In this study, a novel method for design of a direct drive(DD) PM vernier generator (PMVG) is proposed for a gearless, high power and lightweight wind turbine system. In the proposed design method, the base and maximum speeds of the generator are firstly obtained from the nature of aerodynamic power, and then the electrical circuit parameters of the generator are scoped with consideration of the control strategy of PM machines. To determine the generator geometries using the scoped parameters, the relational equations between the parameters and geometries of a PMVG with concentrated windings are derived. A systematic method for design using the derived equations and the parameters is developed, and the 5kW PMVG with outer rotor is designed through the proposed method. The performance characteristics of the designed generator are analyzed with FE simulations and compared with analytically predicted results.
Recently, the interest in renewable energies is very high, and the research and the development on them are more active than ever. Among them, wind-powered generators operate in every size range, from small turbines for battery charging at isolated residences to large wind farms that provide electricity to national electric transmission systems. In particular, wind power generation is being transferred from the ground to the sea, and at the same time, the capacity of generation is growing to the level of gigawatts. These tendencies cause the following compromise; when a mechanical component such as a gear box in an offshore wind turbine breaks down, the maintenance procedure is very complicated and expensive. Therefore, it is beneficial to adopt a direct-drive (DD) generator with no gears, but in this case, the weight of the generator becomes excessively large [1-4]. Hence, to solve these two problems at once, it is necessary to use a DDgenerator having much higher power density.
Thus far, various unique machines for the low speed-high torque applications have been presented such as magnetic gears, vernier machines, flux reversal machines and flux switching machines etc. [4-25], and then it is proven that all these machines are classified into the modulation flux machinery which utilizes the magnetic gear effects,alternatively called the flux modulation effects . Especially, it was proven that the permanent magnet (PM) vernier motor, which is a magnetically coupled structure of the magnetic gear and the PM motor, has theoretically much higher output power density because it uses the main PM flux at the same time as the modulation flux [6, 7].
To improve the problems of the repetitive performance calculation and the resulting inaccuracy in the conventional design procedure of variable-speed electrical machines, this paper proposes a novel design method which is mathematically clear and thus direct and accurate. As a case study, a 5kW direct-drive, outer-rotor PM vernier generator for wind turbine system was designed using the proposed method. The proposed method shows the direct design procedure from choosing the operational speeds of the rotor such as the base and the cut-off speeds to determining the geometries of the generator in details.
The results of this study can be summarized as follows. It has been shown that there are various combinations of circuit constants that satisfy the torque performance required for a wind turbine when a PM generator is represented by a regular electric equivalent circuit. In other words, there may be generators of various different structures. The circuit constant equations expressed in terms of the geometrical structure of the concentrated winding PM vernier generator were derived in consideration of the magnetic gear effects. We have successfully performed a reverse design to determine the shape from the circuit constants by assigning some constraints and reasonable assumptions such as the ratio of the surface current density, the ratio of the gap to the magnet thickness. Then, the reverse design to determine the geometries of a PM vernier generator from the circuit constants has been successfully performed by assigning some constraints and reasonable assumptions such as the ratio of the surface current density, the ratio of the gap to the magnet thickness.