Thermal Investigation of Permanent-Magnet Synchronous Motor for

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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 a surface-mounted permanent-magnet synchronous motor designed for high-temperature aerospace actuation applications. A finite-element package was developed, enabling accurate 3-D transient thermal analysis by considering complex and unsymmetrical actuator housing configurations. Its validity was verified by measurements carried out at different duty cycles and operating conditions. Index Terms—Actuators, aerospace engineering, aerospace industry, computer-aided analysis, electric machines, finite-element (FE) methods, numerical analysis, permanent-magnet machines, thermal analysis, time-domain analysis.

N OMENCLATURE A hi m q qi sij T si AEA PMSM

Sum of the m areas comprising the outer surface of the PMSM (in square meters). ith convection coefficient (in watts per square meter per kelvin). Number of areas comprising the outer surface of the PMSM. Heat transfer rate (in watts). ith heat flux transfer per unit area (in watts per square meter). Stiffness matrix element, i for row and j for column. ith mean surface temperature (in kelvin). All-electric aircraft. Permanent-magnet synchronous motor.

to the resulting localized heating effects and reduced thermal dissipation. This paper describes the thermal analysis of a surfacemounted permanent-magnet synchronous motor (PMSM) designed for high ambient temperatures, inadequate convective heat transfer, and short overload conditions encountered in an aerospace actuation application. Conventional electric-machine thermal analysis includes lumped-parameter networks [4]–[12] and the computational fluid dynamics (CFD) analysis [13]–[15]. Other approaches appearing in the technical literature are the finite-element (FE) method used in conjunction with lumped-parameter models for 2-D [16]–[19] and 3-D [20], [21] steady-state thermal analyses. The aerospace application under consideration involves a highly integrated actuator using a housing of complex and unsymmetrical geometric characteristics. The solution of the particular thermal problem requires detailed transient 3-D analysis where the temperature gradient cannot be ignored. Nevertheless, in this case, the convective heat transfer mechanism of the PMSM is insignificant and the CFD analysis is not necessary to be adopted. The FE method advantage over the CFD analysis is the reduced computational cost. Furthermore, the FE method enables temperature gradient evaluation and complex geometry representation and parameterization in contrast with the lumped-parameter network analysis [22].

I. I NTRODUCTION

II. B RIEF D ESCRIPTION OF THE PMSM

OWADAYS, the application of political, environmental, and economical trends to air transport lead to the allelectric aircraft (AEA) [1]–[3]. The unavoidable elimination of high-pressure hydraulic lines is going to produce a demanding thermodynamic environment for electric actuators due

A Rosenbrock-based optimization algorithm [23] was adopted in order to determine the optimum design configuration of a PMSM working at high ambient temperature (200 ◦ C) and short overload conditions. The design and operational characteristics of the optimum PMSM are shown in Table I. For the specific PMSM, a nonoverlapping all-teeth-wound fractional-slot concentrated-winding configuration and hightemperature samarium cobalt (SmCo) permanent magnets were used [23]. Fig. 1 shows an assembled prototype PMSM on the test rig.

N

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 (FP7/2007-2013) under Grant Agreement AAT.2008.4.2.4-234119 CREAM within the TEIP Consortium Member. The authors are with the School of Electrical and Computer Engineering, National Technical University of Athens, GR-15780 Athens, Greece (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2013.2278521

III. 3-D FE C ODE D ESCRIPTION A. Description of Numerical Techniques 1) FE System Assembly: For the storage of the FE matrices, a modified Morse technique was used that enables quick search of an element of the stiffness matrix. The specific FE system assembly scheme consists in the production of two appropriately

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KEFALAS AND KLADAS: THERMAL INVESTIGATION OF PMSM FOR AEROSPACE APPLICATIONS

TABLE I D ESIGN PARAMETERS OF O PTIMIZED PMSM

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4) Mesh Parameterization: During the optimization process, 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 difference 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 steadystate 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

Fig. 1.

Assembled prototype PMSM on the test rig.

sorted vectors that store the row and column of the diagonal and nonzero elements sij of the sparse stiffness matrix. 2) FE Solver: The improvement of the solution time of the 3-D FE transient analysis was accomplished by using the system assembly procedure integrating the modified Morse technique in combination with the conjugate gradient method with Van der Vorst preconditioning. The heat conduction solver accuracy has been validated by comparing its results to analytical solutions existing in the case of thermal conduction in a cylindrical rod of finite length [24]. 3) Mesh Size Reduction: A significant reduction of the FE mesh size and the corresponding computational cost was achieved by replacing the winding turns, the air–varnish–epoxy composite of the stator slots, and the iron-laminated stator by homogeneous regions with appropriately defined material attributes. In all cases, the thermal properties of the specific regions were evaluated experimentally and by taking into account empirical factors. In particular, the homogenization technique implemented for winding-part discretization reduction in the thermal analysis is detailed in [22]. This technique is used frequently in electromagnetic analysis [25]–[27] and has been adopted for thermal analysis [28], [29].

