The Optimum Design of the Magnetic Orientation of Permanent Magnets for IPMSM under Hot Environments

380 IEEE PEDS 2017, Honolulu, USA 12 15 December 2017 The Optimum Design of the Magnetic Orientation of Permanent Magnets for IPMSM under Hot Environments Noriyoshi Nishiyama 1. Hiroki Uemura 2, Yukio Honda 2 1 Panasonic Corporation, 3-1-1 Yagumo-naka-machi, Moriguchi City, Osaka 570-8501, Japan 2 Osaka Institute of Technology, 1-45, Chaya-machi, Kita-ku, Osaka City, Osaka 530-0013, Japan Email: nishiayam.noriyoshi@jp.panasonic.com Abstract-In this study, we investigate the optimum design for improving the deization resistance of a concentrated winding permanent synchronous motor (IPMSM) by changing the ization direction of the permanent under a high-temperature environment. IPMSMs (flat plate type, V-shaped type, spoke type) are investigated by finite element analysis (FEA) using the same volume of the permanent while changing the s width, and thickness and ic field orientation angle. FEA found that a V angle of 130 and a changed length of 107% (or V angle of 100 and a changed length of 97%) using an oblique ic-field-oriented strike a good balance between deization resistance and torque at 180 C. In the V- shaped arrangement, for a motor with the same length, the deization resistance at a ic field orientation angle α = 20 is better than that at α = 0. The ic field orientation angle α = 20 has a ic flux density distribution closer to a sinusoidal wave form than α = 0 and is thus effective for reducing torque ripple. I. INTRODUCTION The high efficiency of a small-sized permanent synchronous motor is one of its key features. On the other hand, one of its disadvantages is that the coercive force decreases at high temperature and then easily deizes. 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 s are deized. As one countermeasure to improve durability against deization, it is considered effective to reduce the ic field acting in the direction of the permanent s ic field. In this work, we examined a parallelogram-shaped permanent whose ic field orientation was made oblique with respect to the plate thickness direction (Fig. 1). Lm is the length, Am is the crosssectional area and Lg is the air gap length. For obliquely distributed s with an oblique orientation angle α, the ic field orientation inclines from the plate thickness direction with respect to the width Wm and the thickness tm. Furthermore, the operating point of the does not change much from the permeance equation, since the ic field H is acting on the. On the other hand, the reverse ic field component affecting the ization direction of the can be reduced to Hm, thus improving the deization resistance [1], [2]. Previous studies have argued that the ic field orientation influences radial stress [3], measurement of a permanent s ization when tilted in the izing direction [4], and the relationship between the izing direction and irreversible deization [5]. Since insufficient research has been done to clarify the deization characteristics under high-temperature environments, it is crucial to discuss whether ic field orientation improves deization under high-temperature environments. As one countermeasure against lowered motor efficiency due to greater copper loss under a high-temperature environment, it is possible to adopt a concentrated winding motor with low winding resistance. However, in the concentrated winding motor, the number of stator teeth per rotor pole is small, and local irreversible deization of the permanent tends to occur. In this study, by changing the thickness tm and the width Wm while maintaining the same volume, a permanent whose ic field orientation angle is inclined at angle α with respect to the plate thickness direction is changed by varying the arrangement angle of the rotor core. We calculated the deization ratio and the torque ratio (with respect to the flat plate arrangement) by ic field analysis. Furthermore, we made a concentrated winding embedded synchronous motor (IPMSM) with excellent hightemperature deization characteristics and oblique ic field orientation. We report finite element analysis Fig.1 Oblique ic orientated 978-1-5090-2364-6/17/$31.00 2017 IEEE 380

