Performance Analysis of a Direct Drive Permanent Magnet Generator for Small Wind Energy Applications

Abstract

This paper describes performance analysis of a direct drive Axial Flux Permanent Magnet (AFPM) generator developed for small scale wind energy applications. The design process approach began by a field survey intended to understand reasons behind low penetration of Small Wind Turbines (SWTs) in Kenya and failures of existing power transmission systems, before making a decision on the most suitable electrical generator system. The main objective of this study was to design a generator that is cost efficient to improve its adoption, easy to manufacture so that it can be made in small workshops using basic tools and with limited electrical engineering knowledge, to make it attractive as a source of employment for the youth. Starting by elimination among different possible generator structures, a decision was made for the configuration of a core-less (air-cored) double rotor AFPM generator oriented to SWT applications. The design process combines both analytic and Finite Element Analysis (FEA) of the generator. A simple analytic electromagnetic design model, including sizing equations, considering the fundamental lawsgoverning this type of machine was used to achieve and implement a prototype. FEA model and performance equations were used to verify its performance. The generator assembly is reinforced using an innovative strong T-frame to improve generator stability since coreless AFPMs are known to have stability problems. This frame provides a strong reinforcement to the generator assembly and also offers a flexible mounting support to the turbine blades so that the generator can easily be mounted both on a Horizontal Axis Wind Turbine (HAWT) as well as a Vertical Axis Wind Turbine (VAWT). The AFPM generator will be fitted with suitable protective cover to shield the generator from effects of drought such as corrosion and keep debris from becoming lodged in the inner parts. This has the possibility of extending the life of the generator. The generator has also been designed with large number of poles to improve its performance at low wind speeds. The generator efficiency is high with typical efficiency values of between 75-89%.