Guest Editorial: Advances in electric machines - solutions for electro-mobility with improved characteristics

The efforts of mankind focussed on the reduction of global warming and carbon dioxide emissions, achievable through the reduced usage of fossil fuels, strongly influence the development of the entirety of engineering. The means identified to achieve these goals are improved energy efficiency, increased utilisation of renewable energy sources and a broader introduction of electro-mobility. These are all challenges that must be properly addressed in the further development of electric machines. The efforts of industry towards electrification of the transport sector have produced exceptional results. In markets such as the European Union, the share of new electric cars has approached 20% in recent months. The power range of electro-mobility in road transport applications varies from several watts for auxiliary electric drives to several hundreds of kilowatts for main drive trains. Efficiency is imperative for all electrical machines, both conventional and those used in electro-mobility related applications. Electro-mobility introduces new challenges for the research and development of electric machines, where low mass, high power density, high efficiency and reliability, wide speed range, robustness and fault-tolerant design are some of the main targets. These requirements, combined with different supply voltage levels and specific operating cycles, lead to new solutions that incorporate entirely new machine designs and topologies, novelties in thermal management and fault diagnostics and mastering of electromagnetic emissions. We received 12 contributions for this special issue. After a peer review, only six of these met the IET Electric Power Applications criteria and were accepted for inclusion in the special issue. The accepted papers can be grouped into three main categories: design and topology of electrical machines, estimation of thermal conditions and fault diagnosis. The papers in the first category, Nobahari et al. and Feng et al., focus on the effects of the design on torque and other characteristics of electrical machines, while Krall et al. analyse the effects of the stator design on electromagnetic emissions. In the second category, Dong et al. focus their research on the speed-dependent characteristics of heat transfer parameters, while Nawazish et al. address the thermal degradation of speed estimation in a traction machine drive. In the third category, Trabelsi et al. investigate faults diagnosis in a multiphase permanent magnet synchronous motor (PMSM) drive. Nobahari et al. deal with the design of a PMSM for an electric city bus. The unit winding factor is used to improve torque capability, while the fully open slots are used to accommodate low-cost rectangular wire winding. The resulting large torque pulsation is mitigated by the rotor design, for which an axially segmented Halbach pole configuration is proposed. It provides torque smoothing and a large average torque. The optimal parameters of the proposed Halbach pole configuration are determined in an optimisation procedure by using a 2D finite element model. Feng et al. conducted a comparative study of dual 3-phase permanent magnet (PM) machines with coil spans of two slot pitches, analysing the 24-slot/10-pole and 24-slot/14-pole machines with windings displaced by 30°. The winding factor, back electromotive force, average torque, torque ripple, iron and PM losses, short-circuit (SC) current and PM irreversible demagnetisation are analysed and compared for a healthy operation and for a set of three-phase faults, that is, SC or open circuit. The obtained results clearly show that the 14-pole PM machine has a slightly larger torque output, a better overrating torque capability and a significantly lower risk of PM irreversible demagnetisation. The 10-pole machine has smaller iron losses but larger PM losses. The findings of the authors are confirmed experimentally by measurements performed on prototypes of the two machines that are discussed. In luxury cars, the number of auxiliary electric drives can easily exceed 100, which increases the importance of their electromagnetic emissions. Krall et al. analyse the influence of four different stator winding topologies on the electromagnetic emissions and efficiency of fractional horsepower brushless DC drives. Two monofilar windings with wire diameters of 0.15 and 0.27 mm, one untwisted bifilar winding and one twisted bifilar winding are studied. The parasitic coupling capacitance between the stator and winding represents a coupling path that is the main source of common-mode currents and electromagnetic emissions in electric motors. It is considered by introducing the three-media capacitance model. The performed analysis shows that the stator winding topology affects the radiated electromagnetic emissions, the winding resistance and the efficiency. Dong et al. focus their research on speed-dependent heat convection in high-speed electric vehicle motors and its applications in model-based thermal rotor monitoring. The heat convection intensities on the surfaces of the air gap and the end-cavity vary significantly with the rotational speed, which significantly affects the accuracy of the thermal model. The authors investigate the impact of rotational speed on the variation of airflow aerodynamic properties and the heat convection using theoretical, numerical and experimental methods. Based on their findings, the authors updated the constant thermal resistance with a multi-stage characteristic in the thermal model, which resulted in a reduction of the total error in temperature estimation by almost 11%. Nawazish Ali et al. proposed a robust, linear and a parameter-varying observer to estimate the thermal degradation of an induction machine drive performance and its impact on electric vehicle speed estimation