Precise Control of a Three-Phase Squirrel-Cage Induction Motor Using a Practical Cycloconverter

The advent of the silicon controlled rectifier has permitted the design of a practical cycloconverter source of 3-phase electrical power which can be continuously controlled in frequency, from dc to approximately one-half the input power frequency, as required to control the speed of a 3-phase induction motor. The correspondence between the applied stator frequency, the resulting magnetic flux phase rotation, and the mechanical rotor rotation in a 3-phase induction motor is analyzed as a function of slip. A closed-loop feedback system is described for controlling the slip between the rotor and rotating magnetic flux and for optimizing the slip frequency over the full reversible speed range of the motor. The stepless control of angular position, velocity and acceleration, and torque in both the driving and braking modes of operation is discussed.