Improvement in the field-weakening performance of switched reluctance machine with continuous mode M. Rekik, M. Besbes, C. Marchand, B. Multon, S. Loudot and D. Lhotellier Abstract:A new operation mode for switched reluctance motors (SRMs), called ‘continuous mode’,is described. By using this mode, the torque and then power in field-weakening mode can be considerably increased without any hardware modifications. Consequently, powerand torque densities of SRMs become comparable to other technologies (synchronous and induction motors)and with a field weakening operation over a large speed range. This new degree of freedom makes it possible to improve the motor design, by modifying the rotor pole arc size or the windings turnsper pole.Only simulation results are presented here, for a 12/8 SRM. Results confirm that the maximum power is improved (constant poweron a very large speed range) and with a higher efficiency than that in the classical discontinuous mode. 1 Introduction Automotive applications require a very high torque at low speed and a high constant power up to very high rotation speeds, especially for traction motors (electric or hybrid electric vehicles). These requirements often lead to an over- sized inverter rating, thus size and cost. Permanent magnet machines remain the most widely used technology with the ability to operate at constant power over a wide speed range with a minimum impact on power switch and diode rating [1]. Nevertheless, in contrasto these excellent features, permanent magnet machines are disadvantaged by higher currents for field-weakening operation, the magnet costs and high fabrication costs [2]. More robustand cost- efficientmachines, such asswitched reluctance motors (SRMs), have a good potential for automotive applications. The SRM has been widely used not only for low power applications such as fans, pumps and domestic appliances, but also for high-performance actuators such as centrifuges, automotive and aerospace [3 – 6]. The SRM is known to be highly cost-effective and reliable due to its simple structure. Its rotor is made of steel laminations without magnets or windings, providential for harsh environment applications (high temperatures). The stator is also made of steel lami- nations with short-pitched and concentrated windings. As a result, the manufacturing cost of the SRM is relatively low and it can rotate at very high speeds [7]. Furthermore, if well designed and controlled, this motor works well in flux-weakening operation [8], which is par- ticularly interesting for automotive applications. The inver- ter can be sized with minimum current rating thanks to the full voltage waveform used at high speed. The SRM inver used for this study is an asymmetrical half-bridge inverte 2 SRM characteristics, hypothesis, control and sizing 2.1 Machine characteristics and computation of performances The main characteristics of the 12/8 SRM and inverter use in this application are given in Table 1. The magnetic coupling between phases is not taken int account because of the negligible influence and the heav computation. The equations presented are thus based on a single-phase analysis (computation). The phase voltage equation for a switched reluctance machine is V ¼ Ri þ df dt (1) where R is the phase resistance, f the flux linkage per phase and i the phase current. f depends on phase current and rotor position, so V ¼ Ri þ @ f @i di dt þ @ f @ u du dt (2) The phase co-energy is calculated from the flux linkage p phase W 0 m ¼ ð i 0 f di (3) The single-phase torque T 1 (i 1 , u ), which gives the instan- taneous torque value for any given instantaneous current and rotor position, is calculated by deriving the co-energy T 1 ¼ @W 0 m (i 1 , u m ) @ u m ¼ N r @W 0 m (i 1 , u m ) @ u (4) where u m is the mechanical rotor position, uthe electrical angle and N r the number of rotor poles. # The Institution of Engineering and Technology 2007 doi:10.1049/iet-epa:20070069 Paper first received 8th February and in revised form 16th May 2007 M. Rekik, M. Besbes and C. Marchand are with the LGEP/SPEE Labs, CNRS UMR8507,Supelec, Univ.,Pierre etMarie Curie-P6, Univ. Paris Sud-P11, Plateau de Moulon, Gif sur Yvette, F 91192 Cedex, France B. Multon is with SATIE, ENS Cachan, Bretagne, UEB, Av. Robert Schuman, Campus de Ker Lann, Bruz F 35170, France M. Rekik,S. Loudotand D.Lhotellierare with RENAULT Technocentre, Direction Electronique Avance´e, 1, Avenue du Golf, Guyancourt F 78288, France E-mail: rekik@lgep.supelec.fr IET Electr. Power Appl., 2007, 1, (5), pp. 785 – 792 785