Abstract-The constant growth of air traffic, the demand for performance optimization and the need for decreasing both operating and maintenance costs have encouraged the aircraft industry to move towards more electric solutions. As a result of this trend, electric power required on board of aircraft has significantly increased through the years, causing major changes in electric power system architectures. Considering this scenario, the paper gives a review about the evolution of electric power generation systems in aircraft. The major achievements are highlighted and the rationale behind some significant developments discussed. After a brief historical overview of the early DC generators (both wind-and engine-driven), the reasons which brought the definitive passage to the AC generation, for larger aircraft, are presented and explained. Several AC generation systems are investigated with particular attention being focused on the voltage levels and the generator technology. Further, examples of commercial aircraft implementing AC generation systems are provided. Finally, the trends towards modern generation systems are also considered giving prominence to their challenges and feasibility.

Michael (2017) Analytical thermal model for fast stator winding temperature prediction. IEEE Transactions on Industrial Electronics, 64 (8).

Pure copper is an excellent thermal and electrical conductor, however, attempts to process it with additive manufacturing (AM) technologies have seen various levels of success. While electron beam melting (EBM) has successfully processed pure copper to high densities, laser powder bed fusion (LPBF) has had difficulties achieving the same results without the use of very high power lasers. This requirement has hampered the exploration of using LPBF with pure copper as most machines are equipped with lasers that have low to medium laser power densities. In this work, experiments were conducted to process pure copper with a 200W LPBF machine with a small laser spot diameter resulting in an above average laser power density in order to maximise density and achieve low electrical resistivity. The effects of initial build orientation and post heat treatment were also investigated to explore their influence on electrical resistivity. It was found that despite issues with high porosity, heat treated specimens had a lower electrical resistivity than other common AM materials such as the aluminium alloy AlSi10Mg. By conducting these tests, it was found that despite having approximately double the resistivity of commercially pure copper, the resistivity was sufficiently low enough to demonstrate the potential to use AM to process copper suitable for electrical applications. Highlights• Medium powered LPBF machines can process pure Cu to an acceptable level.• Resistivity of as-built Cu increases by 33% depending on initial build orientation.• Resistivity can be reduced by over 50% from as-built conditions via heat treatment.• Electrical resistivity values once heat treated are lower than AlSi10Mg values.

-In electrical machines, a higher torque/force density can usually be achieved by increasing the current density in the windings. However, the resulting increase in copper losses leads to higher temperatures in the coils, especially in the centre of the slots where the thermal resistance to the ambient/cooling surfaces is highest. In this paper a novel, simple technique is presented in which a higher thermal conductivity path between the centre of the slot and the cooling arrangement is created, thus increasing the heat flow away from the slot centre. A lumped parameter thermal model is presented and used along with finite element analysis to investigate the effectiveness of the proposed technique. The lumped parameter model is also used for optimizing the high conductivity path for maximum air-gap shear stress and to obtain a compromise between the reduced slot area and the improved temperature distribution. Experimental validation is then presented to compare the predicted results with the measured results on a purposely built instrumented set-up.Index Terms -Permanent magnet machines, electrical machine windings, high current density, thermal improvements. I. INTRODUCTIONN recent years the need for improvement in terms of size, efficiency and costs of electrical machines has become a main issue which electrical machine designers have to consider through all the design stages [1]. This is most significantly apparent in high performance environments such as the aerospace industry, where torque/force density, fault tolerance and reliability are of vital importance [2], [3].Despite the vastness of the range of electrical machines available today, in general all machines are subject to the constraints imposed by the materials (copper, insulation, soft and hard magnetic materials) from which their main components are constructed [4].These constraints can be considered to fall into two main categories, namely the electro-magnetic limit and the thermal limit. These two limits quantify what force/torque can be obtained from a particular machine. The electro-magnetic limit is set by the soft magnetic material saturation point and Manuscript received April 30, 2011. This work was supported by the "EU FP7 funding via the Clean Sky JTISystems for Green Operations ITD".Michael Galea (eexmg1@nottingham.ac.uk,, Chris Gerada (chris.gerada@nottingham.ac.uk) and Patrick Wheeler (pat.wheeler@nottingham.ac.uk) are with the School of Electrical and Electronic Engineering, University of Nottingham, Nottingham, NG7 2RD, UK). Tsarafidy Raminosoa (t.raminosoa@ieee.org) was with the University of Nottingham until 2011. He is now with GE Global Research, Niskayuna, US.possibly by the maximum armature reaction field so as not to demagnetize the permanent magnets (PM). The thermal limit which is related to the cooling arrangement sets the maximum current density at which a machine can operate safely at. This typically lies below the electro-magnetic limit in steady state and hence extending this thermal limit by adequate coolin...

Abstract-The benefits of implementing a damper winding in salient-pole, synchronous generators are widely known and well consolidated. It is also well known that such a winding incurs extra losses in the machine due to a number of reasons. In order to improve the overall efficiency and performance of classical salientpole, wound field, synchronous generators that employ the traditional damper cage, an improved amortisseur winding topology that reduces the inherent loss is proposed and investigated in this paper. This is essential in order to meet modern power quality requirements and to improve the overall performance of such 'classical' machines. The new topology addresses the requirements for lower loss components without compromising the acceptable values of the output voltage total harmonic distortion and achieves this by having a modulated damper bar pitch. As vessel for studying the proposed concept, a 4MVA, salient-pole, synchronous generator is considered. A finite element model of this machine is first built and then validated against experimental results. The validated model is then used to investigate the proposed concept with an optimal solution being achieved via the implementation of a genetic algorithm optimization tool. Finally, the performance of the optimised machine is compared to the original design both at steady state and transient operating conditions.

No abstract

This paper proposes a capacitor voltage regulation method for the dual converter with a floating bridge for aerospace applications. This topology has previously been reported, but with a constrained voltage utilisation factor due to the need for capacitor voltage regulations. In this paper, the effect of switching states on the voltage variation of capacitor is quantitatively modelled and an enhanced space vector modulation scheme with current feedback is proposed to achieve an active control of the floating capacitor voltages. This proposed method also allows further exploitation and utilisation of converter voltage. The relationship between the allowed modulation index of dual converter and load power factor is obtained and expressed using a fitted polynomial equation. The advantages of the proposed method include boosted voltage utilisation and superior performance in term of capacitor voltage balance. These advantages have been proven through simulation and experimental results on RL loads as well as with an open-end winding induction motor. The proposed modulation scheme can boost the converter voltage utilisation by at least 10% while achieving full four-level operation. More importantly, the higher available voltage allows extending the constant torque region of the motor, the further beginning of field weakening operation could be postponed.

In electrical machine design, thermal management plays a key role in improving performance and reducing size. End-windings are commonly identified as the machine hot-spot. Hence, lowering and predicting end-windings temperature are crucial tasks in thermal management of electrical machines. This paper proposes and investigates a non-invasive but effective cooling method that aims for a uniform cooling of a machine's winding by implementing direct cooling on its end-windings. Modelling and experimental results show that a 25% hot-spot temperature reduction on a particular application can be achieved. To analyse the proposed technique in detail, an accurate but computationally-economic lumped parameter thermal network is developed. Comparison between a 'standard' thermal network and its simplified equivalent (with less nodes) is presented where the models are developed and fine-tuned based on experimental data. All the above is used to investigate the potential of the proposed end-winding cooling method with different configurations of the methodology.