This thesis is concerned with design and analysis of fault tolerant permanent magnet synchronous machines for safety critical applications. In addition to high performance under healthy operations, the fault tolerant machines under consideration should provide satisfactory performance under common faults, good demagnetisation withstand capability and thermal robustness. Firstly, a novel triple redundant 9-phase (3x3-phase), 6-pole, 36-slot permanent magnet assisted synchronous reluctance machine (PMASynRM) with segregated delta-connected winding is proposed based on the same topology with segregated wye-connected winding. The performances of machines with these two winding configurations are comprehensively compared under healthy and fault conditions by finite element analysis (FEA) and equivalent models under various fault conditions, including inter-turn short circuit (SC). It is shown that the delta-connected winding has better fault tolerance due to higher output torque under one phase open-circuit fault and lower inter-turn SC current when the mitigation measure -- 3-phase terminal short-circuit is applied. Subsequently, the demagnetisation withstand capability for the proposed PMASynRM with wye-connected winding is assessed by a continuous demagnetisation model under various critical faults at the peak torque and base speed. The dynamic response and the post demagnetisation performance have been obtained to demonstrate that the machines with both delta- and wye-connected windings have very strong demagnetisation withstand capability. The thermal behaviour of the proposed PMASynRM with wye-connected winding under healthy and fault conditions with asymmetric temperature distribution have been investigated by established transient lumped parameter (LP) and 3-dimensional (3D) thermal models. Further, a directly coupled electromagnetic (EM)-thermal simulations based on 2-dimensional (2D) transient EM and 3D thermal model with aid of a scripting file are also performed to gain a deeper insight of the thermal behaviour of the proposed PMASynRM under various fault conditions, including inter-turn SC faults at different speeds and with different numbers of SC turns when considering 17 strands of the winding conductor as a whole as well as inter-strand SC fault when each strand is modelled separately. The temperature distributions which result with EM-thermal coupled simulations have been comprehensively compared with those under thermal-only simulation with constant losses to demonstrate the necessity of the EM-thermal coupled simulation under various fault conditions. The EM and thermal behaviour of the proposed PMASynRM with wye-connected winding are also assessed against a more realistic insulation deterioration process leading to a full SC fault. In addition, the EM performance obtained by 2D FE model and thermal performance obtained by 3D thermal model have been validated experimentally. Finally, electromagnetic and thermal behaviours of a 2.5 MW, dual 3-phase permanent magnet generator for E-Fan-X demonstrator are assessed by the developed EM-thermal coupled simulation technique to quantify fault severity against a number of potential electric failure modes resulting from insulation breakdown.