 is conducted. With demagnetization factor at β = 80 for β = 90, demagnetization factor 0.39 (2.6 times) at α = 0 decreases to 0.23 (4.3 times) at α = 20. The demagnetization resistance in field weakening control is furr improved. Key words: IPMSM, demagnetization, concentrated winding, high temperature, field weakening. 1. Introduction The high efficiency of a small-sized permanent magnet synchronous motor is one of its key features. On or hand, one of its disadvantages is that coercive force decreases at high temperature and n easily demagnetizes. In recent years, motors for vehicles, high-temperature heat pumps, and extreme-environment-compatible robots have had to operate in severe load environments, which are often extremely high-temperature environments where permanent magnets are demagnetized. In this work, we examined a parallelogram-shaped permanent magnet whose magnetic field orientation was made oblique with respect to plate thickness direction (see 1). Lm is magnet length, Am is magnet cross-sectional area. Corresponding author: Noriyoshi Nishiyama, doctor of philosophy in Engineering, research fields: permanent magnet synchronous motor design. For obliquely distributed magnets with an oblique orientation angle α, magnetic field orientation inclines from plate thickness direction with respect to magnet width Wm and magnet thickness tm. Furrmore, operating point of magnet does not change much from permeance equation, since magnetic field H is acting on magnet. On or hand, reverse magnetic field component affecting magnetization direction of magnet can be reduced to Hm, thus improving demagnetization resistance [1, 2]. Few studies have attempted to examine magnetic field orientation of permanent magnets detailed demagnetization characteristics under high temperature environments [3-8]. A field weakening control that energizes negative d-axis current component can suppress induced voltage of motor and can operate in wide rotational speed range under a limited power supply voltage [9]. The demagnetization in field-weakening control is likely to

The Study of Demagnetization of 145 1 Oblique magnetic orientated magnet. occur at a current phase β = 80 where negative d-axis current component is very large.

control by changing magnetization direction of permanent magnet under a high-temperature environment. 2.

In addition, IPMSM, in which a magnet is embedded in a rotor core, is generally arranged in a rectangular parallelepiped permanent magnet, and by deeply embedding a magnet in rotorr core, magnetic

(1): LmAgσ c= AmLgf Pc where Ag is air across-sectional area, Lg is air gap length, σ is leak coefficient, and f is magneto motive force loss factor.

2 The Study of Demagnetization of Oblique magnetic orientated magnet. occur at a current phase β = 80 where negative d-axis current component is very large. In this paper, we investigate optimum design for improving demagnetization resistance of a concentrated winding IPMSM (interior permanent magnet synchronous motor) accounting for field weakening control by changing magnetization direction of permanent magnet under a high-temperature environment. 2. Demagnetization Analysis A concentrated winding motor is advantageous for use in high-temperaturee environments. In addition, IPMSM, in which a magnet is embedded in a rotor core, is generally arranged in a rectangular parallelepiped permanent magnet, and by deeply embedding a magnet in rotorr core, magnetic field acting on magnet can be relaxed, which improves demagnetization resistance. Permeance coefficient Pc, whichh determines operating point of permanent magnet, is expressed as Eq. (1): LmAgσ c= AmLgf Pc where Ag is air across-sectional area, Lg is air gap length, σ is leak coefficient, and f is magneto motive force loss factor. The improvement of demagnetization resistance is greatly affected by angle difference between direction of working magnetic field and magnet orientation direction. The aim of demagnetization analysis is to study optimum design by changing magnetic field orientation angle α and magnet (1) arrangement angle Va while also changing magnet length Lm and magnet width Wm under a fixed magnet volume at 180 C. A current causing a reversee magnetic field to act on rotor s magnet is passed through winding and air gap, and magnetic flux density before and after energy conduction is obtained by electromagnetic field analysiss software (JMAG-Designer). A neodymium sintered magnet (reversible data of NMX-S 36 UH) is used. [10] The analysis conditions are shown in Table 1, and analysis model is shown in 2. The evaluation indexes are demagnetization limit current and torque. The demagnetization ratio Dr n is defined as Eq. (2): The demagnetization improvement ratio Dir n is defined as Eq. (3): Dr n = 100 (1 - B2/B1) (2) Dir n = Dr n / Dr 0 (3) The torque ratio Tr n is defined as Eq. (4): Tr n = T n / T0 (4) where B1 is air gap s magnetic before reversed magnetic field is applied. B2 is air gap s magnetic flux density after reversed magnetic field is applied. Dr 0 is demagnetization ratio at Table 1 Analysiss conditions. Stator ID 56 mm Rotor OD mm Residual magnetic 1.16 T flux density Br Stack length Magnet volume Winding 32 mm 1135 mm 3 (at one pole) 150 turn, 3Y Coercivity Hcj Electromagnetic steel sheet Temperature flux density 2,387 ka/m 35A C 2 V-shaped magnet arrangement motor model.

