Optimal-current control of convertor-fed DC motors. II. General case

For pt. I see ibid., vol.141, no.5, p. 240-8. (1994). In this paper a new method of firing control for a three-phase thyristor bridge supplying a DC motor in both monitoring and regenerating modes is described. The method uses the firing points as the sampling instants and the `cusp current´ at these instances as the controlled variable. For continuous-current operation, the method enables the current to be changed from one steady-state level to another in two thyristor firings (6.6 ms). The method developed in Part I is extended to apply to normal systems with motor or DC-link resistance present and shows that basically the same continuous-current-control algorithm may be used. It also presents an improved method for current control with discontinuous currents which permits rapid changes and reversals of current to be achieved. Examples of waveforms of step-current changes and reversals are provided for a 3.5 kW practical system operating under microprocessor control. Frequency-response tests up to 90 Hz showed that the current-control dynamic response is equivalent to a finite time delay of 5.2 ms with constant unity gain