Thermal Investigation of Permanent-Magnet Synchronous Motor for Aerospace Applications Themistoklis D

4404 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 8, AUGUST 2014 Thermal Investigation of Permanent-Magnet Synchronous Motor for Aerospace Applications Themistoklis D. Kefalas, Member, IEEE, and Antonios G. Kladas, Senior Member, IEEE

Abstract—This paper concerns the thermal investigation of to the resulting localized heating effects and reduced thermal a surface-mounted permanent-magnet synchronous motor de- dissipation. signed for high-temperature aerospace actuation applications. A This paper describes the thermal analysis of a surface- finite-element package was developed, enabling accurate 3-D transient thermal analysis by considering complex and unsymmetri- mounted permanent-magnet synchronous motor (PMSM) de- cal actuator housing configurations. Its validity was verified by signed for high ambient temperatures, inadequate convective measurements carried out at different duty cycles and operating heat transfer, and short overload conditions encountered in an conditions. aerospace actuation application. Index Terms—Actuators, aerospace engineering, aerospace in- Conventional electric-machine thermal analysis includes dustry, computer-aided analysis, electric machines, finite-element lumped-parameter networks [4]–[12] and the computational (FE) methods, numerical analysis, permanent-magnet machines, fluid dynamics (CFD) analysis [13]–[15]. Other approaches thermal analysis, time-domain analysis. appearing in the technical literature are the finite-element (FE) method used in conjunction with lumped-parameter models for NOMENCLATURE 2-D [16]–[19] and 3-D [20], [21] steady-state thermal analyses. A Sum of the m areas comprising the outer surface of The aerospace application under consideration involves a the PMSM (in square meters). highly integrated actuator using a housing of complex and hi ith convection coefficient (in watts per square meter unsymmetrical geometric characteristics. The solution of the per kelvin). particular thermal problem requires detailed transient 3-D anal- m Number of areas comprising the outer surface of the ysis where the temperature gradient cannot be ignored. Nev- PMSM. ertheless, in this case, the convective heat transfer mechanism q Heat transfer rate (in watts). of the PMSM is insignificant and the CFD analysis is not nec- essary to be adopted. The FE method advantage over the CFD qi ith heat flux transfer per unit area (in watts per square meter). analysis is the reduced computational cost. Furthermore, the FE sij Stiffness matrix element, i for row and j for column. method enables temperature gradient evaluation and complex T si ith mean surface temperature (in kelvin). geometry representation and parameterization in contrast with AEA All-electric aircraft. the lumped-parameter network analysis [22]. PMSM Permanent-magnet synchronous motor.

II. BRIEF DESCRIPTION OF THE PMSM I. INTRODUCTION OWADAYS, the application of political, environmental, A Rosenbrock-based optimization algorithm [23] was and economical trends to air transport lead to the all- adopted in order to determine the optimum design configuration N ◦C electric aircraft (AEA) [1]–[3]. The unavoidable elimination of a PMSM working at high ambient temperature (200 ) and of high-pressure hydraulic lines is going to produce a de- short overload conditions. The design and operational charac- manding thermodynamic environment for electric actuators due teristics of the optimum PMSM are shown in Table I. For the specific PMSM, a nonoverlapping all-teeth-wound fractional-slot concentrated-winding configuration and high- temperature samarium cobalt (SmCo) permanent magnets were used [23]. Fig. 1 shows an assembled prototype PMSM on the test rig. Manuscript received January 30, 2013; revised April 26, 2013 and July 18, 2013; accepted July 27, 2013. Date of publication August 15, 2013; date of current version February 7, 2014. The research leading to these results has received funding from the European Union Seventh Framework Programme III. 3-D FE CODE DESCRIPTION (FP7/2007-2013) under Grant Agreement AAT.2008.4.2.4-234119 CREAM within the TEIP Consortium Member. A. Description of Numerical Techniques The authors are with the School of Electrical and Computer Engineering, National Technical University of Athens, GR-15780 Athens, Greece (e-mail: 1) FE System Assembly: For the storage of the FE matrices, [email protected]; [email protected]). a modified Morse technique was used that enables quick search Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. of an element of the stiffness matrix. The specific FE system as- Digital Object Identifier 10.1109/TIE.2013.2278521 sembly scheme consists in the production of two appropriately

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TABLE I 4) Mesh Parameterization: During the optimization pro- DESIGN PARAMETERS OF OPTIMIZED PMSM cess, the geometry design parameters of the FE model constantly change. In this case, the mesh parameterization technique is employed in order to avoid repeatedly using the mesh generation and FE system assembly procedures. 5) Time-Step Generator: The computational cost of the transient thermal analysis was reduced by using a time-step generator procedure that improved the resolution at the ﬁrst steps and minimized the required number of steps of the transient analysis.

