Electrical power generation in aircraft: review, challenges and opportunities

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

Influence of Insulation Thermal Aging on the Temperature Assessment in Electrical Machines

Thermal modeling and temperature assessment of electrical machines often rely on the use of lumped-parameter thermal networks. A historic limitation of analytical thermal models is their need for an experimental fine-tuning, necessary for selecting the appropriate values of thermal conductivity and convection heat transfer coefficients. This evaluation procedure is commonly carried out at the design stage of a new machine, by assuming that its thermal behavior will remain unchanged throughout its whole lifetime. This paper demonstrates, through an in-depth experimental investigation, how the capability of heat extraction from a machine's hot spot towards the coolant can be strongly affected by the level of thermal aging of its insulation system. Based on the experimental findings, a decrement of the winding equivalent thermal conductivity is noted as the thermal aging accumulates, with a corresponding progressive increment in hot-spot temperature

Lifetime consumption and degradation analysis of the winding insulation of electrical machines

In this paper, a novel multi-stress model which estimates the lifetime of the winding insulation relative to its duty cycle is proposed and investigated. With an adequate implementation of this model, then an electrical machine can be designed not only in terms of its performance requirements, but also considering the associated reliability and lifetime aspects. Since thermal and thermo-mechanical stresses are considered as the main ageing factors, the model is particularly suited for low voltage, low duty cycle machines. The determination of the model parameters is based on the results of accelerated thermo-mechanical ageing tests, whose procedure is thoroughly reported in the paper. The results of the accelerated ageing tests show that the effect of thermo-mechanical ageing is significant even for small size, random wound windings under fast temperature rise

Self-commissioning of interior permanent- magnet synchronous motor drives with high-frequency current injection

In this paper, a simple and robust method for parameter estimation at rotor standstill is presented for interior permanent magnet (IPM) synchronous machines. The estimated parameters are the stator resistance through dc test, the dq inductances using high-frequency injection, and the permanent magnet flux by means of a closed-loop speed control maintaining rotor stationary. The proposed method does not require either locking the rotor or additional/special power supplies. The validity of the suggested method has been verified by implementation on two IPM motor prototypes. Finally, the estimated parameters have been compared against results obtained through finite-element simulations and with machine magnetic characterization, separately performed, to validate the method's effectiveness. Saturation and cross-saturation effects are taken care of through amplitude modulation and cross-axis current application, respectively

Moving Toward a Reliability-Oriented Design Approach of Low-Voltage Electrical Machines by Including Insulation Thermal Aging Considerations

© 2020 IEEE. Electrical machines (EMs) are required to consistently perform their intended mission over a specified timeframe. The move toward transportation electrification made the EMs' reliability an even stringent and predominant requirement, since a failure might cause severe economic losses, as well as endanger human lives. Traditionally, the design procedure of motors conceived for safety-critical applications mainly relies on over-engineering approaches. However, a paradigm shift is recently taking place and physics of failure approaches/methodologies are employed to meet the reliability figures, while delivering an optimal design. This article proposes and outlines a reliability-oriented design for low-voltage EMs. Thermal accelerated aging tests are preliminarily carried out on custom-built specimens. Once the aging trend of the turn-to-turn insulation system is assessed, the thermal endurance graph at several percentile values is determined and lifetime models are developed, for both constant and variable temperature operations. Finally, these models are used to predict the turn-to-turn insulation lifetime of motors meant for aerospace and automotive applications

Analytical thermal model for fast stator winding temperature prediction

This paper introduces an innovative thermal modelling technique which accurately predicts the winding temperature of electrical machines, both at transient and steady state conditions, for applications where the stator Joule losses are the dominant heat source. The model is an advanced variation of the classical Lumped Parameter Thermal Network approach, with the expected degree of accuracy but at a much lower computational cost. A 7-node Thermal Network is first implemented and an empirical procedure to fine-tuning the critical parameters is proposed. The derivation of the low computational cost model from the Thermal Network is thoroughly explained. A simplification of the 7-node Thermal Network with an equivalent 3-node Thermal Network is then implemented, and the same procedure is applied to the new network for deriving an even faster low computational cost model. The proposed model is then validated against experimental results carried on a Permanent Magnet Synchronous Machine which is part of an electro-mechanical actuator designed for an aerospace application. A comparison between the performance of the classical Lumped Parameter Thermal Network and the proposed model is carried out, both in terms of accuracy of the stator temperature prediction and of the computational time required

Integrated design of motor drives using random heuristic optimization for aerospace applications

High power density for aerospace motor drives is a key factor in the successful realization of the More Electric Aircraft (MEA) concept. An integrated system design approach offers optimization opportunities, which could lead to further improvements in power density. However this requires multi-disciplinary modelling and the handling of a complex optimization problem that is discrete and non¬linear in nature. This paper proposes a multi-level approach towards applying random heuristic optimization to the integrated motor design problem. Integrated optimizations are performed independently and sequentially at different levels assigned according to the 4-level modelling paradigm for electric systems. This paper also details a motor drive sizing procedure, which poses as the optimization problem to solve here. Finally, results comparing the proposed multi-level approach with a more traditional single-level approach is presented for a 2.5 kW actuator motor drive design. The multi-level approach is found to be more computationally efficient than its counterpart