146 The Study of Demagnetization of parallel-oriented magnet (α = 0 ) placed in flat arrangement (Va = 180 ).

The demagnetization limit current is maximum current at which reduction of non-conducting air gap s magnetic flux density, before and after application of reversee

increases in V-shaped arrangement, demagnetizationn improvement ratio also decreasess as magnet length ratio decreases. Furr, if 3.

As magnet arrangement angle Va is changed, demagnetization improvement ratio Dr n and torque ratio Tr n are in a trade-off relationship (see 3).

For demagnetization resistance improvement ratio, demagnetization limit current of a rotor with flat-plate magnet arrangement using a parallel-oriented magnet is 100%

3 146 The Study of Demagnetization of parallel-oriented magnet (α = 0 ) placed in flat arrangement (Va = 180 ). T0 is torque at parallel-oriented magnet (α = 0 ) placed in flat arrangement (Va = 180 ). The demagnetization limit current is maximum current at which reduction of non-conducting air gap s magnetic flux density, before and after application of reversee magnetic field current, does not yet reach 1%. The torque is calculated with rated current ( 7.07 Arms) and current phase β = 20. increases in V-shaped arrangement, demagnetizationn improvement ratio also decreasess as magnet length ratio decreases. Furr, if 3. Magnet Length Resistancee and Demagnetization Here, we consider demagnetizationn analysis by an analytical model in which magnet arrangement angle Va is changed from 180 to 60. Based on a 1% demagnetization limit current of flat plate model (Va = 180 ), this value is increased 3 times for Va = 100 model and 8 times for spoke model (Va = 60 ) ). As magnet arrangement angle Va is changed, demagnetization improvement ratio Dr n and torque ratio Tr n are in a trade-off relationship (see 3). In analysis results, horizontal axis showss magnet length ratio and vertical axis showss demagnetization resistance improvement ratio or torque ratio (see Figs. 4 and 5). For demagnetization resistance improvement ratio, demagnetization limit current of a rotor with flat-plate magnet arrangement using a parallel-oriented magnet is 100% (circle within solid line; 1). The torque ratio is ratio of parallel-oriented magnets to flat arrangement rotor s torque (circle within solid line; 1). Under condition of a constant magnet volume, at a magnet length of less than 100%, magnet width is increased. On or hand, at a magnet length of 100% or more, magnet width is constant using an oblique magnetic-field-oriented magnet. Although demagnetization improvement ratio 3 Magnet arrangement angle Va vs. demagnetizationn ratio and torque ratio. 4 Magnet length ratio vs. demagnetizationn improvement ratio. 5 Magnet length ratio vs. torque ratio

4 The Study of Demagnetization of 147 demagnetization improvement ratio is improved, torque ratio decreases. Aiming for demagnetization improvement ratio of 200% or more, torque ratio becomes 90% or less (circle in dottedd line; 2.3). They are in a trade-off relationship. Next, we investigate possibility of improving demagnetization resistance by oblique magnetic field oriented magnets when magnet length ratio of Va = 100 model is less than 100%. In oblique magnetic-field-oriented magnet, even if magnet thickness tm is small, magnet length Lm can be increased, and thus improvement in demagnetization resistance can be expected. Using Va = 100 model offers a lot of latitude in magnet arrangement, so we investigated demagnetization resistance and torque using magnet length Lm as a parameter. A list of studied magnets is given in Table 2, based on a motor model in which magnet 1 is used as a reference. Demagnetization is evaluated as ratio of demagnetization limit current ratio to 1% demagnetization limit current. Torque is evaluated as ratio of magnet s torque ratio to average value of air gap s magnetic flux density at time of non-conduction. The torque of IPMSM is sum of magnet torque and reluctance torque, since maximum torque per current is about 20 in current phase and this is a motor model mainly based on magnet torque. In orderr to clarify influence of using different magnets, here we compare torque with magnet torque ratio. In analysis resultss (see 6), horizontal axis is magnet length ratio, and vertical axis shows demagnetization limit current ratio and torque ratio. The demagnetization limit current ratio is greatly reduced for a motor using magnet 2 or magnet 3 with a magnetic field orientationn angle α = 0 obtained by increasing magnetic width Wm through reducing magnet thickness tm (i.e., reduction of magnet length Lm). On or hand, in a motor in which magnet 4 or Table 2 No Magnet parameters. Magnet length Lm [mm] Magnet width Wm [mm] Magnet length ratio vs. demagnetizationn improvement ratio and torque ratio. magnet 5 with magnetic field orientation angle α = 20 is used, demagnetization limit current ratio is improved over that of motor using magnets with magnetic field orientation angle α = 0. The change in torque ratio increases as magnet width Wm increases, but it is smaller than change in demagnetization limit current ratio. 4. Demagnetizationn Field-Weakening Control magnetic orientation angle α [deg.] Ratio magnet length ratio 100% 97% 91% 97% 90% at The demagnetization ratio of field weakening control (β = 80 ) decreases with respect to negative d-axis of current (β = 90 ), and this ratio is defined as current phase demagnetization ratio Cr n. Cr 80 = Dr 80 / Dr 90 (5) where, Dr 80 is demagnetization ratio at β = 80, Dr 90 is demagnetizationn ratio at β = 90. The smaller current phase demagnetization ratio Cr, more margin is given to demagnetizationn resistance in field-weakening control.

