IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 15, NO. 2, JUNE 2000 211 Bidirectional Starting of a Symmetrical Two-Phase Switched Reluctance Machine Ragi Hamdy, John Fletcher, and Barry W. Williams Abstract—The mechanical and electrical properties of the two-phase 4/2 switched reluctance machine make it suitable for high-speed operation. Its adoption in such applications has been hindered by a perceived starting problem especially if starting in both directions is required. A starting technique is proposed which exploits the natural magnetic asymmetry in the symmet- rical machine geometry and provides for bidirectional torque production at start-up. The approach utilizes mutual coupling in the machine, an effect not previously identified for starting. The technique requires no modifications to either stator or rotor poles, and is suitable for fan applications which require only low starting torques. The starting technique is described and tested with nonoriented silicon steel and GOSS rotors, and starting torque from machine experimentally determined. Index Terms—switched reluctance machine, starting. I. INTRODUCTION S WITCHED reluctance machines (SRM’s) are considered to be mechanically robust. This is by virtue of their simple structure. The stator and rotor typically comprise of a lamination stack displaying appropriate magnetic saliency. The rotor has no windings and stator coils are mounted on individual poles with no overhang thus reducing as far as possible end-winding induc- tance. For high-speed operation, motors with low pole numbers are preferred. This reduces hysteresis and eddy current losses by minimizing the magnetic core pulsations per rotor revolu- tion. Single-phase SRM 2/2 constructions have been proposed but offer poor air-gap utilization as the phase coil only produces positive torque for half a rotor revolution. Further, some auxil- iary feature is required to park the rotor in a torque producing region of the machine to reliably start the rotor. This feature is commonly provided by permanent magnets. The 4/2 topology, the stator has four poles and the rotor two poles, utilizes the air-gap better than the 2/2 construction and is favored for high-speed operation. It also displays starting problems, which researchers have attempted to resolve using modified stator and rotor structures[1]–[3]. It is the purpose of this paper to describe a new techique which produces sufficient starting torque for fan type applications and provides bidirectional starting, but which uses symmetrical rotor and stator lamination geometries. A symmetrical stator structure is important to retain the simple coil construction of the 4/2 SRM and a symmetrical rotor eases mechanical balance issues. Fur- Manuscript received January 27, 1999. This work was supported by the En- gineering and Physical Sciences Research Council (EPSRC). The authors are with the Department of Computing and Electrical Engi- neering, Heriot-Watt University, Edinburgh, Scotland EH14 4AS, UK. Publisher Item Identifier S 0885-8969(00)04525-3. Fig. 1. Geometry of the 4/2 Switched Reluctance Machine. ther, a machine with a symmetrical stator and rotor geometry produces rated performance in both directions. The operation of the starting technique is described and evaluated for both oriented and nonoriented silicon steel rotors. Starting torque is measured on a test machine. These results show that maximum starting torque is obtained with specific phase windings excitation. The starting torque developed by the technique is sufficient to start fan loaded machines—an application area where the 4/2 SRM would compete with existing drives if a simple and low cost starting technique could be devised. II. THE TWO-PHASE SRM STARTING PROBLEM A typical 4/2 SRM construction is shown in Fig. 1. The ma- chine has four coils mounted on stator poles, – . Diamet- rically opposed coils (for example, and coils) are con- nected in series to form a phase winding with turns such that the current in each coil reinforces the magnetomotive force. The two phase windings, Phase 1 and Phase 2, are mechanically or- thogonal. A typical phase inductance profile for each phase winding is shown in Fig. 2(a) as a function of rotor angle, . The rotor angle is measured between the center line of the rotor and the center line of stator poles associated with the phase winding. Minimum phase inductance occurs when the rotor poles are in quadrature to the winding’s stator poles, —the un- aligned position—and maximum inductance occurs when the rotor and stator poles are aligned, . The rotor pole arc, , is typically greater than the stator pole arc, . This causes the plateau in the winding inductance at . 0885–8969/00$10.00 © 2000 IEEE