B. Electromagnetic and Thermal Phenomena Weak Coupling The 3-D thermal FE analysis is weakly coupled with a multiple-slice 2-D FE code already presented [23] for the evaluation of the thermal sources. In fact, the important dif- ference of time constants between thermal and electromagnetic phenomena has been exploited in order to ensure updating of both copper conductivity and permanent-magnet magnetization variation with temperature as proposed in [30]. This is achieved by implementing an appropriate steady- state 2-D FE electromagnetic analysis for several PMSM cross sections at every time step of the transient 3-D FE thermal simulation. Moreover, the temperature variation of the thermal conductivity has been equally taken into account.

C. Particular Boundary Conditions Development Due to the high integration of the actuator, the housing of the PMSM has a complex and unsymmetrical geometry with no 2-D or 3-D symmetry. In order to emulate this housing, a novel numerical method is developed. The proposed method consists in the application of properly defined boundary conditions at the outer surface of the PMSM in contact with the housing. The Fig. 1. Assembled prototype PMSM on the test rig. surface is partitioned into m areas, each representing a different component of the actual housing e.g., a fin or the area between two fins. At each area, a different boundary condition is applied sorted vectors that store the row and column of the diagonal and so that (1) is satisfied, where qi is the heat flux transfer per nonzero elements sij of the sparse stiffness matrix. unit area of the ith area, hi is the convection coefficient of the 2) FE Solver: The improvement of the solution time of ith area, and T si is the mean surface temperature of the ith area. the 3-D FE transient analysis was accomplished by using the The first term of (1) must satisfy (2), where q is the heat transfer system assembly procedure integrating the modified Morse rate, i.e., the sum of the PMSM heat sources, and A is the sum technique in combination with the conjugate gradient method of the m areas with Van der Vorst preconditioning. m m The heat conduction solver accuracy has been validated by · qi = hi T si (1) comparing its results to analytical solutions existing in the case i=1 i=1 of thermal conduction in a cylindrical rod of finite length [24]. m q 3) Mesh Size Reduction: A significant reduction of the q = . (2) i A FE mesh size and the corresponding computational cost was i=1 achieved by replacing the winding turns, the air–varnish–epoxy In particular, for the airgap thermal transfer, approximate composite of the stator slots, and the iron-laminated stator convection coefficients proposed in [31] have been imple- by homogeneous regions with appropriately defined material mented, providing sufficient accuracy for the studied PMSM, attributes. In all cases, the thermal properties of the specific re- as will be shown in Section VI. gions were evaluated experimentally and by taking into account empirical factors. In particular, the homogenization technique D. Code Structure implemented for winding-part discretization reduction in the thermal analysis is detailed in [22]. This technique is used A FE package has been assembled for the 3-D FE transient frequently in electromagnetic analysis [25]–[27] and has been and steady-state analyses of actuators. It consists of seven codes adopted for thermal analysis [28], [29]. that integrate the aforementioned techniques, and it is structured 4406 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 8, AUGUST 2014

Fig. 3. Detail of the 3-D FE mesh of the PMSM consisting of 2 · 106 nodes and 1.2 · 107 elements.

