Open- and closed-loop cycloconverter-drives.
thesisposted on 19.11.2015, 08:59 authored by Peter Geoffrey. Holmes
This dissertation is mainly concerned with cycloconverter -induction-motor variable-speed drives. Hitherto, the cycloconverter has been at a disadvantage compared with other types of solid-state frequency converters because of a) the number of devices required, and b) the tendency for line to line short-circuit faults. In a simple system devised in this work, the number of devices is reduced by 2-phase, 3-thyristors per group basic design and line to line short-circuit faults are prevented by the design of a special stator winding with electrically-separate, magnetically-coupled thyristor group circuits. The requirements of thyristor gate circuitry have been considered and circuits have been devised to meet the requirements of the experimental system. A study of the harmonic content of the cycloconverter output voltage has been made and its effect on the machine performance and efficiency examined. The performance and stability of cycloconverter-induction motor-drives and cycloconverter-reluctance-motor drives have been considered and it has been shown that while a closed-loop, variable- speed induction motor drive is inherently stable, a cycloconverter- reluctance motor drive can become unstable at low speeds and loads. A method of simulating cycloconverter-induction-motor-drives to enable the performance of large power drives to be predicted has been investigated, although it has not been possible to fully implement the simulation. Electrical braking in cycloconverter-induction-motor drives has been investigated and methods of achieving zero-sequence characteristic a.c. braking and optimum low-frequency a.c. braking have been devised.