Axial-Flux Motors and Generators Shrink Size, Weight

Today, most electric machines operate with radial flux (RF). Magnax offers a new concept, though—a machine that operates with axial flux (AF). Figure 1 provides a comparison of the RF and AF technologies.

1. Comparison of radial- and axial-flux machines.

Magnax says the key to the high power density is its motors’ yokeless AF design with two rotors at each side of the machine. The yokeless AF motors have a shorter flux path, and permanent magnets are farther away from the axis, resulting in greater efficiency and leverage around the central axis.

Furthermore, thanks to the axial flux design, very little copper is wasted on overhanging loops on the windings. The motors have zero overhang; that is, 100% of the windings are active.

The technology is scalable from small motors (i.e., electric vehicles) to large generators (i.e., wind-turbine generators).

According to the company, a 100-kW axial-flux permanent-magnet (AFPM) generator for a mid-size wind turbine offers the following benefits:

• Increased efficiency (+96% efficiency, up to 97% for larger generators).
• Reduced length (5X to 8X shorter than traditional wind-turbine drive trains).
• Reduced mass (2X to 5X lighter than geared or traditional geared direct-drive generators).
• Reduced resources requirement (½ to ⅓ of materials required vs. traditional RF direct-drive geneators, which also results in lower costs).

Design features of the Magnax axial-flux machine include:

• Dual permanent-magnet rotors, for the highest possible torque-to-weight ratio.
• Yokeless stator, for the shortest possible flux paths.
• Rectangular cross-section copper wire, for 90% possible copper fill factor.
• Concentrated windings, for the lowest possible copper losses (no coil overhangs).
• High-performance grain-oriented electrical steel (ThyssenKrupp GOES), lowering the core losses by as much as 85%.
• A patented system for cooling the windings, for the lowest possible stator temperatures.
• The entire manufacturing chain is based on low-cost, easy-to-scale production processes.

AFPM machines perform well at a very broad range of rotational speeds, which makes them suitable for high-speed-low-torque and low-speed-high-torque applications.

2. Here’s a comparison of the BMW I3 motor and the Magnax AXF225.

AF machines are more compact because they’re much more effective from an electromagnetic perspective than RF machines, which is often crucial for built-in applications, such as in vehicles. The slim and lightweight structure results in a machine with a higher power and torque density....