Improved current control in thyristor drives for d.c. machines.

Abstract

The accurate and versatile control of a direct current machine's speed using a thyristor converter has led to its use for many industrial applications. However, as will be demonstrated, the current gain characteristic of such a device is not linear and under certain operating conditions can lead to an unacceptable degradation in performance. In order to develop a method of adaptive control that can compensate for the gain changes, the key features and theory of phase controlled thyristor converters are described and used to develop a digital computer simulation of a single phase converter. The d.c. gain of the device as a function of firing angle is then determined and the model used as the basis of a larger simulation of the machine's armature current control loop. The latter is used to investigate a number of control strategies, one of which is developed practically using a combination of analogue and digital electronics. The performance of the adaptive control system is then investigated by means of a comparison with the responses predicted by the software model and also by comparing the responses of the adaptive controller with the current loop responses obtained from a purely linear commercial drive. These show that the implementation of the chosen control strategy, although being very complex, does not achieve ideal responses. However, those obtained are seen under most conditions to be an improvement on the conventional system. Finally, the effect of the different armature current controllers on the speed response of d.c. machines using both linear and variable structure speed controllers is investigated. This is done purely in software following the development of a digital simulation of the speed loop. These show the adaptive qualities of VSS speed control, especially when used with the "faster" adapted current loop.

Date of Award	1985
Original language	English
Awarding Institution	University of Bath