under different thermal and loading conditions in a steady state and during large transients. The stability and robustness of the linear parameter variation (LPV) method are ensured by optimal gains of H∞ control and linear matrix inequalities using convex optimisation techniques. A genetic algorithm is applied to optimise the weighting functions in the LPV design. Comparison of the proposed observer with sensorless field-oriented control and sliding mode observer shows the improved speed performance in an electric vehicle operation. Trabelsi et al. introduced a virtual current vector-based method for detecting open-switch and open-phase inverter faults in a multiphase PMSM drive. Variables called virtual current vectors were defined first, where the projection of the zero-sequence current component onto these variables was used to define two fault indices. The mathematical development of the proposed method is provided and supported by experimental tests conducted on two prototypes of multiphase PMSMs in the laboratory. The experimental results confirm the effectiveness and robustness of the proposed method and its ability to detect single and multiple open-switch and open-phase faults in the electric drive. The selection of papers for this special issue shows that the immense progress in electro-mobility stimulates further research and the permanent development of electrical machines. This applies not only to the most suitable topologies and designs of various electrical machines but also to the control of electromagnetic emissions, the estimation and monitoring of thermal conditions and fault diagnosis. We would like to express our gratitude to the authors for contributing novel ideas and research results to this special issue. Our gratitude goes to the reviewers, whose expertise was necessary for the professional and unbiased evaluation of the submitted manuscripts. We hope that this effort will stimulate the scientific and technological community to contribute to further development in the field of electrical machines for electro-mobility. Finally, we would like to express our appreciation to the editor-in-chief of the journal and the editorial office for their support in preparing this special issue. Gorazd Štumberger was born in Ptuj, Slovenia, in 1964. He received his B.Sc., M.Sc., and Ph.D. degrees from the University of Maribor, Maribor, Slovenia, in 1989, 1992 and 1996, respectively, all in electrical engineering. Since 1989, he has been with the Faculty of Electrical Engineering and Computer Science, University of Maribor, where he is currently a full professor of electrical engineering and dean. He was a visiting researcher at the University of Wisconsin-Madison, Madison, Wisconsin, USA, in 1997 and 2001, and with the Catholic University Leuven, Leuven, Belgium, in 1998 and 1999. His research interests include design, modelling, analysis, optimisation, operation and control of electrical machines and drives, electrical power system elements, smart grids, microgrids and energy management systems. He has been a project team leader of three international, five national and over 30 industry-funded projects. His bibliography contains over 170 articles in scientific journals, over 280 contributions to proceedings of scientific conferences and four international and two national patents. He has been an editorial board member of five scientific journals, a guest editor of two special issues and a reviewer in over 40 scientific journals. He was one of the authors of two works awarded by the Slovenian Research Agency with the Excellence in Science Award. He is a member of IEEE and national societies SATENA, SiEnE and the Slovenian Committee CIGRÉ. Damir Žarko was born in Zagreb, Croatia, in 1972. He received the B.S. and M.S. degrees in electrical engineering from the University of Zagreb, Zagreb, Croatia, in 1995 and 1999, respectively, and the Ph.D. degree from the University of Wisconsin-Madison in 2004. He is currently a full professor at the Faculty of Electrical Engineering and Computing, University of Zagreb, Croatia. His research activities are related to the design, modelling and optimisation of electrical machines and power transformers. He has been involved in 13 scientific projects as a project leader or team member. He has published more than 100 papers in high impact journals and international conferences. He gave two invited talks at international conferences and served as a guest editor for two international journals. He participated in the programme committees of 15 international scientific conferences and was a reviewer for papers in 10 international scientific journals and numerous international conferences. In 2015, he received the Croatian IEEE Section Award for the outstanding technical contributions. He is a member of the Croatian National Committee CIGRÉ, Croatian Academy of Engineering and a senior member of IEEE. Jonathan Bird received the B.S. degree in electrical and computer engineering from the University of Auckland, Auckland, New Zealand, in 2000 and the M.S. and Ph.D. degrees in electrical and computer engineering from the University of Wisconsin-Madison, Madison, WI, USA, in 2004 and 2006, respectively. From 2007 to 2008, he was a Senior Design Engineer with the General Motors Advanced Technology Center, Torrance, CA, USA. From 2009 to 2015, he was an Assistant then Associate Professor with the University of North Carolina at Charlotte, Charlotte, NC, USA. Since 2015 he has been an associate professor at Portland State University, Portland, OR, USA. His primary research interests include magnetic geared electrical machines, electrodynamic wheel maglev technology and variable stiffness magnetic springs. Dr. Bird's research has been primarily funded by the Department of Energy and the National Science Foundation. He is an Associate Editor of IEEE Transactions on Magnetics and a member of IEEE.