381 (FEA) and evaluation results for a prototype machine with optimum design when using s. II. INFLUENCE OF MAGNET ARRANGEMENT ANGLE ON ROTOR CORE A. Deization Analysis A concentrated winding motor is advantageous for use in high-temperature environments. In addition, IPMSM, in which a is embedded in a rotor core, is generally arranged in a rectangular parallelepiped permanent, and by deeply embedding a in the rotor core, the ic field acting on the can be relaxed, which improves the deization proof stress. Permeance coefficient Pc, which determines the operating point of the permanent, is expressed as LmAgσ Pc= (1) AmLgf where Ag is the air across-sectional area, σ is the leak coefficient, and f is the o motive force loss factor. Geometrically, the operating point of an oblique icfield-oriented is almost the same as a rectangular ; however, the improvement of the deization resistance is greatly affected by the angle difference between the direction of the working ic field and the orientation direction (Fig. 1, (1)). The orientation angle and the influence of deization differ depending on the position of the rotor core in the arrangement. By changing the arrangement angle Va of a having the same volume from 180 to 360 /P (P = number of poles), it is possible to form a flat plate arrangement, a V-shaped arrangement, or a spoke arrangement. Figure 2 shows an example of a 6-pole rotor model. The aim of deization analysis is to study the optimum design by changing the ic field orientation angle α and the arrangement angle Va while also changing the length Lm and width Wm under a fixed volume at 180 C. A current causing a reverse ic field to act on the rotor s is passed through the winding and the air gap, and ic flux density before and after energy conduction is obtained by electroic field analysis software (JMAG-Designer). A neodymium sintered (reversible data of NMX - S 36 UH) is used. The analysis conditions are shown in Table I, and the analysis model is shown in Fig. 3. The flat plate model (Va = 180 ) and the spoke model (Va = 60 ) are configured by changing the arrangement angle Va. The evaluation indexes are deization limit current and torque. The deization ratio is defined as the ratio of the air gap s ic flux density at the time of nonconduction before and after the reverse ic field is applied. The torque ratio is defined as the ratio of the paralleloriented (α = 0 ) to the torque of the flat arrangement (Va = 180 ). The deization limit current is the maximum current at which reduction of the non-conducting air gap s ic flux density, before and after application of the reverse ic field current, does not yet reach 1%. The torque is calculated with rated current (7.07 Arms) and current phase β = 20. For a flat plate arrangement, width is based on the chord size of a one-pole rotor. In order to improve deization resistance and to further increase torque, it is effective to expand the width Wm, although in the flat plate arrangement Wm cannot be enlarged beyond the chord size. On the other hand, in the V-shaped and spoke arrangements, the width Wm can be expanded beyond the reference dimension. First, in the V-shaped arrangement, in which the width Wm can be enlarged beyond that of the reference, the is deeply embedded, the deization resistance is improved, the thickness tm is reduced, and Wm is enlarged to achieve high torque. Furthermore, for the width Wm of a basic, we aim to improve deization resistance by increasing the length Lm as an oblique ic field orientation. B. Relationship between deization resistance and torque for V-shaped angle Here, we consider deization analysis by an TABLE I Analysis conditions Stator ID 56 mm Residual ic 1.16 T Rotor OD 54.4 mm flux density Br Stack length 32 mm Coercivity Hcj 2387 ka/m Magnet 1135 mm3 Electroic 35A300 volume (at one pole) steel sheet Winding 150 turn, 3Y Temperature 180 C Magnet arrangement angle Va. Stator core Coil Permanent (a) Flat plate (b) V-shape (c) Spoke (Va = 180deg.) (Va = 60deg.) Fig.2 Magnet arrangement 6-pole rotor models Rotor core Fig 3 V-shaped arrangement motor model 381

