Power loss Analysis of a Permanent-Magnet Machine Based - Starter/Generator Fed by an Active Front-End Rectifier

03/02/2015
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Power loss Analysis of a Permanent-Magnet Machine Based - Starter/Generator Fed by an Active Front-End Rectifier

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application/pdf Power loss Analysis of a Permanent-Magnet Machine Based - Starter/Generator Fed by an Active Front-End Rectifier Tao Yang, Christopher Hill, Puvan Arumugam, Serhiy Bozhko, Christopher Gerada
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Power loss Analysis of a Permanent-Magnet Machine Based - Starter/Generator Fed by an Active Front-End Rectifier

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Power loss Analysis of a Permanent-Magnet Machine Based Starter/Generator Fed by an Active Front-End Rectifier Tao Yang, Christopher Hill, Puvan Arumugan, Serhiy Bozhko, Christopher Gerada 1 : The University of Nottingham, Tao.Yang@nottingham.ac.uk 2 : The University of Nottingham, Christopher.Hill@nottingham.ac.uk 3: The University of Nottingham, Puvan.Arumugan@nottingham.ac.uk 4 : The University of Nottingham, Serhiy.Bozhko@nottingham.ac.uk 5: The University of Nottingham, Chris.Gerada@nottingham.ac.uk Electrical & Electronic Engineering, Nottingham, NG7 2RD, UK, Abstract The More-Electric Aircraft (MEA) has become a major trend in the aircraft industry. One of the main characteristics of the MEA is the use of electrical systems to start the engine. An alternator is utilised as a starter to crank the engine instead of using bleed air from the Auxiliary Power Unit (APU). The alternator is also used as a generator which is driven by the engine during flight mode. This paper studies the rotor losses in the alternator during motoring and generating modes using a Finite Element (FE) Model in MagNet. The effect of switching behaviour from power converters is studied through co-simulation between MagNet and Matlab. Thermal analysis will also be used as guidance when designing the cooling system for the alternator. Introduction An underlying principle of the More-Electric Aircraft (MEA) is the utilization of electrical power to power the non-propulsive aircraft systems. It has been identified as the major trend for the future of aircraft [1]. Many functions that used to be driven by hydraulic, pneumatic or mechanical power will be replaced with electrical subsystems in order to achieve higher reliability, capability, flexibility and lower maintenance cost. The main enabler of the MEA is from the advancement of power electronics, machine drives and control theory. In conventional aircraft, it is common to start an engine is by use of an external air source, air from an APU or by using air from another running engine [2]. In the MEA, an electrical starter will utilised since there is no bleed air available from the engine for this purpose. The alternator used for this purpose requires high torque to rotate the engine until the fuel has ignited and a self-sustaining state is reached. The modern, state-of-art generator is effectively three generators on the same shaft. The rotating windings and diode packs in this compound generator actually limit the speed to a point where it is not practicable to package the rotating elements within the necessary speed and weight constraints. In addition, the MEA will replace the established constant 400Hz supply with a variable-frequency system in order to eliminate the mechanically complex and unreliable Constant Speed Drive (CSD) from the electrical power generating system. The use of an electrical starter/generator thus becomes a realistic option for the MEA Electrical Power System (EPS). Different types of machine, including Induction machine, Switched Reluctance (SR) machine and Permanent Magnet (PM) machine are studied together, with their respective drive topologies, in our previous publication [3]. A surface-mounted PM machine is selected for detailed study, with consideration of mechanical and thermal constraints at high speed, power density, reliability, overall size and weight. A three-level Neutral-Point Clamped (NPC) converter is chosen due to its low power losses and high power quality. In [3], the authors studied the power losses of different types of machine. However, all the presented results were calculated using the assumption that the machine phase currents are always sinusoidal. Thus the power loss from higher harmonics due to the switching behaviour of the power converters is not considered. This paper aims to fill in this gap and focuses on the analysis of power losses of PM machines with consideration of higher harmonics in phase currents. A co-simulation between the FE machine model and the behavioural converter model has been set up for this study purpose. Both motoring and generating modes are studied and the results from this paper give more accurate power losses data for cooling system design of the PM machine. Design of Permanent Magnet Machine The speed-torque characteristic requirement of the starter/generator is shown in Fig. 1. The machine runs as a motor during the engine starting period. A high constant torque is required to crank the engine up to the speed 10krpm (ωstart), at which point the engine ignites. In generation mode, since there is no CSD, the machine speed is directly dependant on the engine shaft. The speed range of the machine is between 19.2krpm (ωmin) up to 32krpm (ωmax). ωbase ωmin ωmax ω (rad/s) T(Nm) Tstart 0 ωstart Fig. 1: Torque-Speed characteristics of the aircraft starter-generator