148 The Study of Demagnetization of 7 shows relationship between demagnetization ratio Dr with respect to current at magnetic orientation angle α = 0 and α = 20 in Va = 130 model.

5 148 The Study of Demagnetization of 7 shows relationship between demagnetization ratio Dr with respect to current at magnetic orientation angle α = 0 and α = 20 in Va = 130 model. At magnetic orientation angle α = 0, when current is 42.5 A, demagnetization ratio at β = 90 (Dr 90) is 0.91%, and demagnetization ratio at β = 80 (Dr 80) is 0.79%. When current phase differs by 10, demagnetization ratio is reduced to times. At magnetic orientation angle α = 20, when current is 70 A, demagnetizationn ratio at β = 90 (Dr 90) is 0.91%, and demagnetization ratio at β = 80 (Dr 80) is 0.67%. If current phase differs by 10, demagnetization factor is reduced to 0.67 times. At α = 20 with respect to α = 0, in addition to large demagnetizing resistance current, demagnetization ratio when β is shifted from 90 to 80 is small, demagnetization resistance ratio is high. improvement ratio Dir = 90% (magnet 5) ), Cr is about same value as magnet 1. At demagnetization improvement ratio Dir = 90% or more, current phase demagnetization ratio Cr is also improved. 6. Measureme ent of Magnetic Flux Density We evaluate magnetic flux density of prototype magnets with magnets were produced by obliquely slicing samariumm field orientationn direction. market demand for neodymium sintered magnets, it is difficult to use m for special prototypes under development; refore, we used samarium cobalt sintered magnets to evaluate difference in orientation angles. oblique orientation. The prototype cobalt sintered magnet with respect to magnetic Since ree is a large 5. Magnet Length and Demagnetization Ratio at Field Weakening Control 8 shows current phase demagnetization ratio Cr with respect to magnet length ratio, which is 1, 2, 3, 4, and 5 in Table 2. With magnetic field orientation angle α = 0, motor using magnet 1 (magnet length 100%) indicates Cr = With magnet length ratio of 97%, motor using magnet 2 indicates Cr = On or hand, with magnetic field orientation angle α = 20, motor using magnet 4 indicates a smaller Cr = With magnet length ratio of 90% and, magnetic field orientation angle α = 0, motor using magnet 3 indicates Cr = 0.54; whereas with magnet orientation angle α = 20, motor using magnet 5 indicates a smaller Cr = At magnet length ratio of 97% (magnet 2, magnet 4), current phase demagnetization ratio Cr is improved. At magnet length ratio of 90%, Cr is as low as 74% (magnet 3), and at demagnetization 7 Current vs. demagnetization ratio. 8 Magnet length ratio vs. current demagnetization ratio. phase