382 (a) Flat plat model Fig.5 Magnet arrangement angle Va vs deization ratio and torque ratio (b) V Shape model deization ratio 5.0 Fig.6 Magnet length ratio vs deization improvement ratio (c) Spoke model 0.0 Fig.4 Deization analysis analytical model in which the arrangement angle Va is changed from 180 to 60. The used is a basic (Fig. 4) in a flat plate model (Fig. 4 (a)), a V-shaped model (V100, Fig. 4 (b)), and a spoke model (Fig. 4 (c)). Based on a 1% deization limit current of the flat plate model, this value is increased 3 times for the V100 model and 8 times for the spoke model. As the arrangement angle Va decreases from 180 to 60, the flux line escapes to the center of the ic pole, and the ic field acting on the decreases (Fig. 5). The deization resistance is higher when the arrangement angle Va is smaller. On the other hand, the ic flux density and the torque are lower. In the analysis results (Fig. 6, Fig. 7), the horizontal axis shows the length ratio and the vertical axis shows the deization resistance improvement ratio or the torque ratio. For the deization resistance improvement ratio, Fig.7 Magnet length ratio vs torque ratio the deization limit current of a rotor with the flat-plate arrangement using a parallel-oriented is 100% (circle within dotted line; 1). The torque ratio is the ratio of the parallel-oriented s to the flat arrangement rotor s torque (circle within dotted line; 1). Under the condition of a constant volume, at a length of less than 100%, the width is increased. On the other hand, at a length of 100% or more, the width is constant using an oblique ic-field-oriented. Considering the centrifugal force of the rotor and the ease of ization, a shallowly embedded permanent in a large V-shaped angle rotor is preferable. From the results for a arrangement angle Va = 130 and a 382

383 length of 107% by the oblique ic-field-oriented, the deization improvement rate for the reference is 250% or more and the torque ratio is 88%, so a good balance is achieved between deization resistance and torque (circle within solid line; 2). As the V- shaped angle decreases, the angle difference increases between the ic field orientation angle and the reverse ic field acting on the s end portion, so the deization resistance is improved. The arrangement angle Va = 100 and the length ratio 91% were obtained as a result of using a with the ic field orientation angle α = 0. This result shows an improvement of 230%, slightly smaller than the 250% deization improvement ratio of V130 using an oblique ic-field-oriented (circle within dashdotted line; 3). However, the torque ratio is 92%, which is larger than the 88% torque ratio of V130. Next, we investigate the possibility of improving the deization durability by oblique ic field oriented s when the length ratio of V100 is less than 100%. C. Effect of oblique orientation in V-shaped arrangement Improvements in deization resistance and torque are studied by changing the thickness tm, the width Wm, and the ic field orientation angle α with a constant volume. In the oblique ic-fieldoriented, even if the thickness tm is small, the length Lm can be increased, and thus improvement in deization resistance can be expected. Using the V100 model with the arrangement angle Va = 100 offers a lot of latitude in the arrangement, so we investigated the deization resistance and torque using the length Lm as a parameter. Analysis conditions and the analysis model are shown in Table I and Fig. 3, respectively. A current causing a reverse ic field to act is applied to the winding, and the deization of the permanent and the air gap s ic flux density before and after energy conduction is obtained by FEA. A list of the studied s is given in Table II, based on a motor model in which 1 is used as a reference. Deization is evaluated as the ratio of the deization limit current ratio to the 1% deization limit current. Torque is evaluated as the ratio of the s Table II Magnet parameters width Wm [mm] No. length Lm [mm] ic orientation angle α [deg.] length ratio 1 1.65 10.75 0 100% 2 1.60 11.1 0 97% 3 1.50 11.8 0 91% 4 1.50 11.8 20 97% 5 1.40 12.7 20 90% torque ratio to the average value of the air gap s ic flux density at the time of non-conduction. The torque of the IPMSM is the sum of the torque and the reluctance torque, since the maximum torque per current is about 20 in the current phase and this is a motor model mainly based on torque. In order to clarify the influence of using different s, here we compare the torque with the torque ratio. In the analysis results (Fig. 8), the horizontal axis is the length ratio, and the vertical axis shows