Low-variable-speed, continuous-circulating current, cycloconverter-induction-motor drives.

The cycloconverter as a direct frequency converter without a d.c. link is well known as a power amplifier in low speed a.c. drives. If continuous circulating current operation can be maintained, reversible energy flow and regenerative braking can be achieved in an induction motor drive without a change in the gate pulse sequence. Continuous circulating current operation also reduces the output harmonic content of the cycloconverter and increases the speed range of the drive. This dissertation describes the use of an induction motor stator as an inter-group reactor maintaining continuous circulating current in the cycloconverter. The same stator winding sets up the conventional rotating air gap-flux and the reactor machine has standard torque-speed characteristics. In the multi-machine drive designed and constructed in this work, one machine requires a special stator winding. Conventional squirrel-cage machines can then be added for tandem speed control and the utilisation of copper improved. Closed-loop speed control and air gap flux control are incorporated on the reactor machine. The standard induction motors added in addition to the reactor machine are shown to be essentially operating under open-loop conditions but a "hard supply" is maintained by closed-loop control.