3934 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 Iron Loss in a Modular Rotor Switched Reluctance Machine for the “More-Electric” Aero-Engine David J. Powell, Geraint W. Jewell, Stuart D. Calverley, and David Howe Department of Electronic and Electrical Engineering, The University of Sheffield, Sheffield. S1 3JD, U.K. This paper investigates the iron loss that results in a novel topology of switched reluctance (SR) machine which has been developed for application as a starter/generator within the high pressure region of an aero-engine. It will be shown that the machine has a significantly lower loss than that of an equivalent SR machine equipped with a conventional rotor and, thus, a lower operating temperature, which is a critical factor. The iron losses in the cobalt-iron stator and rotor laminations are calculated on a per-element basis using flux density data derived from electric circuit-coupled finite-element analysis. Index Terms—Aircraft power systems, cobalt alloys, finite-element (FE) methods, reluctance machines. I. INTRODUCTION P OTENTIALLY, the only means of satisfying the electrical power requirements of “more-electric” aircraft is to inte- grate electrical machines within the engines, when, in addition to supplying loads such as flight control surface actuators, air-conditioning and de-icers, they would also facilitate engine starting and power transfer between the spools of the gas-tur- bine. Given the harsh environment, in terms of the ambient temperature ( C) and rotational speed ( krpm), a switched reluctance (SR) machine is likely to be favored for the high-pressure spool. Due to the space envelope constraints, a high pole number machine is required. However, the diameter of a conventional SR rotor is limited by the hoop stress in the laminations. Therefore, a modular rotor, which comprises lam- inated modules attached to a solid, high strength, nonmagnetic hub, Fig. 1, has been proposed [1]. This enables the diameter to be increased by % for the same mechanical stress. Unlike a conventional SR machine, the stator and rotor poles have the same pitch, the difference in pole number that is necessary for continuous torque production being achieved by appropriate spacing of the rotor modules. Further, since the rotor back-iron is not continuous, two phases on adjacent stator teeth must be excited at any instant, which results in short flux paths. The fundamental frequency is 4.05 kHz for a 24/18-pole SR machine at 13.5 krpm. Due to the high hoop stress and operating temperature, 0.15 mm, 49% cobalt-iron laminations are em- ployed. However, whilst these are heat treated for enhanced me- chanical properties for the rotor laminations and for enhanced magnetic properties for the stator laminations of a conventional SR machine, for the modular rotor SR machine, since the rotor is not subjected to such a high level of stress, both the rotor and stator laminations are heat-treated for enhanced magnetic prop- erties. It will be noted that in order to achieve the lowest iron loss, the coils of the conventional machine are arranged to pro- duce a NNNN-SSSS airgap field as they are excited sequentially, whereas excitation of the coils of the modular rotor machine re- sults in a NSNSNSNS airgap field so as to achieve continuous Digital Object Identifier 10.1109/TMAG.2005.854963 Fig. 1. Modular rotor switched reluctance machine. rotation. Iron loss is a major consideration for both machines, since subsequent exposure to high temperatures can compro- mise the mechanical strength, and cause irreversible changes [2] which, in turn, lead to a higher hysteresis loss [3]. II. SR MODELING APPROACHES The high starting torque requirement (200 Nm) and the wide generating speed range (7–13.5 krpm) result in a compromise in the winding design for both machines. The high-speed power requirement imposes an upper limit on the number of turns due to inductance effects limiting the rate of rise of current, whereas the high starting torque requirement necessitates a high number of turns in order to minimize the phase current. For the conventional machine, the number of turns was determined through a hybrid modeling approach, in which magnetostatic finite-element (FE) analyses are combined with computation- ally efficient, nonlinear circuit simulation, in a similar manner to that employed in [4]. An important consideration when determining dynamic current waveforms in SR machines is the extent to which mutual coupling between phases must be taken into account. Conventional SR machines are generally regarded as operating on a phase-by-phase basis, although there is often considerable overlap of the phase currents. Therefore, due to the low mutual coupling between the phases of a con- ventional SR machine, circuit based simulations, which neglect 0018-9464/$20.00 © 2005 IEEE