the deization limit current ratio and the torque ratio. The deization limit current ratio is greatly reduced for a motor using 2 or 3 with a ic field orientation angle α = 0 obtained by increasing the ic width Wm through reducing the thickness tm (i.e., reduction of the length Lm). On the other hand, in a motor in which 4 or 5 with the ic field orientation angle α = 20 is used, the deization limit current ratio is improved over that of the motor using s with the ic field orientation angle α = 0. The change in the torque ratio increases as the width Wm increases, but it is smaller than the change in the deization limit current ratio. The results of the motor using the 4 and the 5 are shown as 11 and 12 in Fig. 6 and Fig.7. By using the oblique ic-field-oriented with the length ratio 90%, the deization improvement ratio is improved to 270% and the torque ratio is improved to 90%. Further, by using the oblique ic-field-oriented with the length ratio 97%, the deization improvement ratio is 330% and the torque ratio is 87%. Even in the V100 model using the oblique ic field oriented s, deization resistance and torque are improved. III. MEASUREMENT OF MAGNETIC Flux Density of Oblique Magnetic-Field-Oriented Magnets A. Measurement of Magnetic Flux Density Fig.8 Magnet length ratio vs current ratio and torque ratio 383

384 ultrafine probe measuring jig tesla meter Magnetic flux density [T] Magnetic field orientation permanent analyzer Fig.9 Magnet flux density measuring device density Fig.10 Magnet open flux density (α = 20deg.) Here, we evaluate the ic flux density of prototype s with oblique orientation. The prototype s were produced by obliquely slicing a samarium cobalt sintered with respect to the ic field orientation direction. Since there is a large market demand for neodymium sintered s, it is difficult to use them for special prototypes under development; therefore, we used samarium cobalt sintered s to evaluate the difference in orientation angles. Three types of s have the following ic field orientation angles: α = 0, 10, and 20. The prototype size is thickness tm = 1.8 mm, width Wm = 10.8 mm, and axial length L = 31 mm. In order to confirm the orientation of the prototype s, the ic flux densities were measured. The prototype s are placed at 45 intervals on the surface of a measuring jig made of an aluminum cylinder (OD = 150 mm) and rotated at 0.5 min-1 to measure the ic flux density on the prototype s surface (Fig. 9). The ic flux density measuring device is composed of a 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. B. Oblique Orientated Magnets The results of comparing the ic flux density of the oblique ic-field-oriented (α = 20 ) with the measurements are shown in Fig. 10. The results of measuring the ic flux density of the prototype with ic field orientation angles α = 0, 10, and 20 are shown in Fig. 11. Since the prototype s are set on aluminum jigs arranged at sufficient intervals, the ic flux density between the s was confirmed as 0T. Accordingly, the ic flux density of each could be accurately measured without interference from the other s. The results of ic field analysis and the measured values are almost the same. Since the circumference of the is a nonic material space and the ic flux density is measured in the radial direction with an outer diameter of 150 mm, the ic flux density sharply changes from minus to plus Fig.11 Magnet open flux density (measured) Fig.12 Magnetic flux density measured around the s end. The ic field orientation of the measured tilts to the right, and the peak values of the ic flux density at the ends of the s are different because the ic field orientation angle is larger. Magnetic field analysis results and the measured values are almost the same. The ic flux density in which s are arranged in the air (open flux) is large at the corners of the permanent. Due to the influence of the oblique ic field orientation, the ic flux density in the ic field orientation direction is strengthened in the vicinity of the s corner portion. C. Rotor Surface and Motor Air Gap Next, the prototype s were assembled into a rotor with arrangement angle Va = 130, and the ic flux density was measured. Furthermore, the rotor was assembled in a cylindrical dummy stator made of a ic body having the same inner diameter as the stator core, and 384