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 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 so that (1) is satisfied, where qi is the heat flux transfer per unit area of the ith area, hi is the convection coefficient of the ith area, and T si is the mean surface temperature of the ith area. The first term of (1) must satisfy (2), where q is the heat transfer rate, i.e., the sum of the PMSM heat sources, and A is the sum of the m areas m i=1 m i=1

qi =

m

hi · T si

(1)

i=1

qi =

q . A

(2)

In particular, for the airgap thermal transfer, approximate convection coefficients proposed in [31] have been implemented, providing sufficient accuracy for the studied PMSM, as will be shown in Section VI. D. Code Structure A FE package has been assembled for the 3-D FE transient and steady-state analyses of actuators. It consists of seven codes that integrate the aforementioned techniques, and it is structured

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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.

as shown in Fig. 2. The control code calls and passes variables to the remaining six codes as follows: 1) The FE preprocessor procedure is called, which produces the geometry and attributes of the 3-D FE model. The specific procedure parameterizes almost all of the geometry and operating variables of the actuator. 2) A first-order tetrahedral mesh generator is used in order to construct the FE mesh. The quality and size of the mesh are determined by the control program. 3) The control code calls the FE system assembly procedure that integrates the modified Morse technique. This procedure uses as input the Cartesian coordinates of the nodes and the list of the tetrahedra. 4) The input files for the 3-D FE thermal transient solver are the files produced by the mesh generator and the system assembly codes. The specific solver produces an output file containing the temperature distribution of the FE model for every time step of the transient analysis. 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. 5) The control program calls the postprocessor procedure that modifies to an appropriate format the output files of the FE solver. 6) Finally, the postprocessor Gmsh is called, which produces graphic representations of the geometry, mesh, and temperature distribution of the FE model.

Fig. 4.

Cross section of the 3-D FE mesh of the PMSM.

The control program, preprocessor, FE system assembly, FE solver, and postprocessor codes were written by the authors, whereas for the mesh generation and the FE graphics representation, the open-access codes TetGen, TetView, and Gmsh were adopted [32], [33]. IV. FE R ESULTS A. Computational Cost of the 3-D FE Transient Analysis A sensitivity analysis was carried out for mesh sizes ranging from 4 · 105 to 1.2 · 107 elements in order to determine the optimum FE mesh size. The sensitivity analysis showed that a mesh size of 1.5 · 106 was the optimum compromise between computational effort and analysis accuracy. The respective computational effort and relative error for a ten-step FE transient thermal analysis were 1523 s and 2.3%, respectively, using a typical desktop PC. A detail of the densest FE mesh tested is shown in Fig. 3. A cross section of the 3-D FE mesh depicting the regions comprising the model is shown in Fig. 4.

B. Results of Thermal Transient Analysis Using the FE package of Section III, the authors performed 3-D FE thermal transient analyses of the PMSM for different duty cycles and load conditions. The temperature distributions of the PMSM running under maximum load conditions over 140 min is shown in Figs. 5


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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 analysis where the time step ranges from 0.18 s for the first step to 5626 s for the tenth step. More specifically, Fig. 5 shows the temperature distribution of the windings, the stator core, the stator, and the rotor at the

third time step of the analysis. Fig. 6 shows the temperature distribution for three steps of the transient analysis. Fig. 7 shows a comparison of the local maximum and minimum temperature distributions versus time of the PMSM.

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Fig. 9. Fig. 7. Local maximum and minimum temperature variations over 140 min.

Fig. 8.

(a) Experimental setup. (b) DAQ system and measurement instruments.

V. E XPERIMENTAL S ETUP The experimental setup used for evaluating the thermal performance of the PMSM is shown in Fig. 8. It consists of the following components: 1) prototype PMSM; 2) separately excited direct current machine (22 kW);

3) 4) 5) 6) 7)

Prototype PMSM inside the thermal chamber.

data acquisition (DAQ) system; digital and analog instruments; torque gauge; variable electrical loads; 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 (SHC6868-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 total harmonic distortion (THD) values. The second VI was used for capturing the voltage and phase current waveforms for the maximum sampling rate of the DAQ device. The third VI was used for manipulating the acquired voltage and current data [27], [34]. The ambient temperature of the PMSM was controlled by an environmental chamber equipped with a PID controller (Fig. 9). For that purpose, two metal plates attached to the housing of the PMSM were used and a thermal insulation material was placed


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Fig. 10. Thermocouple arrangement for temperature measurements.

