Electric Machine Design Tailored for Powertrain Optimization

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1 Electric Machine Design Tailored for Powertrain Optimization Dieter Gerling 1, Gurakuq Dajaku 2, and Klaus Mühlbauer 1 1 Institute for Electrical Drives, University of Federal Defense Munich, Werner-Heisenberg-Weg 39, Neubiberg, 85579, Germany 2 FEAAM GmbH, Werner-Heisenberg-Weg 39, Neubiberg, 85579, Germany dieter.gerling@unibw.de Abstract In spite of being highly efficient, battery electric vehicles suffer from limited driving range because of the limited battery capacity. This disadvantage can be encountered by optimizing the powertrain using a special electric machine design. The main idea is to increase the efficiency of the electric motor especially in those operating points, mainly used in relevant driving cycles. This means, the motor has to be optimized for low-load operation. In addition to this efficiency improvement the electric motor will be even more cost-effective. The impact on the entire powertrain concerning technical and financial advantages will be shown. Keywords electric motor, efficiency optimization, electric powertrain, cost reduction, low-load operation 1. Introduction A big advantage of a full electric powertrain of future passenger cars is the very high maximum efficiency of the electric drive compared to the efficiency of today s internal combustion engines. But in spite of this very high maximum efficiency, the driving range of electric cars is limited because of two main reasons: Firstly the limited energy capacity of the battery, and secondly the limited efficiency of the electric traction drives at typical driving situations (i.e. low-load operation). Some typical operating points for the electrical traction drive are shown in figure 1 in the torque-speedplane (mainly taken from the FTP 72 driving cycle). It becomes obvious that low-load operation is most prominent: in most cases the necessary power is below 25kW. In addition it has to be emphasized that the high power operating points are used only during extremely short periods during the lifetime. This means that the traction drive must be able to deliver such a high power, but the critical design parameter is the efficiency at low-load. Figure 1: Characteristic operating points for an electrical traction drive. 2. Electric Machine Design The boundary conditions for the design of the electric traction machine were set to: 300V DC-link voltage, 12000rpm maximum speed, 250Nm short-time torque at 4700rpm and less than 5% torque ripple at low speed. In addition, the weight and volume of the machine parts (iron, copper, and magnets) and the entire machine housing, the material qualities, the air-gap width, the demagnetization safety, and the mechanical robustness were kept constant for all machine designs to guarantee a fair comparison. Moreover, the temperature was kept constant during the simulation procedure of all alternative machines (stator 100 C; rotor 70 C). In the following brushless interior permanent magnet machines are investigated, all of them contain threephase star-connected stator windings. As a reference, a motor with 8 rotor poles and 48 stator slots (distributed winding with number of slots per pole and phase q=2; short-pitch of one slot) was designed. This machine design will be referred to in the following as Design No. 1. Starting with this Design No. 1 an improvement for all operation points has been realized by pure modification of the winding scheme. This machine is called Design No. 2. In addition, a new electric machine design was elaborated containing some patent pending features. This new motor design is characterized by low costs (because of better manufacturability compared to the standard design No 1) and high efficiency especially at low-load operation. In the following, this novel electric motor is called Design No. 3. The following figures 2 to 4 show the efficiency calculation results in the torque-speed-plane. These results are based on detailed Finite Element Calculations, please refer e.g. to [1, 2].

2 The differences between these motor designs become even more obvious, if the percentage data of the designs No 2 and No 3 are substracted from the data of design No 1 for each operating points. This leads to the following figures 5 and 6 Figure 2: Efficiency of the reference motor (Design No. 1) as a function of torque and speed. Figure 5: Efficiency improvement of Design No. 2 against Design No. 1 as a function of torque and speed. Figure 3: Efficiency of the new electric machine (Design No. 2) as a function of torque and speed. Figure 4: Efficiency of the new electric machine (Design No. 3) as a function of torque and speed. Figure 6: Efficiency improvement of Design No. 3 against Design No. 1 as a function of torque and speed (the blue area indicates efficiency deterioration). It can be clearly deduced from the above figures, that especially during low-load operation a considerable loss reduction can be achieved because of the increased efficiency. In the case of Design No. 3 an improvement of more than 10 percent points at low-load operation is achieved. The lower efficiency at very high speed and very high power (blue area in figure 6) is not relevant for typical standardized driving cycles or for the driving operation of customers. In addition, the new electric machine (design No. 3) has two more advantages: The torque-versus-position characteristic shows a value of about 3% ripple, whereas for the standard motor (design No 1) the ripple is at about 16%. Consequently, there are means necessary for

3 design No 1 to reduce the ripple (e.g. skewing), which deteriorates the power output and therefore the efficiency. For the same output torque the current density in the stator windings is about 30% higher in design No 1 than in design No 3. Therefore, the motor design No 3 stays cooler or this design can further be optimized for minimum material utilization and consequently minimum costs. 3. Simulation of the Electric Powertrain The efficiency improvement of the electric motor shown in the preceding section has of course additional positive impacts on the losses of the power electronics and the losses of the battery (internal resistance). To judge the influence of all components of the electric powertrain on the performance of the vehicle, a simulation of the entire electric powertrain has been performed. As an example, the well-known FTP 72 driving cycle has been taken (see Fig. 7 for the timedependent torque and speed characteristics; even some other driving cycles may be investigated, e.g. the CADC cycle [3], which show differences in detail but in general give similar results). Evaluating such simulations, the performance of the different parts of the powertrain can be separated. As an example, the cumulative losses of the different electric machine designs are presented in the following Fig. 8. With the new electric machine design No 3 a loss reduction of about 20% during the FTP 72 driving cycle is demonstrated. Similar loss reduction calculations are performed for the other components of the electric powertrain, in particular for the power electronics. In summary, this leads to a considerably decreased required battery capacity (and therefore even battery costs) for a certain driving range of the automobile. speed in rpm time in s Figure 7: Torque and speed versus time characteristics of the FTP 72 driving cycle. Design No 1 loss energy in J Design No 2 Design No 3 time in s Figure 8: Cumulative losses of the different electric machine designs during the FTP 72 driving cycle.

