Characteristics Analysis of Novel Outer Rotor Fan-type PMSM for Increasing Power Density

Transcription

1 Journal of Magnetics 23(2), (2018) ISSN (Print) ISSN (Online) Characteristics Analysis of Novel Outer Rotor Fan-type PMSM for Increasing Power Density Sooyoung Cho 1, Geochul Jeong 1, Jong suk Lim 1, Ye Jun Oh 1, Sang-Hwan Ham 2, and Ju Lee 1 * 1 Department of Electrical Engineering, Hanyang University, Seoul 04763, Republic of Korea 2 School of Electrical Engineering, Kyungil University, Gyeongsan 38428, Republic of Korea (Received 18 January 2018, Received in final form 27 March 2018, Accepted 27 March 2018) Most studies on non-rare-earth motors focus on the inner-rotor-type motors, and little research has been conducted on the outer-rotor-type motors. This paper suggests a novel outer rotor fan-type permanent magnet synchronous motor (PMSM) as a non-rare earth outer rotor motor. To verify the superiority of the efficiency and output power, the novel outer rotor fan-type PMSM is compared with the surface permanent magnet synchronous motor (SPMSM), which is primarily used as an outer-rotor-type motor. The detailed design and characteristics of the outer rotor fan-type PMSM was also studied. Finally, the output characteristics of the outer rotor fan-type PMSM were verified through prototyping. Keywords : Non-rare-earth motor, outer rotor, permanent magnet synchronous motor (PMSM), torque, efficiency 1. Introduction Rare-earth permanent magnet motors are used in many industrial fields because of their advantage of superior output power density. Currently, however, non-rare earth motors have been actively studied owing to the instability of rare-earth supply and price fluctuations. Examples of non-rare-earth motors includes a synchronous reluctance motor (SynRM), a permanent magnet assisted synchronous reluctance motor (PMA-SynRM), a line-start synchronous reluctance motor (LS-SynRM), a wound rotor synchronous motor (WRSM), a spoke-type PMSM, etc. The SynRM is the motor that uses the differences in the d-q axis inductance through the barrier structure at a rotor part, and has only reluctance torque because of not having magnets [1-3]. The PMA-SynRM improves the output power by adding permanent magnets such as ferrite in the barrier structure of the SynRM. Therefore, magnetic torque is added to the existing torque [4, 5]. In addition, the LS-SynRM is the motor in which an induction motor (IM) and the SynRM are mixed. This motor can reach a synchronous speed without control, and a copper loss does not occur in the rotor as it rotates at a synchronous The Korean Magnetics Society. All rights reserved. *Corresponding author: Tel: Fax: , julee@hanyang.ac.kr speed [6]. The WRSM is the motor composed of N and S magnets using windings instead of permanent magnets in the rotor structure. The advantage of this motor is that the field flux can be controlled by varying the current flowing in the rotor winding. However, the disadvantage is that slip rings and brushes are additionally required to supply current to the rotor winding [7, 8]. Finally, the spoke-type PMSM is the motor using non-rare-earth permanent magnets with a low residual magnetic flux density compared to the rare-earth permanent magnets. Therefore, the spoke-type PMSM forms structures that can concentrate the magnetic flux of the permanent magnet to improve the output power. The advantage of this motor is that the output power and efficiency are relatively larger than that of the previous motors [9-11]. However, these non-rareearth motors are primarily inner-rotor-type motors, and little research has been conducted to replace the outerrotor-type rare-earth permanent magnet motors. Accordingly, this paper suggests an outer rotor fan-type PMSM using the non-rare-earth permanent magnets to replace the outer rotor SPMSM, which is primarily used as the outer-rotor type. The outer rotor fan-type PMSM has structures that can concentrate permanent magnetic fluxes similar to the spoke-type PMSM. To verify the superiority of the outer rotor fan-type PMSM, this paper includes a comparative analysis of the output power and efficiency with the outer rotor SPMSMs. Further, it 2018 Journal of Magnetics

