This thesis first presents an overview of induction motors and their starting characteristics under a Direct On-Line (DOL) start. From which it is shown that a DOL start results in large inrush currents and large torque rippling. Conventional soft-start methods reduce the negative starting characteristics of the DOL method by limiting the stator voltage/current, but also reduce the overall torque that can be produced.

The problem with reducing the torque through conventional soft-start is that this may lead to the motor stalling under heavy loads. Discrete Frequency Control (DFC) aims to increase the starting torque whilst still maintaining a reduced starting current, by applying a series of discrete frequencies to the motor whilst still using the soft-starter’s hardware. The generation of sub-harmonics is achieved through the general operating principles of DFC, which is to either include or exclude half cycles of the supply. The DFC research conducted in this thesis is divided into two key topics: first, the generation of balanced sub-harmonics: and second, an assessment of three DFC control methods.

Harmonic analysis of generated sub-harmonics is undertaken using the Fast Fourier Transform (FFT). From this analysis it is confirmed that the methods for generating these sub-harmonics yields the most symmetrically positive three-phase system.

Three DFC methods are assessed, DOL-DFC, Voltage/Frequency-DFC (V /f -DFC), and Current Control-DFC (CC-DFC). Simulations of the three show that the CC-DFC performs best, as it produces the most torque with the least amount of current. Simulations of CC-DFC method and the conventional soft-start method show that the CC-DFC is capable of producing higher torque with less RMS current being required, but this comes at the cost of greater peaks/spikes in current and an increase in torque ripple.