Fig. 2. Structure of the developed FE package. Fig. 4. Cross section of the 3-D FE mesh of the PMSM. as shown in Fig. 2. The control code calls and passes variables The control program, preprocessor, FE system assembly, FE to the remaining six codes as follows: solver, and postprocessor codes were written by the authors, whereas for the mesh generation and the FE graphics represen- 1) The FE preprocessor procedure is called, which produces tation, the open-access codes TetGen, TetView, and Gmsh were the geometry and attributes of the 3-D FE model. The adopted [32], [33]. specific procedure parameterizes almost all of the geometry and operating variables of the actuator. IV. FE RESULTS 2) A first-order tetrahedral mesh generator is used in order to construct the FE mesh. The quality and size of the mesh A. Computational Cost of the 3-D FE Transient Analysis are determined by the control program. A sensitivity analysis was carried out for mesh sizes rang- 3) The control code calls the FE system assembly procedure ing from 4 · 105 to 1.2 · 107 elements in order to determine that integrates the modified Morse technique. This proce- the optimum FE mesh size. The sensitivity analysis showed dure uses as input the Cartesian coordinates of the nodes that a mesh size of 1.5 · 106 was the optimum compromise and the list of the tetrahedra. between computational effort and analysis accuracy. The re- 4) The input files for the 3-D FE thermal transient solver spective computational effort and relative error for a ten-step FE are the files produced by the mesh generator and the transient thermal analysis were 1523 s and 2.3%, respectively, system assembly codes. The specific solver produces an using a typical desktop PC. A detail of the densest FE mesh output file containing the temperature distribution of the tested is shown in Fig. 3. A cross section of the 3-D FE mesh FE model for every time step of the transient analysis. depicting the regions comprising the model is shown in Fig. 4. Furthermore, during the particular stage, the storage of the stiffness matrix is carried out and boundary conditions and material attributes are assigned to the FE model. B. Results of Thermal Transient Analysis 5) The control program calls the postprocessor procedure that modifies to an appropriate format the output files of Using the FE package of Section III, the authors performed the FE solver. 3-D FE thermal transient analyses of the PMSM for different 6) Finally, the postprocessor Gmsh is called, which produces duty cycles and load conditions. graphic representations of the geometry, mesh, and tem- The temperature distributions of the PMSM running under perature distribution of the FE model. maximum load conditions over 140 min is shown in Figs. 5 KEFALAS AND KLADAS: THERMAL INVESTIGATION OF PMSM FOR AEROSPACE APPLICATIONS 4407

Fig. 5. PMSM temperature distribution at 2.54 s. (a) Winding. (b) Stator core. (c) Stator. (d) Rotor. Fig. 6. PMSM temperature distribution. (a) 26.1 s. (b) 82.7 s. (c) 825 s. and 6. The specific results were obtained by a ten-step transient third time step of the analysis. Fig. 6 shows the temperature analysis where the time step ranges from 0.18 s for the first step distribution for three steps of the transient analysis. to 5626 s for the tenth step. Fig. 7 shows a comparison of the local maximum More specifically, Fig. 5 shows the temperature distribution and minimum temperature distributions versus time of the of the windings, the stator core, the stator, and the rotor at the PMSM. 4408 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 8, AUGUST 2014

Fig. 9. Prototype PMSM inside the thermal chamber.

Fig. 7. Local maximum and minimum temperature variations over 140 min. 3) data acquisition (DAQ) system; 4) digital and analog instruments; 5) torque gauge; 6) variable electrical loads; 7) air-forced cooling apparatus. The voltage across the PMSM stator terminals was captured using active differential voltage probes. The specific voltage probes measure floating potentials by converting the high input voltage (≤ 1400 V peak) into a low voltage (≤ 5 V). The 3-dB frequency of the voltage probe is 18 MHz, the rejection rate on common mode at 50 Hz is 90 dB, and the attenuation factor is equal to 205 [27], [34]. Current probes based on the Hall Effect were used for capturing the phase currents. The specific probes provide a galvanic isolation between the primary circuit (high power) and the secondary circuit (electronic circuit) and very good linearity (< 0.2%). The current measurement range is ±36 A peak, and the 1-dB frequency of the probe is 150 kHz. The output of the probes was connected to a noise-rejecting shielded BNC I/O connector block (BNC-2110) via passive probes (TP6060). A noise-rejecting shielded cable (SHC68- 68-EP) connects the DAQ device directly to the BNC-2110 connector block. The DAQ device used was National Instruments (NI) 6143. It is based on a dedicated analog-to-digital converter (ADC) per channel architecture. NI 6143 has eight differential channels, an ADC resolution of 16 bit, a maximum sampling rate of 250 kSamples/s per channel analog input, and a ±5 V analog input signal range. The analysis of the captured data was carried out by using LabVIEW software. Three virtual instruments (VIs) were created with the use of LabVIEW. The purpose of the first VI was the real-time measurement of the voltage and phase current waveforms, fast Fourier transform, peak, rms, and Fig. 8. (a) Experimental setup. (b) DAQ system and measurement instruments. total harmonic distortion (THD) values. The second VI was used for capturing the voltage and phase current waveforms for V. E XPERIMENTAL SETUP the maximum sampling rate of the DAQ device. The third VI was used for manipulating the acquired voltage and current data The experimental setup used for evaluating the thermal per- [27], [34]. formance of the PMSM is shown in Fig. 8. It consists of the The ambient temperature of the PMSM was controlled by an following components: environmental chamber equipped with a PID controller (Fig. 9). 1) prototype PMSM; For that purpose, two metal plates attached to the housing of the 2) separately excited direct current machine (22 kW); PMSM were used and a thermal insulation material was placed KEFALAS AND KLADAS: THERMAL INVESTIGATION OF PMSM FOR AEROSPACE APPLICATIONS 4409

Fig. 10. Thermocouple arrangement for temperature measurements.

