Static four-quadrant control of induction motors.

This work describes a method of controlling a slip-ring induction motor so as to provide characteristics suitable for hoist drive applications. The primary of the machine was supplied from a three-phase voltage regulator which in addition to producing a motoring torque can also provide plug braking by phase reversal and dynamic braking by d.c. injection. The hoist drive characteristics are of a constant torque nature and these were obtained by the control of a variable duty cycle forced commutated thyristor chopper in the secondary circuit of the motor. The operation of the voltage regulator has been examined for a load consisting of a delta wound induction motor. A simple model of the regulator has been developed from which speed-torque characteristics have been determined. Analysis of the operation of the motor and thyristor chopper was used to determine the appropriate relationship between duty cycle and speed necessary to produce a constant torque characteristic. This relationship was realized by a function generator which was used to control the thyristor chopper. Closed loop control of motor speed by the voltage regulator combined with open loop operation of the thyristor chopper as a function of speed gave the required constant torque characteristic. Logic circuits were incorporated in the design to allow automatic selection of the motoring and braking modes. The four-quadrant controller has been designed and successfully operated throughout the required speed range. The operation of the controller has been considered from both steady state and dynamic aspects.