Pedobarographic and kinematic analysis in the functional evaluation of two post-operative forefoot offloading shoes

Background: Forefoot offloading shoes are special orthopaedic footwear designed to protect and unload the injured part of the foot after surgery and for conservative treatments. The offloading action is often achieved by transferring plantar load to the rearfoot via rocker shoes with reduced contact area between shoe and ground. While these shoes are intended to be worn only for short periods, a compromise must be found between functionality and the risk of alterations in gait patterns at the lower limb joints. In this study, the pedobarographic, kinematic and kinetic effects of a traditional half-shoe and a double-rocker full-outsole shoe were compared to those of a comfortable shoe (control). Methods: Ten healthy female participants (28.2±10.0 years) were asked to walk in three different footwear conditions for the left/right foot: control/half-shoe, control/full-outsole, and control/control. Full gait analysis was obtained in three walking trials for each participant in each condition. Simultaneously a sensor insole system recorded plantar pressure in different foot regions. Normalized root-mean-square error, coefficient of determination, and frame-by-frame statistical analysis were used to assess differences in time-histories of kinematic and kinetic parameters between shoes. Results: The half -shoe group showed the slowest walking speed and the shortest stride length. Forefoot plantar load was significantly reduced in the half-shoe (maximum force as % of Body Weight: half-shoe=62.1; full-outsole=86.9; control=93.5; p<0.001). At the rearfoot, mean pressure was the highest in the full-outsole shoe. At the ankle, sagittal-plane kinematics in the full-outsole shoe had a pattern more similar to control. Conclusions: The half-shoe appears significantly more effective in reducing plantar load at the forefoot than a double-rocker full-outsole shoe, which is designed to reduce forefoot loading by using an insole with a thicker profile anteriorly as to maintain the foot in slight dorsiflexion. However, the half-shoe is also associated with altered gait spatio-temporal parameters, more kinematic modifications at the proximal lower limb joints and reduced propulsion in late stance

Functional Evaluation of a Shock Absorbing Insole During Military Training in a Group of Soldiers: A Pilot Study

Abstract Objective Soldiers' lower limbs and feet are frequently affected by overload- and overuse-related injuries. In order to prevent or limit the incidence of these injuries, the use of foot orthoses is often recommended. The aim of this study is to assess the effects of shock-absorbing insoles on in-shoe plantar pressure magnitude and distribution in a group of professional infantry soldiers wearing military boots during standard indoor military training. Methods Twenty male professional soldiers of the Italian Army (age 35.1 ± 6.1 years; BMI 25.2 ± 2.3 kg/m2) were recruited for this study. Each subject underwent clinical examination to assess possible overuse-related diseases of the lower limb and trunk. Subjects with altered foot morphology according to the Foot Posture Index (FPI) were excluded from this study. Twelve subjects were considered eligible and therefore underwent an indoor training routine comprised of marching, running, jumping inside parallel bars and jumping from different heights. Soldiers repeated the training session twice wearing standard military boots along with two types of insoles: the standard prefabricated insole within the boots (STI), and a special shock-absorbing insole (SAI) featuring an elastic medial arch support. A 99-capacitive sensor insole system was used to record plantar pressure distribution in both feet. Analysis of in-shoe pressure parameters at rearfoot, midfoot and forefoot and in the total foot was performed via a custom-software application developed in MATLAB. Perceived foot comfort (VAS 0–15) was also assessed. Results Pressure parameters recorded during walking and running were considered suitable for statistical analysis. In the whole foot region, pressure parameters were 18–22% lower in military boots fitted with the SAI during walking and 14–18% lower during running. SAI resulted in better comfort (+25%) with respect to the prefabricated boot orthotics (median comfort: SAI = 15/15; STI = 12/15; p = 0.0039) both during walking and running. Conclusions Shock-absorbing insoles can be an effective solution when fitted inside military boots. The present functional evaluation shows that wearing a prefabricated shock-absorbing insole can provide a significant amelioration of perceived foot comfort and plantar pressure parameters. Further studies are now needed with a larger population and more demanding exercises

Influence of rotor endcaps on the electromagnetic performance of high speed PM machine

Surface-mounted permanent magnet (SPM) machines are preferred for high-speed aerospace applications over induction and switched reluctance machines, since they combine the advantages of high torque density and efficiency. Also, in aerospace applications, where low rotor weight and inertia are essential requirements, a permeable hollow shaft is proposed to replace the need for rotor back-iron and reduce the overall rotor weight. For rotor mechanical integrity, a retaining sleeve is commonly used, leading to thicker magnetic airgap. Furthermore, when permeable rotor endcaps are applied, an increase of the magnetic end leakage occurs, i.e. end-effect. In this paper, the influence of the rotor endcaps on the mechanical and electromagnetic performance of a high-speed SPM machine is investigated through 3D-finite element analyses (FEA). Also, different endcap thickness and different rotor shaft materials are investigated and compared in this work. Finally, a prototype of the SPM machine under study has been manufactured and tested. The comparison between simulation and experimental results is presented and discussed