Fig. 12. Third and fourth sets of simulated and measured temperature variations.

winding end, the housing center, and the housing end of the first and second sets of measurements. In order to evaluate the thermal impact on the PMSM of rapid load conditions, two more sets of measurements were carried out, each of them consisting of two subcycles. A constant ohmic load was used, ensuring an increased loading of the PMSM for a limited time period at the first subcycle. Then, the load was removed and the air-forced cooling apparatus was used in order to decrease rapidly the PMSM temperature. The third set of measurements was carried out at 1200 r/min, with phase currents of 9.5 A, and torque of 14.2 N · m for 60 min. A centrifugal fan producing 98 m3/h was used for cooling Fig. 11. First and second sets of simulated and measured temperature down the PMSM. For the fourth set of measurements, increased variations. load and air volume flow rates (400 m3/h) were used. It was between the PMSM housing and the plates in order to minimize carried out at 1200 r/min, with phase currents of 13.7 A, line- heat dissipation. to-neutral voltage of 79 V, and torque of 21.8 N · m for 60 min. Fig. 10 shows the arrangement of five type-k thermocouples Fig. 12 shows the comparison of simulated versus measured used for temperature measurements. Three thermocouples were temperature variation at three locations of the PMSM of the attached to the housing of the PMSM: One was attached to third and fourth sets of measurements. the center of a stator tooth, and one was placed on one of the For the fifth set of measurements, a mixed cycle was em- winding ends. When the rotor was stationary, a sixth type-k ployed using variable loads and high air-forced cooling condi- thermocouple was used to measure the shaft temperature [4]. tions. The first subcycle was obtained for constant loads that resulted in a winding-end temperature rise of 10 ◦C/min. The second subcycle was obtained by regulating the loads over VI. COMPARISON OF SIMULATED AND 60 min in order to maintain the winding-end temperature EXPERIMENTAL RESULTS constant. Finally, the third subcycle was obtained by using the Seven experiments were carried out for different duty cycles, air-forced cooling apparatus in order to achieve a winding- operating conditions, and load conditions using the experimen- end temperature decrease of 10 ◦C/min. Fig. 13 shows the tal setup of Section V. The first set of measurements was carried respective comparison of simulated versus measured temper- out for a rotating speed of 750 r/min, phase currents of 4.92 A, ature distributions at three locations of the PMSM for two and line-to-neutral voltage of 49 V. The ambient temperature consecutive duty cycles. was 25◦C, and the measurements were carried out for 120 min Additionally, two sets of measurements were carried out using a constant ohmic load. for different ambient temperatures using the thermal chamber The second set of measurements was carried out for the same described in Section V. Each of them consists of two subcycles ambient temperature at 1000 r/min, phase currents of 6.45 A, of temperature rise and decrease. For the first subcycle, a and line-to-neutral voltage of 65 V for 120 min. In the particular constant ohmic load was used, and for the second subcycle, air- set, an increased load was used in order to raise the steady- forced cooling conditions were employed. The sixth set was state temperature of the PMSM. Fig. 11 shows a comparison carried out at 1200 r/min for 120 min. An ambient temperature of simulated versus measured temperature variations at the of 25 ◦C was controlled by the thermal chamber. Figs. 14 4410 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 8, AUGUST 2014

Fig. 13. Fifth set of simulated and measured temperature variations. Fig. 16. Comparison of simulated versus measured temperature variations over 40 min (housing center, ambient 70 ◦C). set of measurements was carried out at 1200 r/min at an ambient temperature of 70 ◦C for 40 min. Fig. 16 shows the comparison of simulated and measured temperature variations of the PMSM housing. In all seven experimental sets, the simulated and measured temperature variations agree within 1% to 4% for all measured points.

VII. CONCLUSION The 3-D FE transient thermal analysis presented in this paper was developed as part of a project concerning a new actuator development ensuring aileron guidance and emergency aerodynamic brake functions in aerospace applications. The heat sources of the actuator are calculated by convenient multislice 2-D FE electromagnetic analysis weakly coupled Fig. 14. Comparison of simulated versus measured temperature variations with thermal analysis. Also, the proposed thermal analysis over 120 min (housing center, ambient 25 ◦C). includes an innovative technique enabling consideration of the cooling provided by the particular complex and unsymmetrical housing of the considered actuator. The aforementioned techniques were integrated to a FE package presenting task parallelism computing capabilities. By using the speciﬁc FE package, a ratio of 5.4, of real time to transient analysis time, per processor core was achieved.

