This paper reviews the current technologies used in high-speed electrical machines through an extensive survey of different topologies developed and built in the industry and academia for several applications. Developments in materials and components, including electrical steels and copper alloys, are discussed, and their impact on the machines' operating physical boundaries is investigated. The main application areas pulling the development of high-speed machines are also reviewed to better understand the typical performance requirements.

Hairpin windings are gaining increasing popularity in recent years due to their advantages in improving electrical machine performance while reducing manufacturing time and costs. Their geometrical features introduce new challenges and opportunities on thermal management. In particular, spray cooling is increasingly being used, since hairpin windings open up regular and accurately defined gaps in the end-windings compared to the traditional random windings. To date, technical literature on the effectiveness of spray cooling with hairpin windings is lacking, with no practical guidelines available to researchers. In this article, taking an existing hairpin-wound stator, a test rig is developed to investigate the cooling ability of different spray cooling setups on the end-windings. Three types of commercial spray nozzles are chosen to carry out a series of experiments, varying the oil flow rate, pressure, outlet velocity, and number of nozzles. The winding temperature and heat transfer coefficients are presented and discussed. Furthermore, the efficiency of spray cooling is reviewed based on the experimental results. Finally, suggestions on designing similar cooling setups and increasing the cooling efficiency are also provided.

This paper deals with the considerations associated with the design of high-speed high-power-density laminated-rotor induction machines (IMs). The considerations discussed are described by the design of an actual 10-kW machine, which runs at speeds of up to 75 kr/min with a rated power density of 28 MW/m ³ for an electrically assisted turbocharger. Using a developed multidomain design environment which puts equal weight on the electromagnetic, mechanical, and thermal aspects, the rotor split ratio, electric and magnetic loadings, lamination material, rotor-bar material, and rotor-bar shape are identified as important and sensitive parameters in the design of high-speed IMs. Finally, general guidelines for designing high-speed high-power-density IMs are presented.

The rapidly increasing demand on power density levels of electric vehicle (EV) drive systems is pushing the boundaries of traction motor performance. Hairpin windings are becoming a popular option for EV motors due to their reduced dc losses and improved heat dissipation capability when compared to traditional random windings. In this article, a comprehensive design approach of hairpin winding layouts is first presented. The flexibility and limitation of end-winding patterns are thoroughly investigated in terms of basic pin connections, special jumpers, transposition, parallel branches, terminal positions, phase shift, winding pitches, as well as slot–pole combinations. To address the challenge of much reduced practical layout options with increased slot number per pole per phase, two novel hairpin winding designs are proposed. A 160-kW, 18 000-r/min permanent magnet (PM) traction motor featuring the new winding layout with 54-slot, 6-pole is developed using a multidomain design platform, which puts special focus on the conductor size optimization. The advantages of the designed motor are clearly revealed by comparison with the more traditional 48-slot, 8-pole counterpart. Finally, a corresponding stator prototype with the proposed hairpin winding is built to validate its manufacturability.

This article describes the underpinning research, development, construction, and testing of a 4-MW multithree phase generator designed for a hybrid-electric aircraft propulsion system demonstrator. The aim of the work is to demonstrate gravimetric power densities around 20 kW/kg, as required for multi-MW aircraft propulsion systems. The key design choices, development procedures, and tradeoffs, together with the experimental testing of this electrical machine connected to an active rectifier, are presented. A time-efficient analytical approach to the downselection of various machine configurations, geometrical variables, different active and passive materials, and different thermal management options is first presented. A detailed design approach based on the 3-D finite element analysis (FEA) is then presented for the final design. Reduced power tests are carried out on a full-scale 4-MW machine prototype, validating the proposed design. The experimental results are in good agreement with simulation and show significant progress in the field of high-power-density electrical machines at the targeted power rating.

While the electromagnetic aspects of hairpin windings are actively being investigated and discussed in recent literature, including the design rules together with the loss calculation and reduction techniques, the thermal performance and modeling aspects have received less attention to date. In hairpin windings, the conductors (pins) are comparatively larger and arranged as separate components in parallel within the slot. In contrast, conductors randomly overlap and contact each other for traditional random windings. The differences in the aforementioned winding physical characteristics result in a different methodology to develop the thermal network. This article presents a 3-D lumped parameter thermal network (LPTN) approach for an oil-spray cooled hairpin winding, which includes the slot thermal model configuration, the end-winding connections, together with different methodologies of analyzing the end-winding sprayed oil characteristics. The aforesaid thermal model captures unique features related to the winding technology and cooling mechanism, such as the nonuniform end-winding temperature caused by the uneven oil-spray cooling effects. Finally, taking an existing propulsion drive hairpin stator and a bespoke-designed test setup, the presented steady-state thermal modeling approach is experimentally validated covering various experimental tests, including different spray conditions.