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.

BackgroundChondral lesions of the knee represent a challenge for the orthopaedic surgeon. Several treatments have been proposed with variable success rate. Recently, new therapeutic approaches, such as the use of mesenchymal stem cells, have shown promising results. The adipose tissue is a good source of these naturally occurring regenerative cells, due to its abundance and easy access. In addition, it can be used to provide cushioning and filling of structural defects. The 1-year safety and outcome of a single intra-articular injection of autologous and micro-fragmented adipose tissue in 30 patients affected by diffuse degenerative chondral lesions was evaluated.MethodsMicro-fragmented adipose tissue was obtained using a minimal manipulation technique in a closed system. The safety of the procedure was evaluated by recording type and incidence of any adverse event. The clinical outcomes were determined using the KOOS, IKDC-subjective, Tegner Lysholm Knee, and VAS pain scales taken pre-operatively and at 12 months follow-up. A level of at least 10 points of improvement in the scores has been selected as cut-off representing a clinically significant difference.ResultsNo relevant complications nor clinical worsening were recorded. A total median improvement of 20 points has been observed in IKDC-subjective and total KOOS, and a higher percentage of success was found in VAS pain and Tegner Lysholm Knee, where the total median improvement was 24 and 31 points, respectively.ConclusionThe results of this study show the safety and feasibility of using autologous and micro-fragmented adipose tissue in patients affected by diffuse degenerative chondral lesions. The technique is safe, minimally invasive, simple, one-step, with low percentage of complications, and compliant with the regulatory panorama.Electronic supplementary materialThe online version of this article (10.1186/s40634-017-0108-2) contains supplementary material, which is available to authorized users.

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.

Electrical machines for transportation applications need to be highly reliable, particularly if they drive safety-critical systems. At the same time, another main requirement is represented by the significant torque density, especially for aerospace, where weight constraints are extremely stringent. For achieving high peak torque, an effective strategy consists in supplying the windings with a current greater than the rated value; thus, thermally overloading the machine for limited time periods. However, if the insulation is overheated, the machine lifetime is shortened and reliability issues can arise. This paper experimentally investigates the influence of short-time thermal overload on the insulation lifetime for low voltage, random wound electrical machines. The analysis is performed on round enamelled magnet wire coils, which are aged by accelerated thermal cycles. The obtained results are statistically processed through a two parameter Weibull distribution. According to the findings of the experimental data post-processing, a lifetime prediction model is built. This model is employed for predicting the lifetime consumption of a motor embedded into an electromechanical actuator for aerospace application.

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