AC Ward Leonard drive systems: Revisiting the four-quadrant operation of AC machines

In this paper, the problem of controlling the speed of AC machines in four quadrants is revisited from a completely new viewpoint, based on the idea of powering an AC machine with a synchronous generator that generates a variable-voltage–variable-frequency supply. This is a natural, mathematical, but not physical, extension of the conventional Ward Leonard drive systems for DC machines to AC machines. As a result, AC drives can be regarded as generator-motor systems, which facilitate the analysis of AC drives and the introduction of other special functions because a system consisting of a generator and a motor is easier to be handled than the conventional AC drive that consists of an inverter and a motor. Control strategies, with and without a speed sensor, are proposed to implement this idea and the experimental results are presented to demonstrate the feasibility.