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Kefalas (M’09) was born in Greece an internal permanent magnet synchronous machine by means of com- in 1977. He received the Electrical Engineering Ed- putational fluid dynamics,” IEEE Trans. Ind. Electron., vol. 59, no. 12, ucator degree from the School of Pedagogical and pp. 4568–4578, Dec. 2012. Technological Education, Athens, Greece, in 1999 [16] L. Alberti and N. Bianchi, “A coupled thermal-electromagnetic analysis and the Diploma and the Ph.D. degree in electrical for a rapid and accurate prediction of IM performance,” IEEE Trans. Ind. engineering from the National Technical University Electron., vol. 55, no. 10, pp. 3575–3582, Oct. 2008. of Athens, Athens, in 2005 and 2008, respectively. [17] A. Tenconi, F. Profumo, S. E. Bauer, and M. D. Hennen, “Temperatures Since 2010, he has been an Adjunct Assistant evaluation in an integrated motor drive for traction applications,” IEEE Professor with the School of Pedagogical and Tech- Trans. Ind. Electron., vol. 55, no. 10, pp. 3619–3626, Oct. 2008. nological Education. Since 2008, he has been an [18] W. Li, J. Cao, and X. Zhang, “Electrothermal analysis of induction motor Adjunct Lecturer and Research Associate with the with compound cage rotor used for PHEV,” IEEE Trans. Ind. Electron., School of Electrical and Computer Engineering, National Technical University vol. 57, no. 2, pp. 660–668, Feb. 2010. of Athens. His research interests include electric machine and transformer [19] T. A. Jankowski, F. C. Prenger, D. D. Hill, S. R. O’Bryan, K. K. Sheth, design, modeling, and optimization. E. B. Brookbank, D. F. A. Hunt, and Y. A. Orrego, “Development and Dr. Kefalas is a member of the Technical Chamber of Greece. validation of a thermal model for electric induction motors,” IEEE Trans. Ind. Electron., vol. 57, no. 12, pp. 4043–4054, Dec. 2010. [20] I.-C. Vese, F. Marignetti, and M. M. Radulescu, “Multiphysics approach to numerical modeling of a permanent-magnet tubular linear motor,” IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 320–326, Jan. 2010. [21] S. Dwari and L. Parsa, “Design of Halbach-array-based permanent- magnet motors with high acceleration,” IEEE Trans. Ind. Electron., Antonios G. Kladas (S’80–A’99–M’02–SM’10) vol. 58, no. 9, pp. 3768–3775, Sep. 2011. was born in Greece in 1959. He received the Diploma [22] A. Boglietti, A. Cavagnino, D. Staton, M. Shanel, M. Mueller, and in electrical engineering from the Aristotle Univer- C. Mejuto, “Evolution and modern approaches for thermal analysis of sity of Thessaloniki, Thessaloniki, Greece, in 1982 electrical machines,” IEEE Trans. Ind. Electron., vol. 56, no. 3, pp. 871– and the D.E.A. and Ph.D. degrees from Pierre and 882, Mar. 2009. Marie Curie University (Paris 6), Paris, France, in [23] E. M. Tsampouris, M. E. Beniakar, and A. G. Kladas, “Geometry opti- 1983 and 1987, respectively. mization of PMSM comparing full and fractional pitch winding configu- He was an Associate Assistant with Pierre and rations for aerospace actuation applications,” IEEE Trans. Magn., vol. 48, Marie Curie University from 1984 to 1989. From no. 2, pp. 943–946, Feb. 2012. 1991 to 1996, he was with the Public Power Cor- [24] S. Whitaker, Fundamental Principles of Heat Transfer. New York, NY, poration of Greece, where he was engaged in the USA: Pergamon, 1977. System Studies Department. Since 1996, he has been with the School of [25] T. D. Kefalas, P. S. Georgilakis, A. G. Kladas, A. T. Souflaris, and Electrical and Computer Engineering, National Technical University of Athens, D. G. Paparigas, “Multiple grade lamination wound core: A novel tech- Athens, Greece, where he is currently a Professor. His research interests include nique for transformer iron loss minimization using simulated annealing transformer and electric machine modeling and design, and the analysis of with restarts and an anisotropy model,” IEEE Trans. Magn., vol. 44, no. 6, generating units by renewable energy sources and industrial drives. pp. 1082–1085, Jun. 2008. Dr. Kladas is a member of the Technical Chamber of Greece.