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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.