Growing aircraft transportation has brought the convenience for travelling around, as well as increasing benefits to transport related industries and commercial companies. However, this also has brought more greenhouse gas emission than before, leaving an urgent issue to balance between benefits and pollutions in the aircraft transport sector. More electric aircraft systems, as a solution for replacing conventional aircraft systems, help to reduce the greenhouse gas emissions, improve energy efficiency and reduce maintenance costs. Multiphase machines in aircraft systems reduce the power per electronic device, reducing its size and cost but with improved system reliability on fault condition and power density. This thesis evaluates the performance of multiphase synchronous machines for a DC power network, where particularly harmonic components and winding configurations for the machine under healthy and fault conditions are studied. The analytical method involved in this thesis is the complex harmonic analysis, which is a frequency domain analysis tool for machine design, considers how the harmonic components change with different machine geometry and winding configurations in order to improve the machine performance. The experimental and finite element validation for the complex harmonic analysis are conducted in this thesis to illustrate the limit of the complex harmonic analysis together with some improvements for this method to account a better fit for the machine modelling. The complex harmonic analysis has been extended with novel work addressing saliency modelling, representation of saturation and evaluation of DQ modelling. There is also new work in the end winding leakage calculations. This thesis also presents an example of the complex harmonic analysis applied to a reconfigurable fifteen phase generator-rectifier system with different layouts to maximise the performance and fault tolerance. Specifically, the 15-phase, three sets of 5-phase and five sets of 3-phase along with series or parallel stacked diode rectifier sets are considered. Although this thesis is focused on a specific machine, the conclusions drawn from steady state healthy and fault performance are generalised for other winding configurations, for example, the presence of zero-sequence current and lowered winding factor in short-pitched windings leads to lower peak power capability and the parallel path in parallel stacked diode rectifier sets leads to severe interference between healthy and faulted subsets.