Speed control of switched reluctance motors

Nowadays, switched reluctance motors (SRMs) attract more and more attention. The switched reluctance motor is simple to construct. It not only features a salient pole stator with concentrated coils, which allows earlier winding and shorter endturns than other types of motors, but also features a salient pole rotor, which has no conductors or magnets and is thus the simplest of all electric machine rotors. Simplicity makes the SRM inexpensive and reliable, and together with its high speed capacity and high torque to inertia ratio, makes it a superior choice in different applications. However, the control of the SRM is not an easy task. The motor's double salient structure makes its magnetic characteristics highly nonlinear. The motor flux linkage appears to be a nonlinear function of stator currents as well as rotor position, as does the generated electric torque. Apart from the complexity of the model, the SRM should be operated in a continuous phase-to-phase switching mode for proper motor control. The torque ripple and noise as a result of this commutation are the other two awkward issues which have to be tackled. All these make the control of the SRM a tough challenging. This thesis attempts to investigate the control of the switched reluctance motor from the motor's structure properties, model equations, operation principle, power converter topology and commutation algorithms to starting problem. Apart from the general introduction, this thesis focuses on speed control of an 8/6 SRM based on a simplified model. This simplified model limits the operation of the motor completely into its linear flux region. According to this model, two different commutation strategies, two-phase-exciting and single-phase- exciting methods, are discussed in details. It has been observed that, not like the two-phase-exciting, the turn-on angle for single-phase-exciting is not trivial for it will affect the system performance. Consequently, an optimized single-phase-exciting method is proposed. This method optimizes the whole system's performance from the point of view of power efficiency of the whole system. By optimizing the average power of the whole system, the motor's transient as well as steady-state performance for speed tracking can be improved simultaneously. Experimental results supported the proposed method. These results compared the influences of the two-phase-exciting strategy, optimized and non-optimized single-phase-exciting method on speed tracking capacity and stator currents. It showed that the optimized single-phase has almost the same speed tracking capacity as two-phase-exciting strategy, and the peak value of their stator currents is at the same level under the same condition. However, the performance in speed tracking by the non-optimized single-phase-exciting method is much worse than the other two under the same condition. The reason for this phenomenon has been analysed