Magnetic Forces and Vibration in Wind Power Generators: Analysis of Fractional-Slot Low-Speed PM Machines with Concentrated Windings

The PhD research work presented in this thesis deals with radial magnetics forces and vibration in low-speed fractional-slot permanent magnet (PM) machines with concentrated windings. One of the applications of such machines is a direct-drive energy conversion system for wind turbines. Due to the presence of the low spatial harmonic orders in the radial forces distribution, fractional slot machines with concentrated windings can potentially have a higher vibration level than the traditional PM machines. In order to investigate the magnetic vibration characteristics, flux density distribution is computed using finite element (FE) analysis. Then, the radial and tangential force densities are calculated using the Maxwell stress tensor. Spatial harmonic analysis of the radial force density distribution provides important information regarding the vibration behavior of the machine. The radial force distribution on the stator teeth are calculated and employed as input to a structural FE analysis. In this analysis, deformations in the stator bore due to the exciting magnetic forces are computed. Experimental work is performed on a prototype 120-slot/116-pole PM generator to validate the magnetic and structural simulations. The influence of pole and slot combinations is investigated by comparing PM machines with 120 slots and 80, 112, 116 and 118 poles. Radial forces and vibration, the cogging torque and the torque ripple characteristics of these machines are studied. The effects of the slot harmonic are investigated in the prototype machine. It is discussed how the vibration behavior is changed in the case of different slot closures. The influence of loading is studied systematically; it is discussed how the d-axis and q-axis currents change the amplitude of the lowest spatial harmonic of the radial forces, and consequently the vibration level of the prototype machine