The frequency of a Pulse Width Modulated (PWM) power source can be adjusted to control the speed of an AC machine. Machine response in this mode of operation depends strongly on the quality of the chosen PWM switching strategy. Several methods of PWM waveform synthesis are presented and new methods are introduced. A novel Relaxed Spectrum (RS) algorithm based on a Fourier series expansion of the PWM signal is presented. The algorithm computes sinusoidal and non-sinusoidal switching patterns from a specified set of harmonic coefficients. The RS method is suitable for both full- and half-bridge PWM invertors. Unwanted harmonics inherent in a PWM signal, are exploited to induce excitation into the rotor of a synchronous machine. Rotor mounted diodes are used to rectify the induced currents. 'Rvo novel induced excitation methods are presented which permit accurate control of excitation levels. Whilst the first method uses a single Excitation Harmonic Voltage (EHV) for rotor excitation, an EHV pair is used in the second method to obtain improved machine response. The RS algorithm is applied to obtain a PWM waveform which approximates closest to a given specification. PWM switching patterns are obtained for most possible excitation conditions. Voltage Control Limit Functions (VCLF) are computed which define control limits for several excitation schemes. The response of a computer simulated machine model to an Ideal Test Voltage (lTV) is investigated to assess the performance of the new methods of machine excitation. Simulation reveals that the use of an EHV pair minimises motor torque pulsation. Optimum excitation ratios are then defined in terms of the harmonic indices of a given EHV pair. These ratios are shown to be largely independent of machine parameters. The RS method is used t(] compute generalised sets of PWM switching sequences which can subsequently be applied to a wide range of differently rated machines. Analogue Symmetrical Sample (SS) PWM waveform synthesis methods for induce( excitation are compared with their digital equivalents. It is shown that digital P~ techniques compare favourably and can provide higher excitation levels when a largl fundamental drive voltage is required.

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