Acoustic Noise of Axial Flux Permanent Magnet Generators in Locally Manufactured Small Wind Turbines

Abstract

During the last decade, the local manufacturing of small wind turbines has become a common approach in rural electrification applications, especially among international networks of renewable energy practitioners. Locally manufactured small wind turbines (LMSWTs) used in off-grid battery based renewable energy systems, typically utilize coreless axial flux permanent magnet generators (AFPMGs) due to their simple manufacturing. Typically, low cost three phase uncontrolled diode bridge rectifiers are used for the direct coupling of the AFPMG to the battery bank, which increases significantly the total harmonic distortion (THD) content in the stator current. LMSWT users have occasionally reported AFPMG noise as contributing to the overall acoustic noise emissions of the small wind turbine, especially during low wind speeds. In this paper the relation between current harmonics and acoustic noise emissions in AFPMGs is investigated and verified experimentally, while appropriate passive filtering techniques are utilized in order to reduce their effect. Furthermore, the overall acoustic noise emissions of a LMSWT are measured experimentally in the field and are compared to noise standards for residential areas. Finally, electromagnetic and aerodynamic sources of acoustic noise are investigated experimentally in the field, while design recommendations are developed for LMSWTs towards the reduction of acoustic noise emissions.