2 248 Characteristics Analysis of Novel Outer Rotor Fan-type PMSM for Increasing Power Density Sooyoung Cho et al. analyzes the characteristics of the detailed designed outer rotor fan-type PMSM according to the current phase angle. Finally, the test results from the prototype are shown. 2. Performance Comparison of the Outer Rotor PMSM with Different Rotors As shown in equation (1), the PMSM torque consists of the magnetic torque by the permanent magnet flux and the q-axis current, and the reluctance torque by the differences in the d-q axis inductances. 3 T = p ( ) (1) 2 Φ i + L L i i f q d q d q = T + T m r where p is the number of pole pairs and Φ f is the PM flux. The basic model and specifications of the outer rotor fan-type PMSM using ferrite are shown in Fig. 1 and Table 1, respectively. The magnetization direction was determined as shown in Fig. 1 such that the magnetic flux Fig. 2. (Color online) Comparison model of the outer rotor SPMSM. (a) Same pole-arc proportion and parallel magnetization (b) Same magnet usage per pole and parallel magnetization (c) Maximization of magnet usage per pole and parallel magnetization (d) Same pole-arc proportion and radial magnetization (e) Same magnet usage per pole and radial magnetization (f) Maximization of magnet usage per pole and radial magnetization Fig. 1. (Color online) Base model of the outer rotor fan-type PMSM. Table 1. Specifications of the base model of the outer rotor fan-type PMSM. Contents Value Unit Outer diameter of the stator 111 Outer diameter of the rotor 151 Air gap length 1 Pole-arc proportion (α m ) (at Rotor edge) 0.79 Magnet usage per pole (A m ) Phase current 4 A rms Current density 4.97 A rms / 2 Base speed 2,400 rpm of the permanent magnets could be concentrated to improve the output power. The teeth and shoes of the rotor were constructed to allow for the most magnetic flux to pass through the air-gap. The comparison model to confirm the suitability of the outer rotor fan-type PMSM as the structure of non-rareearth motors is shown in Fig. 2. In this case, the permanent magnet material, stator, outer diameter of the rotor, and air-gap length are the same. Therefore, the comparison was performed by changing the pole-arc ratio, usage of the permanent magnets, and magnetization direction of the permanent magnets No-load air-gap flux density Figure 3 shows the no-load air-gap magnetic flux density of the outer rotor fan-type PMSM and outer rotor SPMSM models (a) to (f) during the electrical one cycle. It is shown that the outer rotor fan-type PMSM contributed greatly to increasing the amount of air-gap magnetic flux because the magnetic flux generated by the permanent magnets was concentrated by the high-permeable teeth structure placed in the rotor. Figure 3(b) shows the results of the air-gap magnetic flux density by the fast Fourier transform (FFT). The outer rotor fan-type PMSM showed the highest fundamental harmonic, followed by model (f).

3 Journal of Magnetics, Vol. 23, No. 2, June Fig. 4. (Color online) Torque of the outer rotor fan-type PMSM and comparison model. Fig. 3. (Color online) Air-gap flux density. (a) Air-gap flux density wave according to electrical angle (b) Harmonic distribution of the air-gap flux density Moreover, when only the magnetization differed under the same conditions among the comparison groups (a) to (f), the radially magnetized magnets generate relatively higher fundamental harmonics than the parallel magnetized magnets because the radially magnetized magnets generate the same amount of magnetic flux in every position Total torque and efficiency Figure 4 shows the output torque of the outer rotor fantype PMSM and the comparison models (a) to (f) at the rated operating point. As shown, the output characteristics of the outer rotor fan-type PMSM with the magnetic flux concentration structure are relatively superior. Further, Table 2 compares the no-load linkage flux, the cogging torque, the average torque, efficiency, and d-q axis currents of each model. As shown, the outer rotor fan-type PMSM has a non-zero d-axis current that provides the maximum torque at the rated operating point owing to its salient pole. However, the cogging torque of the outer rotor fantype PMSM is larger than that of the other motors owing to the salient pole. The outer rotor fan-type PMSM has a relatively high linkage flux at no load because it has the magnetic flux concentration structure. In addition, we confirmed that the outer rotor fan-type PMSM is not only excellent in the average torque but also in efficiency. 3. Detailed Designed Model of the Outer Rotor Fan-type PMSM The outer rotor fan-type PMSM has a structure that can concentrate magnetic flux. If the housing is not insulated from the outer diameter of the rotor, the magnetic flux can leak. Therefore, Fig. 5 shows the detailed rotor design Table 2. Comparison of the result parameters of the outer rotor fan-type PMSM and comparison models. Contents Value Fan-type (a) (b) (c) (d) (e) (f) d-axis current [A] q-axis current [A] Linkage flux at no load [mwb] Cogging torque (peak to peak) [mnm] Average torque [Nm] Efficiency [%]

4 250 Characteristics Analysis of Novel Outer Rotor Fan-type PMSM for Increasing Power Density Sooyoung Cho et al. Table 3. Specifications of the detailed designed model of the outer rotor fan-type PMSM. Contents Value Unit Outer diameter of the stator Outer diameter of the rotor Airgap length Thickness of the insulation Thickness of the housing Phase voltage limit Phase current limit Current density Base speed ,400 Vpeak Arms Arms/2 rpm Fig. 5. (Color online) Outer rotor fan-type PMSM model for detailed rotor design. of the outer rotor fan-type PMSM, and Fig. 6 shows the results from analyzing the rotor parameters of the rotor using the response surface method. Through this process, the final design specifications are shown in Table 3, and the torque of the final model is Fig. 7. (Color online) Torque of the detailed designed outer rotor fan-type PMSM. Table 4. FEA results of the detailed designed outer rotor fantype PMSM. Fig. 6. (Color online) Analysis of rotor parameters using response surface method. Contents Value Unit Torque ripple Average torque Efficiency % Nm % Fig. 8. (Color online) Flux density magnitude and flux vector of the outer rotor fan-type PMSM at the MTPA point.

