Guest Editorial: Advances in High-Speed Machines for Electric Drives, Power Generation and Energy Storage Systems

importance in several application fields thanks to various factors. For example, it is often desirable to get rid of gear-boxes between high-speed turbines or compressors and the coupled electric machinery in favour of a direct-drive arrangement for better efficiency, higher reliability and easier maintenance. At the same time, raising the speed of the electric machine is an effective way to reduce its torque, and hence its size and weight, for any given power rating. This especially applies to electric motors and generators to be used in hybrid or electric vehicles and in moreelectric aircrafts, where room and weight restrictions make high power density a crucial design target. The field of high-speed electric machinery is very broad encompassing a large variety of technologies, applications, power ratings and performance requirements. In any case, the design of these machines is particularly delicate because materials and components in them are subject to extraordinary thermal, mechanical and electromagnetic stresses and tend to work close to their physical operating limits. For instance, high rated frequencies cause large magnetic losses in the stator core and eddy-current losses in stator conductors and rotor active parts, resulting in possibly dangerous temperatures; rotor surfaces may overheat also due to air friction losses at high rotational speeds. On the other hand, centrifugal forces induce mechanical stresses in rotating components causing wear, fatigue and possible early failures. Finally, the need to reach very high speeds may cause the rotor to temporarily cross or approach its critical speeds, resulting in possible vibrations and lateral dynamic instability of the whole shaft line. Accurately evaluating all of these aspects is mandatory for a safe design and must require a multi-physics approach due to the close interactions among electromagnetic, thermal, ventilation and mechanical phenomena. The design process is made ever more challenging by the frequent requirement to minimise the machine cost and maximise its power density together with other performance indices, leading to the need for a multi-objective constrained optimisation approach. This implies that hundreds or thousands of designs are to be comparatively explored in search for the optimal solutions and, to make such a wide exploration feasible, computationally-efficient methods need to be used for the analysis of each design. This Special Issue features thirteen peer-reviewed papers which provide some specific technical insights into the general topics and challenges mentioned above regarding the design, analysis and operation of high-speed electric motors and generators for state-ofthe-art and emerging applications. The first paper, ‘Maximisation of Power Density in Permanent Magnet Machines with the Aid of Optimisation Algorithms’, by F. Cupertino et al., clearly addresses the potentials and limits of power density maximisation in high-speed surface permanentmagnet machines for aeronautical use. It emphasises how, as the rated speed grows, the retaining sleeve thickness needed to secure the permanent magnet against centrifugal force grows as well, leading to larger air-gaps and thus posing a limit on the power density increase. An optimisation process, including both electromagnetic 2D finite-element analysis (FEA) simulations and analytical mechanical formulas, is proposed to identify the speed that produces the maximum achievable power density. The power density optimisation of a surface-permanent magnet machine for aeronautical use is also addressed in the second paper, ‘Optimisation Method to Maximise Torque Density of High-Speed Slotless Permanent Magnet Synchronous Machine in Aerospace Applications’, by D. Lee et al. Here the focus is on an outer-rotor machine topology with a slotless stator and a Halbach-array permanent-magnet arrangement. The optimisation approach is different as the speed is treated as a constraint, together with stator copper losses, rotor mechanical stress levels, inner and outer machine radii and core length. The internal machine dimensions, as well as the magnetisation directions of Halbach-array magnetic segments, are taken as design variables to maximise the power density through a 2D FEA-based optimisation. Design optimisation is covered again in the third paper, ‘Magnetic Circuit Designing and Structural Optimisation for a Three Degree-of-freedom Hybrid Magnetic Bearing’, by Z. Xu et al., but this time applied to magnetic bearings as a key component of many high-speed machines. A correct magnetic bearing design, targeting suitable load capacity and stiffness values, is essential to guarantee a satisfactory rotor-dynamics behavior of the high-speed shaft line. The magnetic bearing is analytically modeled through the magnetic equivalent circuit technique so as to speed-up the particle-swarm optimisation process. The optimal design finally selected is then investigated in more detail through 3D FEA simulations. An innovative approach to achieve magnetically-suspended rotors in high-speed machines as an alternative to conventional magnetic bearings is presented in the fourth paper, ‘1 kW/60,000  min−1 Bearingless PM Motor with Combined Winding for Torque and Rotor Suspension’, by D. Dietz et al. The high-speed motor is equipped with a six-phase stator winding. The multiple degrees of freedom offered by multiphase windings are exploited to generate the torque through a conventional field-oriented control and, at the same time, to produce the radial force required for rotor magnetic levitation. The solution is implemented into a prototype and successfully validated through various tests. The multi-disciplinary nature of high-speed motor design is illustrated in the fifth paper, ‘Design of High Speed Interior Permanent Magnet Motor