Generally, the size and cost of electric machines (motors or generators) are more closely related to the machines’ torque rating than to their power rating. Thus, if two machines are rated for the same power at different speeds, the higher-speed machine will be smaller and less expensive than the lower-speed machine. Therefore, it is common to use gearing to reduce the machine size and cost in many systems. Mechanical gearing, however, introduces acoustic noise, maintenance requirements and reliability concerns into the system. On the plus side, magnetic gears can perform the same function as mechanical gears, transferring power between low-speed, high-torque rotation and high-speed, low-torque rotation, without relying on mechanical contact for this power transfer. This non-contact operation allows magnetic gears to avoid some of the issues associated with mechanical gears, and reduce the size and cost of an electric machine, especially in applications where minimal maintenance and high reliability are important (e.g., wind turbines and electric vehicles).
Additionally, by integrating the magnetic gear with the electric machine, the system size and cost can be further reduced. The Advanced Electrical Machines & Power Electronics Lab (EMPE) at Texas A&M University invented a configuration that places an axial flux machine in the bore of an axial flux magnetic gear to produce a very compact device capable of producing very high torques at low speeds.
Exploded view of a compact axial flux magnetically geared machine
To build the prototype, the EMPE lab designed a magnetic gear to build around a commercially available axial flux motor. It used ANSYS Maxwell electromagnetic field simulation software to evaluate the torques and forces in thousands of axial flux magnetic gear designs. Next, to size the structural components to withstand the strong magnetic forces between the two rotors, the lab employed ANSYS Mechanical structural simulation software. The resulting prototype performed very smoothly and produced torques that agreed with simulated values within one percent.
Simulated flux densities in the magnetic gear
Mechanical deformation simulations of the prototype (front and back views)
Compact axial flux magnetically geared machine on its test bed
Comparison of simulated and measured torque on the prototype’s low-speed rotor