Air Gap Elimination in Permanent Magnet Machines

• Main Author: Judge, Andy
• Other Authors: Alexander E. Emanuel, Committee Member
• Format: Others
• Published: Digital WPI 2011
• Subjects: magnets; electromagnetics; ferrofluid; electric motors; generators
• Online Access: https://digitalcommons.wpi.edu/etd-dissertations/123; https://digitalcommons.wpi.edu/cgi/viewcontent.cgi?article=1122&context=etd-dissertations

In traditional Permanent Magnet Machines, such as electric motors and generators, power is transmitted by magnetic flux passing through an air gap, which has a very low magnetic permeability, limiting performance. However, reducing the air gap through traditional means carries risks in manufacturing, with tight tolerances and associated costs, and reliability, with thermal and dynamic effects requiring adequate clearance. Using a magnetically permeable, high dielectric strength material has the potential to improve magnetic performance, while at the same time offering performance advantages in heat transfer. Ferrofluids were studied as a method for improved permeability in the rotor / stator gap with a combined experimental and computational approach. Results show promise for the ferrofluid technique. An off-the-shelf motor system showed improved performance with ferrofluids vs. fluids of equivalent viscosity, and improved performance vs. an air gap at low RPM. New generator designs showed design dependent performance gains, although some potential for negative performance effects. A proof of concept generator was built and tested, with increased voltage vs. RPM predicted through virtual prototyping, and validated through experimentation, showing ~10% improvement on voltage vs. RPM at the <600 RPM range. More repeatable engineering tests demonstrated a ~30% increase in the voltage / RPM relationship for designs with an isolated stator chamber and a large stator - rotor gap. However, the effects were negative for a similar system with a small stator-rotor gap due to leakage flux effects. New contributions to the body of knowledge in this area include: • Application of the ferrofluid technique to axial flux designs. • Development of a virtual prototype, including variations in the fluid viscosity due to ferrohydrodynamic effects. • Consideration of negative effects of ferrofluid immersion, such as shear losses and increases in leakage flux. • Optimization of the design to eliminate increased viscous losses. The improved design has been designed, built, and tested, featuring isolation of the ferrofluid from the rotating region. This offers all of the performance gain of improved magnetic permeability, while minimizing the offsetting losses from increased shear effects.