Optimal commutation angles for current controlled switched reluctance motors

Abstract

The paper considers the problem of choosing the turn-on and turn-off angles in current control of switched reluctance motors. The maximisation of mean torque is chosen as a primary objective, the expected benefits being a wider torque-speed envelope and faster transient response. With the assumption of flat-topped currents, the contributions of the turn-on and turn-off processes are shown to be independent, and can be optimised separately. The graph of mean torque as a function of turn-off angle has a well defined maximum. An analytical expression for the optimum turn-off is derived which takes saturation into account and is convenient for off-line generation of look-up tables giving the optimum turn-off angle as a function of speed and reference current. The predicted optimum turn-off angles agree satisfactorily with measurements on two motors taken over a wide range of operating conditions. The graph of mean torque as a function of turn-on angle is flat-topped and does not have a well defined local maximum. However, measurements show that over the range of turn-on angles defined by this flat-top, the efficiency varies monotonically with the turn-on angle. This suggests that the maximisation of efficiency for a given mean torque by varying the turn-on angle is a reasonable secondary objective, and one that can be done using existing techniques.