Design of a three-phase multistage axial flux permanent magnet generator for wind turbine applications

Abstract

Presently, axial flux permanent magnet machines are becoming popular and are being deployed actively for low speed applications. This paper presents an improved model of a multistage axial flux permanent magnet generator (AFPMG). The multistage AFPMG consists of multiple stator and rotor discs. There are three identical 1-phase stator discs and four in-phase rotor discs in the proposed multistage AFPMG. In this research, 4 case studies were analyzed on the design of the multistage AFPMG. First, a phase shift model (PSM) positions the three 1-phase stator discs to behave as a 3-phase generator. Actually, the PSM computes phases for three stator discs in order to establish phase shift of 120$^{\circ}$ between each two phases. The implementation of the presented model in the multistage AFPMG reduces the diameter of the stator disc three times as compared to the conventional 3-phase AFPMG with identical rated specifications. Second, the voltage waveform of the AFPMG was analyzed for harmonic contents and the percentages of 3rd and 5th harmonics were computed. The test results show that 3rd and 5th harmonics were reduced to 10.7% and 0.54%, respectively, in voltage waveform. Third, the proposed multistage AFPMG was designed considering begin-to-end winding connections of the stator disc. While adopting begin-to-end connection, the number of poles of the AFPMG are doubled, which ultimately increases air-gap flux density and thus the terminal voltage of the stator disc and operating shaft speed is halved. The test results show that the torque-to-weight ratio parameter of the designed AFPMG was improved by using a begin-to-end connection for the stator disc. Fourth, the increased air-gap flux density also improves the power density parameter of the AFPMG with a begin-to-end winding connection. Moreover, a prototype model of a 1200-W multistage AFPMG was designed and fabricated while following PSM and begin-to-end winding connection and tested. Thus the test results verify the proposed model of the multistage AFPMG for wind turbine applications.