4 4. Impact on the Power Electronics, Battery, and Entire Powertrain As said before, the motor design No 3 needs considerably lower current density than design No 1. Concerning the power electronics, this results in at least 10% less phase current for the same output torque. Consequently, even the power electronics losses are reduced. As a rough estimate, the switching losses are nearly constant, but the conduction losses are proportional to the squared phase current and therefore reduced by far. In total, a reduction of the power electronics losses by about 10% can be estimated. These reduced losses lead to less heating and therefore smaller housing and heat sink can be used. As a consequence, the costs are reduced as well. Designing an electric vehicle for a specific driving range means that the necessary battery capacity depends on the mechanical driving energy as well as the energy losses during driving. The energy losses can be reduced considerably using the presented new motor design, because of loss reduction in motor and power electronics. Conservative estimates deduced from the driving cycle simulations show that at least 1.5% of battery capacity reduction is possible, resulting in a likewise reduction of battery costs. The impact on the entire powertrain is as follows: Because of the reduced losses, all components of the electric powertrain (battery, power electronics, motor, cables) can be downsized and therefore save weight and volume. There are cost savings for all major components: the motor and power electronics are both estimated to be at least 10% cheaper, the battery at least 1.5%. Assuming a cost distribution of 20% motor costs, 30% power electronics costs, and 50% battery costs means that the total costs of the electric powertrain can be reduced by about 5.75%. Further assuming electric powertrain costs of means that for each powertrain about can be saved. Forecasting this for a production number of cars per year means savings of at least 40 million per year. The boundary conditions for this financial calculation were chosen very conservative. For example, the positive feedback of lower weight of all components of the powertrain was not considered so far. In addition, further optimization of the drive with regard to efficiency, power density, and tailoring to specific driving cycles is possible. 5. Conclusion and Outlook It could be demonstrated in this paper that a considerable improvement of the electric powertrain of future battery electric vehicles is possible. Because of the low-cost electric motor and the reduced power losses the overall cost savings of the electric powertrain (motor, power electronics, and battery) can be estimated to be at least 5.5%. It has to be emphasized that additional loss reduction and cost savings are possible due to optimization by further tailoring the electric motor to the needs of the regarded driving cycle (e.g. torque level, focusing the efficiency improvement to the most relevant operating points, etc.). Savings up to 10% seem to be realistic. Moreover, not only PM motors are possible candidates for the described improved motor design, but even induction motors or electrically excited synchronous motors may be utilized. 6. References [1] Dajaku, G.: Electromagnetic and Thermal Modeling of Highly Utilized PM Machines, Ph.D. Dissertation, University of Federal Defense Munich, Germany, [2] Dajaku, G.; Gerling, D.: Analysis of Different Permanent Magnet Machines for Hybrid Vehicles Application, ANSYS Conference & 26th CADFEM Users' Meeting, October 2008, Darmstadt, Germany. [3] Gerling, D.; Dajaku, G.; Mühlbauer, K.: Cost-Effective Electric Traction Drive with High Efficiency at Low-Load Operation, International VDE Congress emobility, November 2010, Leipzig, Germany 7. Authors Prof. Dr.-Ing. Dieter Gerling Institute for Electrical Drives, University of Federal Defense Munich, Werner-Heisenberg-Weg Tel: dieter.gerling@unibw.de URL: Born in 1961, Prof. Gerling got his diploma and Ph.D. degrees in Electrical Engineering from the Technical University of Aachen, Germany in 1986 and 1992, respectively. From 1986 to 1999 he was with Philips Research Laboratories in Aachen, Germany as Research Scientist and later as Senior Scientist. In 1999 Dr. Gerling joined Robert Bosch GmbH in Bühl, Germany as Director. Since 2001 he is Full Professor and Head of the Institute of Electrical Drives at the University of Federal Defense Munich, Germany.

5 Dr.-Ing. Gurakuq Dajaku FEAAM GmbH, Werner-Heisenberg-Weg Tel: URL: Born in 1974 (Skenderaj, Kosova), Dr. Dajaku got his diploma degree in Electrical Engineering from the University of Pristina, Kosova, in 1997 and his Ph.D. degree from the University of Federal Defense Munich in Since 2007 he is Senior Scientist with FEAAM GmbH, an engineering company in the field of electric drives. From 2008 he is a Lecturer at the University of Federal Defense Munich. His research interests include the design, modelling, and control of permanent-magnet machines for automotive application. Dr. Dajaku recived the Rheinmetall Foundation Award 2006 and the ITIS (Institute for Technical Intelligent Systems) Research Award Dipl.-Ing. Klaus Mühlbauer Institute for Electrical Drives, University of Federal Defense Munich, Werner-Heisenberg-Weg Tel: Klaus.muehlbauer@unibw.de URL: Klaus Muehlbauer was born in Tirschenreuth (Bavaria, Germany) on September 30, He graduated from the Friedrich-Alexander-University in Erlangen, Germany, in 2008 with the diploma degree. During his study he stayed abroad at the University of Bristol for one semester. He passed his diploma thesis at the research and innovation center of BMW AG in Munich with the focus on the system design of hybrid electric vehicles. Since July 2008 he works as a research assistant at the University of Federal Defense in Munich at the chair in Electrical Drives and Actuators.