The machine will be required to deliver high torque during starting mode, with Tstart= 50Nm, while also being able to work at high speeds of up to 32krpm during generating mode. A Surface-Mounted Permanent Magnet (SMPM) machine was chosen considering mechanical and thermal constrains at high speed, power density, reliability and overall drive size. A Finite Element model was built in the software MagNet. Different slot-pole combinations were investigated in view of the overall drive system losses and performance. This required a compromise in the design process in order to meet the operational requirements as both an engine starter and generator. Three-level converter In order to improve power quality and reduce power losses, the converter for this starter/generator system uses a Neutral-Point Clamped (NPC), three-level converter, as shown in Fig. 2. The three-level neutral- point-clamped (NPC) converter provides significant advantages over the conventional two-level converters in terms of power quality and thermal management [4]. In addition, the voltage across the switches is only half the dc bus voltage. This feature effectively doubles the power rating of the converter for a given power semiconductor device. When used as an Inverter, the first group of voltage harmonics on the AC side is centred around twice the switching frequency. This feature enables further reduction in size and cost of passive components while at the same time improving the quality of the output waveform. a iNP ia ic ib C C + - E + - E b c S2 S1 S4 S3 NP va vb vc vpn Fig. 2 Circuit Schematic of a three-level converter One of the key issues for the three-level NPC converter is neutral-point voltage balancing. This has been comprehensively studied by N. Celanovic et al in [4] and will not be detailed here. Co-simulation Studies The co-simulation of MagNet and Simulink is performed through the MagNet-Simulink plug-in package. The MagNet FE model can be interfaced with Simulink using a plug-in component as shown in Fig. 3 Fig. 3 Magnet and Simulink interface The input and output pairs are dependent on the MagNet FE model of machine, that is, the input can be phase voltages or currents; the output is complementary. The Simulink plug-in interface has been designed to operate properly when the Simulink step size is fixed. The MagNet software step size should be an integer multiple of the Simulink step size. ia ib ic PI PI _ _ abc dq abc dq * sd v * sq v isd * sd i * sq i a m   isq b m c m PMSM r ω dc v Fig. 4 MagNet and Matlab/Simulink The co-simulation diagram is shown in Fig. 4, where the PMSM is a FE model and the three-level converter is a behavioural switching model. Current controllers are applied to the system in order to control the id and iq currents to the demanded operation points, as shown in Table 1. Table 1 Tested operating point of the PMSM Speed [rpm] Torque [Nm] Id [A] Iq [A] 8,000 54.0 0 334.8 19,200 22.8 -171.3 133.9 32,000 13.5 -245.2 83.7 Simulation results The power losses at three operational points are given in Table 2. The power losses are calculated with the assumption that the PM machine uses Inconel sleeves. It can be seen that the total rotor loss increases simultaneously with increased machine speed. This is mainly due to the fact that the eddy currents in the sleeves increase and thus the corresponding power losses increase. Table 2 Power loss of the PMSM at different operation points Speed[RPM] 8,000 19,200 32,000 iq [A] 334.8 133.9 83.7 id [A] 0 -171.3 -245.2 Inconel sleeves Total Rotor Losses with harmonics (W) 78.6 113.9 334.1 Total Rotor losses without Harmonics (W) 30.5 76.32 271.1 Carbon Fibre sleeves Total Rotor Losses with harmonics (W) -- -- 76 The difference between total rotor power losses, with and without consideration of harmonics, is between 22% and 60%, as illustrated in Table 2. Considering the 200% build factor when designing a machine, the cooling system for this machine with Inconel sleeves should be designed to dissipate 700W power. In order to reduce the rotor loss, non-conductive sleeves can be used. In the worst case, when the machine is running at 32,000rpm, the total rotor loss can be reduced to 76W by using carbon fibre sleeves. Conclusions This paper studied the effect of switching harmonics of power converters on the power losses in the alternator. The analysis is based on co-simulation between a FE model of a machine and a behavioural switching model of the converter. The simulation results reveal that with conductive sleeves, the harmonics can contribute up to 60% of the total power loss in the rotor. Therefore it has been shown that non-conductive sleeves would provide much better performance by significantly reducing the power loss in the rotor. References [1] K. Emadi and M. Ehsani, "Aircraft power systems: technology, state of the art, and future trends," Aerospace and Electronic Systems Magazine, IEEE, vol. 15, pp. 28-32, 2000. [2] I. Moir and A. Seabridge, Aircraft Systems: mechanical, electrical, and avionics subsystems integration 3rd ed.: John Wiley & Sons, 2008. [3] M. Degano, P. Arumugam, W. Fernando, Y. Tao, Z. He, J. B. Bartolo, et al., "An optimized bi-directional, wide speed range electric starter-generator for aerospace application," in Power Electronics, Machines and Drives (PEMD 2014), 7th IET International Conference on, 2014, pp. 1-6. [4] N. Celanovic and D. Boroyevich, "A comprehensive study of neutral-point voltage balancing problem in three-level neutral- point-clamped voltage source PWM inverters," Power Electronics, IEEE Transactions on, vol. 15, pp. 242-249, 2000.