Integrative methodologies to improve operation of Starter/Generator system in More Electric Aircraft

03/02/2015
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Integrative methodologies to improve operation of Starter/Generator system in More Electric Aircraft

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application/pdf Integrative methodologies to improve operation of Starter/Generator system in More Electric Aircraft Constanza Ahumada, Nicolas Schneider, Patrick Wheeler, Mohand Hamiti, Seamus Garvey
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Integrative methodologies to improve operation of Starter/Generator system in More Electric Aircraft

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Integrative methodologies to improve operation of Starter/Generator system in More Electric Aircraft Constanza Ahumada (1), Nicolas Schneider (1), Patrick Wheeler (2), Mohand Hamiti (2), Seamus Garvey (2), Tao Yang (2) University Of Nottingham, Institute for Aerospace Technology, Triumph Road, NG1 2TU Nottingham 1 : INNOVATE Project, constanza.ahumada@nottingham.ac.uk, nicolas.schneider@nottingham.ac.uk 2: University of Nottingham, patrick.wheeler@nottingham.ac.uk, mohand.hamiti@nottingham.ac.uk, seamus.garvey@nottingham.ac.uk, tao.yang@nottingham.ac.uk Abstract With the introduction of the More Electric Aircraft (MEA), and consequently the increase in the power system and Starter/Generator (S/G) size of aircrafts, new challenges have also appeared. This is the case of the performance of the S/G system in terms of vibrations. As a solution to these challenges, this paper introduces an integrative methodology that combines the use of machine design, and power system operation. These technologies can be used in controlling the damping of the system, and therefore improve vibration effects. In this paper, it is shown how the integration of these two methods allows for a reduction in the size and weight of the S/G system of the aircraft. Introduction The More Electric Aircraft (MEA) concept appears as a solution to reduce the weight and fuel consumption of aircrafts. To achieve this, the MEA replaces the pneumatic, hydraulic and mechanical systems by electrical ones [1-5]. The use of electrical systems in the MEA entails a more flexible design and operational capability, but also a redesign of the generation system, due to increased power demand. Higher loads need larger generators, resulting in larger starters. In addition, the connection of higher loads at a given time produces higher torque impacts over the shaft connecting the starter and generator, which makes fundamental the study of the performance in terms of vibrations over the shaft and the redefining of the Starter/Generator (S/G) system [6, 7]. A solution to the this, is the concept of the in line electrical starter generator [6], developed in the late 80s, where it was proposed that the most appropriate machine technology for the different requirements of the starter generator would be a switched reluctance machine (SRM). This is mainly due to the robust nature of the SRM as its rotor can be a simple, low-cost, solid structure. The absence of permanent magnets and windings in the rotor allow the machine to be used at high temperatures and can potentially be driven to considerably high speeds. An important aspect of the SRM is that its a highly fault tolerant machine[8]. The main objective of this paper is thus:  To study the performance of the starter/generator system in terms of improving vibrations. The methodology adopted is shown in Fig. 1. In it, the following methods are introduced in order to reach the objectives of this research:  Machine Technologies: the design used in the machine can decrease the vibrations over the system. The use of a bearingless SRM topology is proposed as a solution.  Power System Operation: The electromechanical interaction between the power system and starter/generator system affects the vibrations over the shaft. This paper introduces the idea of minimizing the vibrations over the shaft through control of the electric power system. M Engine-Generator System Machine Technologies Power System Operation Fig. 1: Methodology Diagram. The rest of the paper is organized as follow: First, the electrical machine design solution is studied. Then the effect of the power system over the shaft through electromechanical interaction is described. Finally, the conclusions are presented. Machines Technologies As mentioned above, the concept of the in line electrical Starter generator [8] has been around since the 1980s. The requirements of the SRM are listed in [9-14]. Reference [9] presents the different advantages of using an in-line electrical starter generator which are as follows:  Elimination of mechanical components, such as the Auxiliary Gearbox, which has substantial weight benefits impacting on the performance of the engine.  An oil-free engine  Providing the generating and starting function which would replace the current air starting system But this kind of machine has to comply with different requirements and limitations:  The machine has to fit in the confined environment  The machine has to withstand high temperatures reaching up to 350 degrees  Fault tolerance  high rotating speed [8, 13] Another potential change in the more electrical aircraft would be to have this electrical starter generator bearingless. Fig. 2: Basic SRBM topology. Switched reluctance bearingless machines (SRBM) offer an easy way to control the position of the rotor with topologies shown on top. In this topology [15] there are two windings, one creating the torque and another one for the suspension force. The one shown here is the very basic structure of a SRBM but it can be improved by separating the two flux paths; torque and suspension force, from each other [16]. The main advantage would be not to replace the bearings inside the engine but use the starter/generator (S/G) as a controllable damping system. It would allow to limit vibrations as well than passing through critical speeds of the shaft more safely by changing the stiffness and damping matrix of the shaft. In the final paper, some aspects of the design of the SRM will be shown and presented. Power System Operation As shown in Fig. 3, the mechanical energy necessary to produce rotation in the generator of an aircraft is obtained from the engine (or starter), and transferred through the shaft to it. Successively, the generator transforms the mechanical energy in electrical and feeds the aircraft power system. Therefore, as the electrical and mechanical system are connected, disturbances in one of them can affect the other [7, 17]. M Control Engine Shaft Generator Power System Fig. 3: Electromechanical interaction on airplane. Through equation (1), it is observed how the electrical current affects the rotation of the shaft. The rotational angle of the shaft,