A two-body buoys power generation system using the field-modulated tubular linear permanent magnet generator (FM-TLPMG) is proposed for direct-drive wave energy conversion in this paper. The traditional linear generator has disadvantages of large weight and low-power density caused by the slow speed of ocean wave. The FM-TLPMG can overcome these disadvantages in some extent through accelerating the speed of the traveling magnetic field. In order to design a suitable FM-TLPMG for the direct-drive wave energy conversion in a short time, ocean wave theory is applied to analyze hydrodynamic problems of the two-body buoys and calculate the operating velocity of FM-TLPMG in the sea. A 2-D analytical model for FM-TLPMG design is presented to obtain the electromagnetic field distribution by solving the Laplace and Poisson equations for each region. The analytical result has been compared with the finite-element analysis and shows its validation for the design.

An outer-permanent magnet tubular linear generator has been designed to increase the power density and distance of relative movement. The design of the two buoys has been completed. This paper describes the detailed design process of the proposed generator. The magnetic network model of the proposed generator is implemented to analyze the performance. Considering the cogging force and voltage, the linear generator has been optimized and the final parameters have been determined. Based on the design results, the integrated wave converter system has been manufactured and put into China's Yellow Sea. The measured voltage is highly consistent with the simulation data. All of results show that the proposed generator is well suitable for wave energy conversion.

A turn-to-turn short circuit fault usually causes excessive fault current because of very low impedance associated with a few fault turns. It should be dealt with promptly to avoid further damages to the machine, especially for a permanent magnet (PM) machine. In order to limit the turn fault current, a novel turn fault mitigation strategy based on current injection technique is investigated for a triple redundant 3x3-phase PM synchronous reluctance machine (PMA SynRM). First, the flux linkage of the faulty phase where the fault turns are located is estimated considering the influence of PM and currents in the healthy and faulty 3-phase sets. This flux linkage, including that of the fault turns, is subsequently reduced by injecting specific currents to the faulty 3-phase set, leading to much smaller fault current. The proposed current injection method does not affect the operation of the healthy 3-phase sets which continue to produce torque. The effectiveness of the proposed method is validated by extensive finite element (FE) simulation and experimental tests on a prototype 3x3-phase fault tolerant PMA SynRM drive.

Direct-drive wave energy converter (DD-WEC) is limited by low power density caused by low direct-drive speed. A tubular hybrid field-modulated linear generator (HFMLG) is proposed to improve the performance of DD-WEC. The generator adopted a field-modulated design to increase the output voltage of the linear generator. A hybrid excited field coil can be used to adapt to different wave situations and effectively improve the output performance of this kind of generator. The operation of the proposed generator is analyzed, and the generator performance is evaluated through finite element method and compared with those of three other generators, namely, a field modulated linear generator with parallel magnetic-circuit, a secondary surface PM field modulated linear generator, and a secondary surface PM fractional-slot linear generator. Results show that the proposed generator exhibits high power density and low cogging force. The steady output power and output voltage of are marginally higher in the proposed generator than in the conventional generators.

Transverse flux permanent magnet linear generator (TF-PMLG) is widely used in direct drive wave energy converter (DD-WEC) because of its high power density. Traditional transverse flux machine (TFM) is designed and built in form of translator-PM. But translator-PM configuration needs a large amount of PM and the cost is high in long stroke application, like DD-WEC. Recently, the stator-PM linear machines have gained more attention for reducing the PM volume and improving the generator performances under low-speed conditions. In this paper, a novel stator-PM TF-PMLG for DD-WEC has been proposed. The fundamental configuration and operation principle of the generator are illustrated. Then, the expressions of the back EMF and the electromagnetic thrust are derived by magnetic circuit analysis. Main dimension parameters, such as PM thickness, central angle of the outer stator shoe and pole pitch are determined and optimized based on finite element analysis (FEA). This topology has advantages of low PM consumption and high power density, and is a suitable candidate for long stroke applications, like DD-WEC, in which a high power per PM volume is usually needed to reduce the amount of permanent magnet and ensure high power density.

The conventional linear permanent magnet generator (CLPMG) for direct-drive wave energy conversion (WEC) has experienced many drawbacks that are difficult to overcome such as low power density and bulky system volume. To improve power density, this paper proposes a linear magnetic-geared interior permanent magnet generator (LMGIPMG) with tubular topology, which artfully incorporates a linear magnetic gear into a linear permanent magnet generator. The operating principle of the LMGIPMG is introduced, and a detailed analysis of air gap flux density, thrust force characteristics, and no-load and load performances are presented and discussed by using finite element method. The CLPMG, which produces the same power as the LMGIPMG, has about four times the volume of the latter. A prototype CLPMG is manufactured to verify simulation results against experimental tests. The design method and the operation conditions of LMGIPMG and CLPMG are both consistent; thus, the performance of LMGIPMG meets the operation requirements of the direct-drive WEC.