The development of a controlled-current inverter system for use in high performance AC motor speed controllers.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
AC induction motor speed control has traditionally involved controlling the motor's input voltage and frequency through use of a three phase pulse-width-modulating (PWM) controlled-voltage inverter, where the inductive motor load acts as an integrating filter to smooth out the motor input currents. This report describes the development of a novel controlled-current inverter system in which the three phase motor input currents are independently monitored the inverter phases are independently switched to ensure that the three phase motor currents are balanced three phase sinusoids at all times. It is shown that a controlled-current inverter is capable of efficiently controlling an AC induction motor's speed and torque through varying the frequency and amplitude of the motor's input currents, respectively. Prototype controlled-current inverter systems have been constructed and extensive testing carried out, with the systems' performance being compared with that obtainable from controlledvoltage inverter systems. It has been found that the controlled-current system results in lower motor losses and much reduced levels of audible motor noise. Since it is intended to develop these prototype designs into commercially viable industrial motor controllers, facilities have been incorporated into the designs that are necessary features of all modern motor controllers which are designed for use in an industrial environment.