Direct-drive applications have been increasingly applied in modern industrial applications, including the direct-drive electric ship propulsion (ESP), electric vehicles, electric robotics, power generation systems, and so on. Compared with the indirect-drive counterpart, the direct-drive system removes the gearing transmission system, which accommodates the benefits of lower frictional loss, lower vibration, higher efficiency, higher drive stiffness, less maintenance and longer service life. On the other hand, without the speed and torque adjustment from gearing sets, the direct-drive machines are required with the higher torque density and efficiency.

With the merits of compact configuration, large torque capacity and high reliability, the double-stator (DS) permanent-magnet (PM) brushless machine is regarded as an applicable candidate for direct-drive applications. Also, to reach the wider speed range and higher flux-modulation capability, the field-windings can also be included as the auxiliary excitation in DS-PM machines. This thesis is to present the design, analysis and control of DS-PM brushless machines for direct-drive applications, which accommodate the advantages of improved torque density, efficiency, flexibility, reliability, wide operation ability, and so on.

Firstly, in Chapter 2, this thesis presents an overview of DS brushless machines, mainly focusing on the emerging DS-PM and DS hybrid machines. The machine design, electromagnetic characteristics, operation principles and comparative studies are mainly included in the overview.

Secondly, the design of the emerging DS-PM brushless machines is introduced in Chapter 3. A unified design principle is first proposed for the emerging DS-PM salient brushless machines, as the theoretical basis of the novel DS-PM machine design. Second, the multi-objective genetic algorithm (MOGA) strategy is introduced, which is well employed in the contradictory-objective optimization of the proposed DS-PM machines. Third, three new DS-PM brushless machines are proposed, including the DS V-shape-PM vernier (V-PMV) machine, DS flux-reversal (FR) PM machine and DS hybrid flux-switching (FS) PM machine. The three machine designs can be regarded as effective references for the DS rotor-PM machines, DS stator-PM machines and DS-PM hybrid machines, respectively. Based on the unified design principle, the operation philosophy fitting for the corresponding machine types are specifically deduced. Also, based on different machine configurations, the corresponding multi-step MO optimization strategies are proposed. Thus, the optimization is carried out with high efficiency and accuracy. Then, the machine performance analysis is presented, with auxiliary of finite element method (FEM).

Thirdly, in Chapter 4, the analysis of DS-PM brushless machines is presented, including the analytical modelling and performance comparison analysis. First, the permeance and inductance modelling of DS-FSPM machines are conducted, with consideration of hybrid excitation. Also, multi-physics performance comparison is carried out among DS-PM vernier brushless machines with different inserted-PM (IPM) structures, which serves as a comprehensive assessment and reference for DS-IPM brushless machines.

Fourthly, the control of DS-PM brushless machines is presented in Chapter 5, which mainly focuses on the utilization of the special DS structure. First, the DC-biased current control on a new DS-FSPM brushless machine is proposed. Through directly injecting the DC current into the slot armature windings, the DC-biased strategy contributes both the flux-enhancing and flux-weakening capabilities to the machine, without occupying extra insulations or machine space. Second, based on the mathematical modelling of DS-PM machines, experiments of the proposed DS-PM brushless machine prototypes are implemented with field-oriented control (FOC) strategy, including the DS-V-PMV, DS-spoke-PV and DS hybrid FSPM machine prototypes. Thus, the machine designs are further validated.

Fifthly, the DS-PM machine direct-drive applications are proposed in Chapter 6, including motor and generator applications. First, the new DS-PMV motors are applied to the direct-drive ESP system. Utilizing the DS configuration, the multi-cruise-mode and multi-charging-mode are realized on the ESP system. Specific summary and discussions of the multi-operating modes are proposed, with experiment implementation as the validation. Second, combing the DS winding configuration and terminal connection diversity, a new DS winding control strategy is proposed and employed on a DS toroidal-winding PMV generator for direct-drive wind-photovoltaic (W-PV) hybrid generation system. In this way, the multi-mode operation is realized with high modulating continuity, which partially solves the voltage fluctuation from the indeterminate wind speed without further control strategy.

Finally, conclusions and recommendations are drawn in Chapter 7, which specifically list the key contribution of this thesis and the future recommendations.