An investigation of efficient control strategies for a PWM inverter driven induction motor

Recent developments in power electronics switching devices have led to significant improvements in AC drives which, coupled with the obvious advantages of squirrel-cage induction motors, have generated a customerled demand for an increase in AC drive performance. This thesis describes the design and construction of a 3-phase pulsewidth modulated inverter using gate turn-off (GTO) thyristor switching devices, which drives a 0.75 kW 3-phase squirrel-cage induction motor. The inverter control circuit comprises a purpose-built large-scale integrated circuit, which generates the 3-phase pwm drive signals and allows the output voltge and frequency to be varied independently. When operating in open-loop, the drive system is capable of reverse operation, and the maximum rate of acceleration and deceleration of the motor may be controlled. Compensation for resistive voltage drop is provided when the motor is running at low speed. An analogue closed-loop proportional-integral-derivative speed controller is described, and for efficient operation under both no-load and on-load conditions torque feedback is also included. This provision both reduces the no-load losses in the motor and improves the torque-speed characteristic under load conditions. The improved closed-loop performance also includes power factor correction when the motor is lightly loaded,.together with an automatic boost to the motor voltage when loads are applied at low speed. A comparison is made between the performance of the analogue system and a digital real-time control implemented using a microcomputer. A series of computer programs are presented which simulate the performance of the drive system and which are suitable for running on the University mainframe computer. The programs enable the effects of the modulation technique and the inverter frequency on the pwm inverter steady-state output to be studied, and the performance of the induction motor to be investigated. Throughout the work, the theoretical predictions are supported by considerable experimental results.