Optimal turn-off angle control in the face of automatic turn-on angle control for switched-reluctance motors

Abstract

A new approach to the automatic control of excitation parameters for the switched-reluctance motor (SRM) is presented. The excitation parameters include the turn-on angle, the turn-off angle and the magnitude of the phase current. The objective is to develop an easily implementable control algorithm that automatically maintains the most efficient excitation angles in producing the required current to produce the electromagnetic torque. The control algorithm determining the turn-on and turn-off angles supports the most efficient operation of the motor drive system. The turn-on angle and turn-off angle controllers work independently and harmoniously with the speed controller. The turn-on angle controller consists of two pieces: the first piece of the control technique monitors the position of the first peak of the phase current (thetasp) and seeks to align this position with the angle where the inductance begins to increase (thetasm). The second piece of the controller monitors the peak phase current and advances the turn-on angle if the commanded reference current cannot be produced by the controller. The first piece of the controller tends to be active below base speed of the SRM, where phase currents can be built easily by the inverter and thetasp is relatively independent of thetasm. The second piece of the controller is active above base speed, where the peak of the phase currents tends to naturally occur at thetasm regardless of the current amplitude. The two pieces of the controller naturally exchange responsibility as a result of a change in command or operating point. The turn-off angle controller works independent of the turn-on angle controller. Through modelling of an experimental SRM and extensive simulation, it is seen that the optimal-efficiency turn-off angles can be characterised as a function of peak phase current and motor speed. Accordingly, the optimal-efficiency turn-off angle is determined from an analytic curve fit. It has been shown that a curve fit using only four optimised points gives very close estimation to the most efficient turn-off angle at any given operating point. The SRM, inverter and control system are modelled in Simulink to demonstrate the operation of the system. The modelling is based on the finite element data that include spatial nonlinearities and magnetic saturation. The control technique is then applied to an experimental SRM system. Experimental operation documents that the technique provides for efficient operation of the SRM system through tuning the controller at only four operating points.