385 ic flux density [T] 0.4 0.3 0.2 0.1 0-0.1 α = 0deg. α = 20deg. 0 10 20 30 40 50 60-0.2 angle [deg.] Fig.13 Rotor surface ic flux density (measured) Fig.14 Air gap ic flux density (measured) the ic flux density of the air gap was measured (Fig. 12). The measurement results of the rotor surface s ic flux density are shown in Fig. 13. Although s with ic field orientation angle α = 0 or α = 20 are embedded in cores of the same shape and their ic flux density distributions are similar, the details differ. The ic flux densities near the center of the ic pole (15 to 45 ) are almost the same. For α = 20, the change is steep at the inter-ic pole part (0 to 8 or 52 to 60 ), and the change around 13 or 47 is slightly smooth. In the V- shaped arrangement, the rotor core made of ic material is thick on the outside of the at the center of the ic pole, and thus the ic flux density is relaxed and the difference in ic field orientation is alleviated. On the other hand, at the inter-ic pole, since the rotor core outside the is thin, the influence of the difference in ic field orientation is very clear. Next, we discuss the results of the air gap s ic flux density measured for the rotor assembled in the dummy stator (Fig. 14). Around 13 or 47, there is a slight change in the measurement of the ic flux density on the rotor surface, but this appears as a remarkable difference when incorporated in the dummy stator. Moreover, the maximum value of the center of the ic pole shows a significant difference. As a result of measuring the air gap s ic flux density, it was found that the maximum ic field orientation angle decreased slightly, but the ic flux density distribution approached a sinusoidal wave form and is thus effective for reducing the torque ripple. IV. CONCLUSIONS In this paper, we examined the optimum design for oblique ic field orientation by expanding the arrangement angle Va from a flat plate type to a spoke type of IPMSM driven under a high-temperature environment, changing the thickness tm and width Wm while keeping a constant volume. We obtained the following findings by finite element analysis (FEA) and investigation of our prototype. 1) In the case of a concentrated winding IPMSM with 6 poles and 9 teeth, the arrangement angle Va is preferably V 130 to V 100, so that both deization resistance and torque achieved good balance. In the V130 model, the length ratio of 107% achieved the deization improvement ratio of 250% and the torque ratio of 88%. In the V100 model, the length ratio of 90% achieved the deization improvement ratio of 270% and the torque ratio is 90%. 2) In the V-shaped arrangement with oblique icfield-oriented s (e.g. α = 20 ), the deization resistance is better than that of the s with α = 0 at the same length Lm. 3) For the V-shaped rotor with the oblique ic-fieldoriented s, the ic flux density distribution of the air gap of the rotor and stator more closely approaches a sinusoidal form wave and is thus effective for reducing the torque ripple. REFERENCES [1] N. Nishiyama, Y. Honda, Development of IPMSM for high temperature conditions using inclined ic field orientation and V- shape arrangement IEEJ The Papers of Joint Technical Meeting on Magnetics and Linear Drives MAG-16-2010,LD-16-145,pp (2016) (in Japanese) [2] H. Uemura, N. Nishiyama, and Y. Honda, The Effeteness of Magnetic field directions of V shaped embedded s for IPMSM, 2017 Ann. Meet. Rec. IEEJ, V, pp15. (2017) (in Japanese) [3] N. Nishiyama, H. Uemura, and Y. Honda, The Study of Highly Deization Performance IPMSM under Hot Environments unpublished. [4] Y. Asano, Y. Honda, Y. Takeda, and S. Morimoto, Reduction of Vibration on Concentrated Winding Permanent Magnet Synchronous Motors with Considering Radial Stress., IEEJ Trans. on Ind. Appli, Vol.l21, No.11, pp.1185-1191 (2001) (in Japanese). [5] Y. Maeda, S. Urata, and H. Nakai, The Evaluation of Deizing Characteristics of the Permanent Magnet in Arbitrary Directions. 2016 Ann. Meet. Rec. IEEJ, II, pp.124-125 (2016) (in Japanese). [6] R. Akune, K. Akatsu, K. Kume, T. Yamamoto, and S. Saito, The Anti Deization Method for Permanent Magnet Synchronous Motor Focused on Magnetized Direction of Permanent Magnet and Basic Experiment, 2016 IEE-Japan Industry Applications Society Conference (JIASC2016), 3-54, (in Japanese) 385