AC motor control with special reference to the induced excitation of synchronous machines.
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.