Instantaneous torque control of electric motor drives

In the control of adjustable speed drives, the performance of inexpensive digital integrated circuits is approaching the stage where traditional control algorithms may be displaced by newer algorithms that better exploit their speed and the functional capabilities of their software. This paper introduces the concept of “instantaneous torque control” as an objective worth pursuing in the application of such digital IC's to drive systems. Instantaneous torque control would in principle permit the fastest possible response and the elimination of torque ripple, along with many other advantages not possible with conventional control algorithms, most of which are set up to control a time-averaged torque. This paper develops some of the fundamental principles of instantaneous torque control for the switched reluctance motor, which is used as an example because, like the brushless dc permanent-magnet motor with concentrated windings, it has the potential for rapid response but it can have appreciable torque ripple with unfavorable firing angles. Neither of these machines satisfies the conditions for the existence of a reference-frame transformation that will eliminate the rotor position from the voltage and torque equations, and this opens up a number of interesting questions as to the generality of instantaneous torque control algorithms, and whether they can be incorporated into the general or unified theory of electrical machines.