6 The Study of Demagnetization of 149 Two types of magnets have following magnetic field orientation angles: α = 0, 20. The prototype magnet size is magnet thickness tm = 1.8 mm, magnet width Wm = 10.8 mm, and axial length L = 31 mm. The magnetic flux density measuring devicee is composed of a magnet analyzer (MAD-300R, DMT), a tesla meter (TM-4700, DMT), and an ultrafine probe (w 0.75 mm - t 0.28 mm F-075, DMT) with a resolution of 21,600 (see 9). Next, prototype magnets were assembled into a rotor with magnet arrangement angle Va = 130, and magnetic flux density was measured. Furrmore, rotor was assembled in a cylindrical dummy stator made of a magnetic body having same inner diameter as stator core, and magnetic flux density of air gap was measured (see 10). We discuss results of air gap s magnetic flux density measured for rotor assembled in dummy stator. As a result of measuring air gap s magnetic flux density, it is found that maximum magnetic field orientation angle decreased slightly, but magnetic flux density distribution approached a sinusoidal wave form and is thus effective for reducing torque ripple. 11 shows air gap magnetic flux density of motor by FEA. The magnetic field orientation angle α = 0 indicated by solid line and a = 20 indicated by broken line are non-energized air gap magnetic flux densities. The magnetic flux density of 47 to 53 and 87 to 10 Air gap magnet flux density (measured). 11 Air gap magnet flux density (analysis). 90 on horizontal axis is distorted by open slot of stator. The air gap magnetic flux density of α = 20 is as small as 7% as compared with α = 0, close to a sinusoidal wave from 66 to 72 on horizontal axis without effect of open slot. It is similar to actual measurement at air gap magnetic flux density of cylinder dummy stator. 7. Conclusion ns 9 Air gap magnet flux density measuring device. In this paper, we examined optimum design accounting for field-weakening control oblique magnetic field orientation of IPMSM driven under a high-temperature environment, changingg magnet thickness tm and magnet width Wm while maintaining a constant magnet volume. We obtained that demagnetizationn factor Cr at β = 80 for β = 90, Cr = 0.39 (2.6 times) at α = 0 decreases to Cr = 0.23 (4.3 times) at α = 20 by FEA (finite element analysis) and investigation of our prototype. The demagnetizationn

7 150 The Study of Demagnetization of resistance of oblique magnetic orientation magnets IPMSM in field-weakening control is furr improved. References [1] Nishiyama, N., and Honda, Y Development of IPMSM for High Temperature Conditions Using Inclined Magnetic Field Orientation and V-Shape Magnet Arrangement. In Proceedings of IEEJ Joint Technical Meeting on Magnetics and Linear Drives MAG , LD , pp (in Japanese) [2] Uemura, H., Nishiyama, N., and Honda, Y The Effeteness of Magnetic field directions of V shaped embedded magnets for IPMSM. In Proceedings of 2017 Ann. Meet. Rec. IEEJ, V, p. 15. (in Japanese) [3] Nishiyama, N., Uemura, H., and Honda, Y The Study of Highly Demagnetization Performance IPMSM under Hot Environments. In International Conference on Electrical Machines and Systems 2017 Proceedings, [4] Asano, Y., Honda, Y., Takeda, Y., and Morimoto, S Reduction of Vibration on Concentrated Winding Permanent Magnet Synchronous Motors with Considering Radial Stress. IEEJ Trans. on Ind. Appli. l21 (11): (in Japanese) [5] Maeda, Y., Urata, S., and Nakai, H The Evaluation of Demagnetizing Characteristics of Permanent Magnet in Arbitrary Directions. In Proceedings of 2016 Ann. Meet. Rec. IEEJ, II, pp (in Japanese) [6] Akune, R., Akatsu, K., Kume, K., Yamamoto, T., and Saito, S The Anti Demagnetization Method for Permanent Magnet Synchronous Motor Focused on Magnetized Direction of Permanent Magnet and Basic Experiment. In 2016 IEE-Japan Industry Applications Society Conference (JIASC2016), (in Japanese) [7] Peng, P., Xiong, H., Zhang, J., Li, W., Leonardi, F., Rong, C., Degner, M. W., Liang, F., and Zhu, L Effects of External Field Orientation on Permanent Magnet Demagnetization. In Proceedings of IEEE Energy Conversion Congress and Exposition 2016, [8] Galea, M., Papini, L., Zhang, H., Gerada, C., and Hamiti, T Demagnetization Analysis for Halbach Array Configurations in Electrical Machines. IEEE Trans. on Magn. 51 (9): [9] Kawano, S., Murakami, H., Nishiyama, N., Ikkai, Y., Honda, Y., and Higaki, T High Performance Design of an Interior Permanent Magnet Synchronous Reluctance Motor for Electric Vehicles. In Proceedings of Power Conversion Conference, Nagaoka, Vol. 1, pp [10] Nishiyama, N., Uemura, H., and Honda, Y The Optimum Design of Magnetic Orientation of Permanent Magnets for IPMSM under Hot Environments. In Proceedings of IEEE International Conference on Power Electronics and Drive Systems, pp

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