Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Electrical and Computer Engineering


Electrical Engineering

First Advisor

David Jiles


Direct-drive permanent magnet generators (DDPMGs) offer many benefits over traditional geared doubly-fed induction generators for large and offshore wind turbines. However, DDPMGs are used in less than 1% of utility scale wind turbines (>100 kW) in the U.S. wind industry due to two major barriers: significant scaling of size and mass with rated torque and power, and the use of rare earth NdFeB permanent magnets. The former is due to the need to generate high torque at low speeds by increasing the volume of the PMG, while the latter is a concern due to the high cost of the critical rare earth materials.

In this work, finite element methods were used to investigate methods to increase magnetic contribution to torque and allow for significant reduction in the PMG volume, or the use of hard ferrite permanent magnets that do not contain critical rare earth materials.

To achieve these goals, the magnetic loading, or average magnetic flux density over the rotor surface of the PMG, must be increased to compensate for torque not generated from either the PMG volume or high energy density NdFeB permanent magnets. The magnetic properties of permanent magnets needed to achieve significant size reduction in a 10 MW PMG were calculated to provide insight into suitable material developments, and mechanisms by which permanent magnets contribute to torque and power were investigated. Practical approaches to concentrating the magnetic flux over the rotor surface were also investigated. The use of Halbach arrays and a novel rotor design incorporating electrical steel flux collectors in a 3.5 kW PMG were found to allow for up to 35% or 46% reduction in the outer diameter and axial length respectively compared to a PMG with surface mounted permanent magnets, or the use of hard ferrite, strontium iron oxide permanent magnets. Existing permanent magnet topologies were also investigated to determine which provided the highest magnetic loading and torque density to allow for size reduction. Finally, the political, environmental and social barriers to the lack of U.S. legislative action to secure a long-term, sustainable supply of rare earths were investigated.


Copyright Owner

Helena Arifa Khazdozian



File Format


File Size

222 pages