Control of hybrid-excited permanent magnet machines

This thesis focuses on the control of hybrid-excited permanent magnet machines (HEPMMs) in both flux-enhancing and flux-weakening operations, taking due account of both efficiency improvement and fault protection. High torque at low speed and wide operating speed range can be achieved by the proposed control strategy for the hybrid-excited switched-flux permanent magnet machine (HESFPMM) with iron flux-bridge. The torque can be enhanced by utilising the positive field excitation current. In the flux-weakening region, the field excitation current is controlled towards zero rather than negative, while the d-axis current is also utilised. The efficiency can thus be significantly improved. Also, three flux-weakening control methods for the HEPMMs, namely using field excitation current alone, using armature current alone, and the optimisation method, are compared and experimentally verified. All three methods can achieve the same torque in the constant-torque region due to the field excitation currents being kept the same, whilst the optimisation method provides the highest torque and efficiency in the flux-weakening region compared to other methods. Maximum efficiency can be achieved by applying an appropriate field excitation current according to the given torque and speed conditions rather than fixing it at the maximum value. Two control strategies, optimisation and maximum efficiency tracking (MET), are proposed in order to improve the efficiency of HEPMMs. These methods have demonstrated experimentally the enhanced performance under dynamic conditions, in which the load torque is changed, compared with the method which utilises the maximum field excitation current for all operating regions. Finally, since the PM-flux of the prototype HEPMM is inherently short-circuited when the field excitation current is not fed into the excitation windings, the open-circuit back-emf is almost zero. As a result, the machine has an inherent fault protection capability which can be utilised to prevent dangerous excess voltage issues from uncontrolled generator faults (UCGF) at high speed operation. An original method of fault protection for the HEPMM is therefore proposed and experimentally verified based on the utilisation of field excitation current control.