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

Fig. 11. First and second sets of simulated and measured temperature variations.

between the PMSM housing and the plates in order to minimize heat dissipation. Fig. 10 shows the arrangement of five type-k thermocouples used for temperature measurements. Three thermocouples were attached to the housing of the PMSM: One was attached to the center of a stator tooth, and one was placed on one of the winding ends. When the rotor was stationary, a sixth type-k thermocouple was used to measure the shaft temperature [4]. VI. C OMPARISON OF S IMULATED AND E XPERIMENTAL R ESULTS Seven experiments were carried out for different duty cycles, operating conditions, and load conditions using the experimental setup of Section V. The first set of measurements was carried out for a rotating speed of 750 r/min, phase currents of 4.92 A, and line-to-neutral voltage of 49 V. The ambient temperature was 25◦ C, and the measurements were carried out for 120 min using a constant ohmic load. The second set of measurements was carried out for the same ambient temperature at 1000 r/min, phase currents of 6.45 A, and line-to-neutral voltage of 65 V for 120 min. In the particular set, an increased load was used in order to raise the steadystate temperature of the PMSM. Fig. 11 shows a comparison of simulated versus measured temperature variations at the

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 down the PMSM. For the fourth set of measurements, increased load and air volume flow rates (400 m3 /h) were used. It was carried out at 1200 r/min, with phase currents of 13.7 A, lineto-neutral voltage of 79 V, and torque of 21.8 N · m for 60 min. Fig. 12 shows the comparison of simulated versus measured temperature variation at three locations of the PMSM of the third and fourth sets of measurements. For the fifth set of measurements, a mixed cycle was employed using variable loads and high air-forced cooling conditions. 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 60 min in order to maintain the winding-end temperature constant. Finally, the third subcycle was obtained by using the air-forced cooling apparatus in order to achieve a windingend temperature decrease of 10 ◦ C/min. Fig. 13 shows the respective comparison of simulated versus measured temperature distributions at three locations of the PMSM for two consecutive duty cycles. Additionally, two sets of measurements were carried out for different ambient temperatures using the thermal chamber described in Section V. Each of them consists of two subcycles of temperature rise and decrease. For the first subcycle, a constant ohmic load was used, and for the second subcycle, airforced cooling conditions were employed. The sixth set was carried out at 1200 r/min for 120 min. An ambient temperature of 25 ◦ C was controlled by the thermal chamber. Figs. 14

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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. C ONCLUSION

Fig. 14. Comparison of simulated versus measured temperature variations over 120 min (housing center, ambient 25 ◦ C).

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 with thermal analysis. Also, the proposed thermal analysis 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. R EFERENCES

Fig. 15. Comparison of simulated versus measured temperature variations over 120 min (winding end, ambient 25 ◦ C).

and 15 show the comparisons of the simulated and measured temperature variations for two locations, the motor housing obtained by thermocouple 1 and the winding end obtained by thermocouple 5. Figs. 14 and 15 show that the winding end is the critical temperature component of the PMSM. The seventh

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Themistoklis D. Kefalas (M’09) was born in Greece in 1977. He received the Electrical Engineering Educator degree from the School of Pedagogical and Technological Education, Athens, Greece, in 1999 and the Diploma and the Ph.D. degree in electrical engineering from the National Technical University of Athens, Athens, in 2005 and 2008, respectively. Since 2010, he has been an Adjunct Assistant Professor with the School of Pedagogical and Technological Education. Since 2008, he has been an Adjunct Lecturer and Research Associate with the School of Electrical and Computer Engineering, National Technical University of Athens. His research interests include electric machine and transformer design, modeling, and optimization. Dr. Kefalas is a member of the Technical Chamber of Greece.

Antonios G. Kladas (S’80–A’99–M’02–SM’10) was born in Greece in 1959. He received the Diploma in electrical engineering from the Aristotle University of Thessaloniki, Thessaloniki, Greece, in 1982 and the D.E.A. and Ph.D. degrees from Pierre and Marie Curie University (Paris 6), Paris, France, in 1983 and 1987, respectively. He was an Associate Assistant with Pierre and Marie Curie University from 1984 to 1989. From 1991 to 1996, he was with the Public Power Corporation of Greece, where he was engaged in the System Studies Department. Since 1996, he has been with the School of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece, where he is currently a Professor. His research interests include transformer and electric machine modeling and design, and the analysis of generating units by renewable energy sources and industrial drives. Dr. Kladas is a member of the Technical Chamber of Greece.