AC motor variable frequency drives-consideration of the most economic and technically viable convertor power circuits for drives above 500 kW

In the general sphere of industrial applications there has been a large increase in interest in AC variable frequency drives. The price of such systems has been gradually decreasing (with the improving capabilities of thyristors) at a time when energy costs have in general been increasing, so that it is now relatively easy to make out an economic case for driving plant at variable speed. The moves in the direction of AC drives have also been helped by the general improvement in overall efficiency and by the potential for a reduction in maintenance caused by the use of brushless systems. The author discusses three such systems: the synchro converter system using naturally commutated converters feeding brushless synchronous motors; the capacitor self-excited induction motor system; and the cycloconverter low frequency system. Future systems and briefly discussed