5 Journal of Magnetics, Vol. 23, No. 2, June Fig. 9. (Color online) Torque and efficiency of the outer rotor fan-type PMSM. shown in Fig. 7. In addition, the finite element analysis (FEA) results of the detailed designed proposed motor are shown in Table 4. This torque is smaller than the output torque obtained in Fig. 4. This is because a small amount of leakage flux is passed to the housing, as shown in Fig. 8. Figure 9 shows the magnetic torque, reluctance torque, total torque, and efficiency curve according to the current phase angle (β) of the outer rotor fan-type PMSM. These results show that the outer rotor fan-type PMSM has inverse salient-pole characteristics. Therefore, the current phase angle that generates the maximum torque has positive values. 4. Experiment Figure 10 presents the prototype of the outer rotor fantype PMSM designed in chapter 3 and Fig. 11 shows the experimental setup. The electromotive force (EMF) waveform obtained by a no-load test is used to compare the results of the FEA, as shown in Fig. 12. We confirmed that the FEA values of Fig. 11. Experimental setup. Fig. 12. (Color online) Line-to-line EMF of experiment and FEA. Table 5. Comparison of FEA torque with experimental torque at rated speed. Content FEA Experiment Unit Line-to-line EMF at no load Input phase current Toque Vrms Arms Nm the outer rotor fan-type PMSM are almost the same as those of prototype. Table 5 compares the FEA torque and the experimental torque at the rated operating point. Consequently, we confirmed that the design value and the experimental value differed by approximately 1.2 % only. 5. Conclusion Fig. 10. (Color online) Prototype of the outer rotor fan-type PMSM. This paper presents the outer rotor fan-type PMSM as a non-rare-earth motor. In addition, the outer rotor fan-type PMSM is compared with the SPMSMs, which is primarily

6 252 Characteristics Analysis of Novel Outer Rotor Fan-type PMSM for Increasing Power Density Sooyoung Cho et al. used as the outer rotor type motor. By comparing the characteristics of the motors, we confirmed that the outer rotor fan-type PMSM is superior to the SPMSMs in terms of output torque as well as efficiency. Further, the final model of the outer rotor fan-type PMSM was designed using the response surface method. From the prototype, the final model was verified against the design values and the superiority of the outer rotor fan-type PMSM was confirmed. Therefore, if the outer rotor fan-type PMSM is applied to outer rotor applications such as the direct-drive motor in the future, it is expected to be competitive in output power, efficiency, and price. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2017R1D1A1B ). This work was supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No ). References [1] R. R. Moghaddam and F. Gyllensten, IEEE Trans. Ind. Electron. 61, 5058 (2014). [2] A. A. Arkadan, M. N. ElBsat, and M. A. Mneimneh, IEEE Trans. Magn. 45, 956 (2009). [3] J.-K. Lee, D.-H. Jung, J. Lim, K.-D. Lee, and J. Lee, IEEE Trans. Magn. 54, Article no (2018). [4] W. H. Kim, K. S. Kim, S. J. Kim, D. W. Kang, S. C. Go, Y. D. Chun, and J. Lee, IEEE Trans. Magn. 45, 4660 (2009). [5] P. Niazi, H. A. Toliyat, D.-H. Cheong, and J.-C. Kim, IEEE Trans. Ind. Appl. 43, 542 (2007). [6] H.-C. Liu and J. Lee, IEEE Trans. Ind. Electron. 65, 3104 (2018). [7] Q. Ali, T. A. Lipo, and B.-I. Kwon, IEEE Trans. Magn. 51, 542 (2015). [8] R. Wang, S. Pekarek, M. Bash, A. Larson, and R. Maaren, IEEE Trans. Energy Convers. 30, 821 (2015). [9] S. Cho, H. Ahn, H. C. Liu, H.-S. Hong, J. Lee, and S.-C. Go, IEEE Trans. Magn. 53, Article no (2017). [10] M. R. Mohaad, K.-T. Kim, and J. Hur, IEEE Trans. Magn. 49, 2397 (2013). [11] S. G. Lee, J. Bae, and W.-H. Kim, IEEE Trans. Appl. Supercond. 28, Article no (2018).