Some methods of control of induction motor systems using thyristors.

In terms of cost and maintenance the polyphase induction motor has advantages compared with its direct current counterpart, but the control of the output torque and speed is more involved. The advent of high power thyristors has allowed a new appraisal of the induction motor for variable speed drives (e.g. winches, cranes, and traction), and some of the well established, but inefficient methods (in terms of cost, control, and power consumption) have been given a new look. This thesis compares some of these methods of control, namely variable frequency, stator voltage, and rotor control and suggests a combination of stator voltage and rotor control to increase the versatility of the machine and to extend the torque/speed envelope of operation without the complexity of a fall inverter system. It is shown that the harmonics produced by a square wave inverter output can be reduced by increasing the number of pulses per half cycle output if the individual pulses have a given relationship. The influence of this harmonic reduction in the output of the inverter fed induction motor is investigated. An equivalent circuit is developed to represent the induction motor when fed with voltage waveforms of high harmonic content. This equivalent circuit is solved using a finite difference technique and is shown to give first order results with a variety of voltage waveforms. First order performance predictions are shown to be possible of an induction motor system, stator fed from a three phase alternating current supply via "back-to-back" pairs of thyristors using a simple chart method. The chart in this method gives a "derated" value of current available from the production of output torque, based on the uncontrolled theoretical performance of the motor and the given firing angle of the thyristors and the power factor of the motor at the given speed. The prediction of the performance of an induction motor with a thyristor direct current chopper in the rotor circuits in both the motoring and braking regions is achieved using the concept of an effective rotor resistance. The procedure for the calculation of this resistance from given parameters of known values is given, and the theoretical performance compared with that of an experimental system.