Electric and hybrid-electric aircraft propulsion are rapidly revolutionising mobility technologies. Air travel has become a major focus point with respect to reducing greenhouse gas emissions. The electrification of aircraft components can bring several benefits such as reduced mass, environmental impact, fuel consumption, increased reliability and quicker failure resolution. Propulsion, actuation and power generation are the three key areas of focus in more electric aircraft technologies, due to the increasing demand for power-dense, efficient and fault-tolerant flight components. The necessity of having environmentally friendly aircraft systems has promoted the aerospace industry to use electrically powered drive systems, rather than the conventional mechanical, pneumatic or hydraulic systems. In this context, this paper reviews the current state of art and future advances in more electric technologies, in conjunction with a number of industrially relevant discussions. In this study, a permanent magnet motor was identified as the most efficient machine for aircraft subsystems. It is found to be 78% and 60% more power dense than switch-reluctant and induction machines. Several development methods to close the gap between existing and future design were also analysed, including the embedded cooling system, high-thermal-conductivity insulation materials, thin-gauge and high-strength electrical steel and integrated motor drive topology.

For a continuous mode of operation, insulating material in an electrical machine is subject to constant thermal, electrical, mechanical and environmental stresses where thermal stress is a major cause of gradual insulation deterioration, which leads to ultimate winding failure. To guarantee a satisfactory lifetime, electrical machines are designed to operate winding temperatures well below their thermal class, which results in an oversized design. Standard methods for thermal lifetime evaluation of electrical machines are based on accelerated aging tests that require several months of testing. This paper proposes an alternative approach relying on a supervised neural network that significantly shortens the time demanded by accelerated aging tests for thermal lifetime evaluation of electrical machines. The supervised neural network is based on a feedforward neural network trained with Bayesian Regularisation Backpropagation (BRP) algorithm. The network predicts the wire insulation resistance with respect to its aging time at aging temperatures of 250°C, 270°C and 290°C, which reveals a good match of prediction outcomes against the experimental findings. The mean time-to-failure at each aging temperature is extracted using the Weibull probability plot in order to compare the Arrhenius curves for both conventional and proposed method and a relative error of 0.125% is achieved in terms of their temperature indexes. In addition, the analysis shows a time saving of 1680 hours (57% time saved of experimental test procedure) when the thermal life of the insulating material is predicted using BRP neural network.

Estimating the lifetime of enameled wires using the conventional/standard test method requires a significant amount of time that can endure up to thousands of testing hours, which could considerably delay the time-to-market of a new product. This paper presents a new approach that estimates the insulation lifetime of enameled wire, employed in electrical machines, using curve fitting models whose computation is rapid and accurate. Three curve fit models are adopted to predict the insulation resistance of double-coated enameled magnet wire samples, with respect to their aging time. The samples' mean time-to-failure is estimated, and performance of the models is apprised through a comparison against the conventional `standard method' of lifetime estimation of the enameled wires. The best prediction accuracy is achieved by a logarithmic curve fit approach, which gives an error of 0.95% and 1.62% when its thermal index is compared with the conventional method and manufacturer claim respectively. The proposed approach provides a time-saving of 67% (83 days) when its computation time is compared with respect to the `standard method' of lifetime estimation.

In safety-critical applications such as aerospace and marine, electrical machines are limited to meet several stringent requirements such as physical, electrical, magnetic and thermal isolation between two three-phase channels, in order to be considered for fault-tolerant operation throughout its service life. This paper presents novel sectored fault-tolerant PM synchronous machines which include single, double and four sectored designs where one pole pair section of the machine’s stator is cut out to meet the fault-tolerant constraints. A detailed electromagnetic analysis is carried out in terms of their electromagnetic performance which is then compared with the benchmark 30s/10p combination for a given output power of 50kW. Due to the heightened power density in the innovative design, a highly effective thermal management system became necessary. This led to the identification of a novel and efficient cooling configuration. Leveraging the four gaps between the four sectors, an efficient cooling system was successfully implemented. As a result, the current density increased by 18.2%, representing an increase of 2A/mm² compared to the benchmark machine design.

A close functional and structural integration of passive elements is required to improve the power density of motor drives. Such power dense motor drives are prerequisite in aerospace and automotive applications. This paper presents a permanent magnet motor with integrated filter inductor, which not only eliminates inductor losses but also removes its associated weight and volume. The motor with integrated filter uses motor's inherent magnetics in order to use it as a filter inductance instead of adopting an external inductor option, which is traditionally placed outside the motor. A vector-controlled model, taking modulation and switching effect into account, has been developed by using MATLAB/Simulink tool. To experimentally validate the concept of the motor with integrated filter inductor, the winding connections of an existing motor are modified. The comparative analysis, between the traditional and integrated motor drive systems, is carried out in terms of magnitude of switching component, total system losses, weight and volume. The total losses in the motor with integrated filter are reduced by 34.2% at 2100rpm and 3Nm load, when compared to the motor with traditional filter inductor, whereas, its weight and volume in the motor with integrated filter inductor is eliminated completely.

This article shares with the aerospace community a case study of turboprop mild hybridization using a recently developed integrated drive system in the University of Nottingham, U.K., within the ACHIEVE project under EU H2020 CleanSky 2 program (project No. 737814). The developed drive system enables the green taxiing of a turboprop aircraft while on the ground with its engine OFF and as an electrical generator when the turboprop is in the air. The entire system is designed to be able to integrate within the power auxiliary gearbox (PAGB) of a turboprop aircraft. Some of the key features of the developed system include a high-speed permanent magnet machine (up to 14200 rpm) with a dual-three-phase design, silicon carbide (SiC)-based high power density (11.8 kW/L for the power converter, and 35.3 kW/L and 7.2 kW/kg for the machine active parts), integrated cooling design for high-temperature operation ( $>130~^{\circ }\text{C}$ ambient temperature), fault tolerance consideration with dual-channel operation capabilities, and sensorless control for entire operational conditions. This article is giving an overview of the design process of the electrical machine, power converters, and the cooling of the entire drive. The numerical analysis [finite element method (FEM) and computational fluid dynamics (CFD)] and some experimental results are presented to demonstrate the effectiveness and the desired performance of the developed integrated drive system.