Ripple Free Torque Operation in Switched Reluctance Motors

Abstract

This thesis investigates the control of current and torque at different speed of the switched reluctance motor observing the motor constructional properties, mathematical model equations, operating principle and power converter topologies. The switched reluctance motor has gained popularity in industry with the fact that it is simple to construct and suitable for harsh environment. The electromechanical energy conversion is produced from rather simple machine design than as it is done in conventional motors. The other advantages are its rugged construction, low maintenance cost, and ability to operate over a wide speed range at constant power. The merits of switched reluctance motors are marred by the requirement of expensive control and its innate torque ripple issue, which is also a cause of vibration and acoustic noise. Torque ripple is produced from successive excitation of phase currents. The current through phase windings produces magnetic force with tangential and radial components where former component contributes to the average torque production and the latter is the major cause of vibration and acoustic noise in switched reluctance motors. Torque ripple reduction leads to smooth operation, thus it also improves the vibration and acoustic noise. This reduction can be achieved either through better machine design or with superior control techniques. Better machine design involves basic design parameters like number of stator poles, rotor poles and also their shape which is generally determined using detailed magnetic analysis. In this piece of work literature review of both approaches is included but the latter current control approach is adopted for detailed research. This thesis also discusses the split stator winding technique to improve reduction in torque ripple. This is unique strategy to introduce a compensation torque and remarkable reduction in torque ripple. This technique requires the slight change in shape of stator. This technique has ability to eliminate the negative torque generation and also the starting problem in switched reluctance motors. The control techniques deal with the electronic part of complete drive systems. The torque and current are directly related to each other, so the torque control can be implied by current control methods. This dissertation discusses the new current control technique with efficient bipolar power converter for torque ripple mitigation control. This piece of work also elaborates the new unique strategy of split stator winding for torque ripple reduction. In this thesis motor mathematical model is simulated using new control strategy along with two power converter topologies, conventional asymmetric half bridge converter and proposed symmetric full bridge converter. In addition split stator winding technique is simulated with suggested bipolar drive to show the improvement in ripple reduction. The experimental setup is also included in this thesis, where specific motor is operated using complete drive system. The design and implementation of this drive system is discussed in detail. This drive system is capable to perform both conventional and bipolar operations. At the end, practical results and simulated results are acquired and compared. The effect of excitation and de-excitation angles on torque production is observed and simulated. Torque and current relationship is observed to conclude which drive is suitable for smooth torque operation and have better current control at different speeds. The apparent cost difference is also analyzed and it is found that full bridge drive is more suitable for general purpose solution in industry with bearable increase in cost.