Design of a Fault-Tolerant 6-phase Switched Reluctance Motor for Electric Power-Assisted Steering Systems C. Martis*, C. Oprea*, I.A. Viorel* and J. Gyselinck** *Technical University of Cluj Napoca 15, Daicoviciu, Cluj-Napoca, RO-400020, Romania **Université Libre de Bruxelles 50, Avenue Franklin Roosevelt, Brussels, B-1050, Belgium Abstract – In this paper a six-phase switched reluctance motor for electric power-assisted steering systems is proposed. Based on the specification data the main motor dimensions are obtained via a dedicated sizing-design procedure. The motor parameters and characteristics are computed by using three methods, namely using a finite element model, a magnetic equivalent circuit model and using analytical formulas. The flux linkage and electromagnetic torque as a function of rotor position and phase current are calculated, showing that the steady state motor characteristics are adequate for the purposed drive. I. INTRODUCTION Electric actuation is one of the actual trends in the automotive industry, due to its high reliability, energy efficiency and controllability [1]. The use of electrical and electronic systems, such as electronic automatic climate control, entertainment systems, antilock brakes (ABS), traction control systems, enhances customer comfort, convenience and safety. New electric systems eliminate the traditional pumps, hoses, hydraulic fluid, drive belt and pulley on the engine, significantly increasing fuel economy. Electric power-assisted steering (EPAS) systems have already begun replacing the hydraulic power steering ones, simplifying vehicle assembly and offering more room in the engine compartment. An EPAS system incorporates a steering gear, assist mechanism, an electric motor and its electronic controller to provide responsive steering assist. Normally, sensors measure two primary inputs: applied torque on the steering wheel and the position of the latter. EPAS solutions can be separated into categories according to the location of the electric motor that provides steering assistance, as: column- type, pinion-type, rack-type, and double pinion-type [1, 2]. Some aspects have to be considered during the design of the EPAS drive systems: reliability, performance, thermal and acoustic behavior, energy efficiency and cost. These applications require high performance motors with high torque/volume ratio, low inertia, high dynamics, low torque pulsations and low radial forces. Different types of electrical motors have been proposed and used for EPAS applications so far. The first "all electric" power-steering system used a brush DC motor placed concentrically around the rack [1]. But the DC motor has some unattractive attributes, such as brush arcing and commutator/brush friction, resulting in wear, lower overall power density and EMI problems. As a result, the robust induction motor was chosen to replace brush DC motor in EPAS applications. The difficulties associated with extracting heat from the rotor, efficiency problems over a wide speed range if high slip is required, long end-turns, and a more expensive manufacturing process due to the distribution of windings are some of the unattractive features of the induction machine that made researchers look for other solutions. The permanent magnet synchronous machine (PMSM) represents a competitive solution for the considered application. High torque density, low cogging torque, small torque ripple and high energy efficiency are the key assets of PMSMs for this kind of application [2]. However, high power density permanent magnets are extremely sensitive to the temperature increase, the stability temperature limit being well below 120°C. This is to be taken into consideration since in automotive applications the ambient temperature limits are from -40°C up to 125°C and more. The PMSMs also raise concerns about failure modes and, without an adequate design and control, can produce quite important cogging torques. Switched reluctance machines (SRMs) are easy to construct and contain no permanent magnets, resulting in excellent high- temperature performance and high reliability. Due to the phase independence, SRMs are able to operate under partial phase failure conditions and therefore are a competitive candidate for fault tolerant EPAS applications, where safety is the most important consideration. The analysis of the specifications for automotive EPAS reveals also some other interesting motor challenges: given overall dimension, low supply voltage, wide range of ambient temperature, and low motor torque ripple. The fault tolerance of an EPAS system can be focused either on the power electronic converter, or on the electrical machine. Concerning the latter, the effort can be oriented towards the windings and the number of phases, without increasing the complexity of the drive. The minimization of the torque ripple 993 978-1-4244-4252-2/09/$25.00 ©2009 IEEE