Performance Analysis of a Novel Fall-Back Transverse-Flux Permanent-Magnet Generator with Outer Rotor Design Suitable for Direct-Coupling Wind Turbine.

摘要

A fall-back transverse-flux permanent magnet generator (FB-TFPMG) with inner rotor design preferred for wind power applications, employs half the number of PMs (as against conventional TFPMG), elliptical shaped stator core and toroidal shaped coil. In this manuscript, a novel concept of FB-TFPMG with outer rotor design, suitable for direct coupling to wind turbine, is explored with a possibility to improve the power to volume ratio in comparison with the inner rotor FB-TFPMG. In the proposed configuration, the blades of the wind turbine are directly fastened to the drum to perceive the direct coupling between the wind turbine and the outer rotor FB-TFPM generator. This leads to immediate benefit of lower weight and better cooling. The dynamic performance of a novel concept of the proposed topology is analysed using 3-D finite element tool and the results are compared with the inner rotor FB-TFPMG. With expeditious penetration of wind power generator technologies and remarkable wind power generation capacity installed worldwide, numerous concepts of wind generator have been proposed and assembled. In [1]-[2], possible future wind generation systems and topologies are reviewed. A conventional transverse-flux permanent-magnet generator (TFPMG) with U-shaped stator core [3]-[4] is chosen as the best generator among all direct-drive PM generators because of its higher power density and simple coil design. The main drawbacks of conventional TFPMG has a complex design, an uneven magnetic flux distribution and high flux leakage due to multi-dimensional magnetic flux pattern. An iron bridge is employed between adjacent stator cores, as a solution to prevent the excessive leakage flux [5]-[6]. An improvement to the concept of conventional transverse-flux permanent-magnet generator, with a fall-back rotor is presented in [7]. A fall-back transverse-flux permanent-magnet generator (FB-TFPMG) with extit{inner} rotor design reduces the number of PMs to half, possess elliptical shaped stator cores and toroidal shaped coil in comparison to conventional TFPMG. Fall-back path of the rotor offers several advantages, importantly it avoids the losses in inactive magnets and the reduction in overall cost of the generator. Various outer rotor configurations of permanent magnet generators are deliberated in the literature i.e. radial flux, axial flux, transverse flux design, claw pole type and superconducting generators [8]-[10]. Transverse flux PM generator with outer rotor designs are compared in [11]-[12]. In this paper, a novel concept of FB-TFPMG is extended with outer rotor design, suitable for direct-coupling of wind turbine. This design is explored with a possibility to improve the performance and power to volume ratio in comparison with the inner rotor FB-TFPMG. The detailed cut section of the one phase of a novel FB-TFPM generator with outer rotor design is depicted in Fig. 1. The FB-TFPMG is basically a multi-phase configuration with circumferential toroidal shaped coil per phase in the stator. It possesses half of the total magnets with outer rotor for benefits of the availability of the space in it and possibility to modify the PM rotor pole position for better performance. It comprises of inner stator core assembly, which is mounted on the fixed shaft. Soft magnetic composite material is used for stator structure with even number of U-shaped magnetic circuits (cores) placed circumferentially inside a rotor core assembly. Rotor possesses number of magnetic pole pairs (NdFeB) exactly equal to the number of stator cores. The rotor back is made of mild steel. PM fluxes of all N-poles add up in the stator core in one direction and as the rotor travels one PM pole angle, all fall-back part (S-poles) add up their fluxes in the stator core in the reverse direction. As the rotor travels, the magnets and fall-back part of the rotor change heir polarities and hence alternating emf is induced in the toroidal shaped coil. PM flux paths are bi-directional in the stator and three-dimensional in the rotor. The results of a novel outer rotor FB-TFPMG are focused on the most important parameters such as electromagnetic field analysis and induced emf under no-load condition and on-load condition. The induced emf plots under no-load and on-load condition are shown in Fig. 2 and are in good agreement with the inner rotor FB-TFPMG. Outer rotor design with 10.27% reduction in volume gives the equivalent output power as compared with the inner rotor design.