Based on Multi-Physics Fields’, by F. Zhang et al., which presents the design process for a high-speed interior permanent-magnet motor. The need for a multi-physics approach is emphasised, stressing the importance and interdependence of the electromagnetic, structural, rotor-dynamics, heat-transfer and fluid-dynamics analyses which need to be performed in the design of a high-speed machine. An insight into the rotor-dynamics analysis in high-speed machine design is given in the sixth paper, ‘Rotor-Dynamics Modelling and Analysis of High-Speed Permanent Magnet Electrical Machine Rotors’, by Z. Huang and Y. Le. Predicting the natural frequencies associated with the rotor bending modes (especially the first two) is, in fact, essential to ensure that all steady-state operating points are sufficiently far from critical speeds and avoid the occurrence of dangerous vibrations and mechanical failures. The integration of mechanical and electromagnetic calculations in the design of high-speed synchronous reluctance motors is addressed in the seventh paper, ‘Design Methodology for HighSpeed Synchronous Reluctance Machines’, by C. Babetto et al. IET Electr. Power Appl., 2018, Vol. 12 Iss. 8, pp. 1065-1066 © The Institution of Engineering and Technology 2018 1065 The paper proposes a semi-analytical procedure for the optimal design of high-speed synchronous reluctance machines intended to achieve the maximum power density and the minimum torque ripple while keeping centrifugal force stresses within safe margins in all rotor regions. This is mainly achieved by an appropriate dimensioning of rotor flux barriers and tangential rib thickness based on both electromagnetic and mechanical calculations. Synchronous reluctance machines are an example of magnetfree solutions for high-speed motors and generators. The benefits of removing permanent magnets are significant in terms of cost reduction and improved reliability above all, at the expense of a lower power density. However, other kinds of magnet-less topologies are emerging as discussed in the eighth paper, ‘Overview of Magnetless Brushless Machines’, by C.H.T. Lee et al. Some of them are suitable for low-speed high-torque applications only, while others may have a potential in the field of high-speed drives and generation systems. Examples are switched reluctance and flux-switching DC machines thanks to their simple and robust rotor structure. The ninth paper, ‘High-Speed Solid Rotor Induction Motor Design with Improved Efficiency and Decreased Harmonic Effect’, by M.O. Gulbahce and D.A. Kocabas considers what is probably the most common and traditional high-speed machine; the solidrotor induction motor. A new asymmetrical stator winding design, including slots and coil sides of different sizes, is presented to reduce the air-gap armature field space harmonics, which are responsible for eddy-current losses in the solid rotor surface. The solution may be helpful to increase motor efficiency and reduce rotor surface overheating risks. A high-speed induction motor is also considered in the tenth paper, ‘Nine-phase IM for Hybridisation of a Compact Vehicle by Parallel TTR Architecture’, by R.Á. Silva et al. The paper discusses the high-speed motor use for the hybridisation of an existing car to reduce its fuel consumption and polluting emissions. A simple hybrid parallel arrangement is proposed for the car refitting with the original combustion engine driving front wheels and the electric motor used for rear axle propulsion and regenerative breaking. Showing the results of driving test campaigns, the authors prove that significant improvements can be obtained with the simple proposed refitting scheme in terms of vehicle efficiency and reduced pollution. The eleventh paper, ‘Influence of Rotor Endcaps on the Electromagnetic Performance of High-Speed PM Machine’, by A. Al-Timimy et al., investigates high-speed surface-permanent magnet machines focusing on the impact of rotor ferromagnetic endcaps used to guarantee rotor mechanical integrity. The authors show that the presence of endcaps significantly reduces the output torque due to an increase in end leakage fluxes. It is also shown how this performance degradation cannot be predicted using 2D FEA, while it is in excellent agreement with 3D FEA simulation results. The previous paper stresses the importance of using accurate 3D FEA models to capture some parasitic effects that may affect high-speed motor performance. However, for the purpose of design optimisation, the use of 3D FEA models would lead to inacceptable computational burden and simplified (analytical or semi-analytical) models are needed for a fast first-attempt performance prediction. To this end, the twelfth paper, ‘Calculation of Rotor Losses in PM Machines with Retaining Sleeves Using Transfer Matrices’, by J. R. Anglada et al., presents an analytical approach for the fast prediction of eddy-current losses in high-speed surface permanentmagnet machines with a partly-conductive retaining sleeve. The paper proposes a computationally-improved approach to the solution of the Helmholtz's equation in the rotor domain under the hypotheses of neglecting end effects and magnetic saturation. Finally, the last paper, ‘Parameters and Performance Analysis of a Dual Stator Composite Rotor Axial Flux Induction Motor by an Analytical Method’, by C. Hong et al., proposes an analytical method to rapidly compute the performance of high-speed axialflux induction machines to be used, for example, as flywheels for kinetic energy storage devices. As in the previous paper, the Helmholtz's equation is solved in the rotor domain which is modelled according to a multi-layer, multi-slice scheme. The solution is particularly interesting as it takes into account the magnetic saturation as well as end effects. The reliability of the proposed analytical calculation method is assessed by comparison with both 3D FEA and experimental results.