Developing fault-tolerant control strategies for a five-phase permanent-magnet machine drive system

Many critical five-phase permanent magnet synchronous machine (PMSM) drive systems require operation after either a single or dual open-phase fault occurs. Some of these drive systems use the open-end winding inverter topology as it allows for the injection of unbalanced phase currents, i.e. currents that do not sum to zero. This allows for better optimization of the machine torque after open-phase faults occur. The post-fault phase currents are often controlled using the field-oriented control (FOC) technique. However, all previously proposed FOC transformations yield non-dc references when controlling unbalanced phase currents. Therefore, these techniques will experience tracking errors, with the PI current controllers, that create undesired distortions in the phase currents. This thesis focuses on proposing post-fault FOC transformations that yield dc references when controlling multiple sets of unbalanced phase currents. These sets of currents include three unbalanced sinusoidal phase currents, four unbalanced sinusoidal phase currents and three unbalanced phase currents with fundamental and third harmonic components. These current sets are for different post-fault optimization approaches in the event of a single or dual open-phase fault in a five-phase PMSM system. The proposed techniques are significant in that they do not suffer from tracking errors, like the previous FOC transformations, when controlling these currents. Example FOC transformations are developed for each case and are used to experimentally demonstrate, using a specialized experimental testbed, that these transformations can be used in an FOC architecture to accurately control the post-fault phase currents with zero tracking error. This thesis also proposes a generalized post-fault optimization algorithm for the event of an open-phase fault in a multi-phase PMSM system. This technique includes any desired constraints on the current form whilst determining the optimal phase currents which yield the desired torque waveform. Also, a new modulation method is proposed which accurately generates the zero-sequence voltage reference when operating in the over-modulation region. This avoids obtaining high current distortion when operating in the over-modulation region with the five-phase open-end winding configuration. The work performed in this thesis is significant to the field of post-fault control of multi-phase PMSMs and the control of the open-end winding inverter topology