Steady-state performance assessment of three- and five-phase permanent magnet generators connected to a diode bridge rectifier under open-circuit faults

Small-scale wind turbines often utilise permanent magnet generators that supply power via an uncontrolled diode rectifier. In such systems, a custom generator design is required to optimise the aerodynamic and electromechanical performance; therefore the phase number of the generator is not restricted. This study assesses and compares the performance of prototype 700 W five- and three-phase permanent magnet generators connected to a diode bridge rectifier and their performance under open-circuit fault conditions. The simulation and experimental results demonstrate that the five-phase system can utilise a smaller DC-link capacitor and develops lower shaft torque ripple compared to the three-phase system. The three-phase generator can only tolerate one single-phase open-circuit failure. However, three possible open-circuit phase failure modes are considered in the five-phase machine: a single open-circuit phase and two open-circuit phases, where the two phases can be adjacent or non-adjacent. It is shown that the shaft torque ripple and output power are superior in the five-phase system with both the single-phase failure mode and the non-adjacent phase failure mode compared to the three-phase system with a single-phase failure. However, the adjacent phase failure mode is more extreme and generates less output power than the three-phase system. Simulation and finite-element analysis results are verified by experiments on the prototype 30-slot, four-pole, double-layer, surface-mounted permanent magnet generator.