Axial flux? Transverse flux? What do these terms mean? Why do they matter? What is flux, anyway? Here’s a vocabulary primer for electric marine propulsion.
Radial Flux Motor
To visualize a radial flux motor, think of two iron cylinders. One is fitted with magnets set around its outside and an axle running through its center (the rotor). Surrounding this cylinder is a larger one (the stator). Inside the stator cylinder is a series of ridges wound with copper wire. The wires are connected to a switch—the commutator—that alternately changes the current direction.
The alternating current created by the commutator induces magnetic fields around the stator’s windings. These alternating pulses oscillate between south and north at 60 cycles per second (aka hertz). This force—the flux—acts at right angles to the rotor’s permanent magnets, forcing it to turn. Because the pulses come so quickly, the rotor turns at full power immediately, providing the instant torque and acceleration for which electric vehicles are known.
While relatively bulky and heavy because of the metal yokes around their stators and rotors, radial flux motors are easy to manufacture and require minimal maintenance over long service lives. Torqeedo manufactures radial flux marine motors.
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Axial Flux Motor
Next, imagine an electric motor with a flat disk of nonferrous material serving as the stator. Set around its perimeter are a dozen half-inch-high stubs wound with wire. As before, alternating current feeds the windings. Facing the stator (like stacked pancakes) is another 6-inch disk—the rotor—with its permanent magnets fastened around its perimeter. The oscillating magnetic poles of the stator react with the poles in the rotor’s permanent magnets, attracting and repulsing them. The flux force created causes the rotor and its shaft to turn, but notice that its flux acts in parallel—axial—with the axis of the shaft.
The lack of a yoke and locating the magnets away from the central axis result in higher power-to-weight ratios than equivalent radial flux motors. The EVOA E1 and Acel outboards use axial flux motors.
Transverse Flux Motor
A transverse flux motor runs on more-complex three-dimensional paths of magnetic flux. Instead of copper wire wound around stator ridges, their coils run circumferentially, at right angles to the axis of rotation. The 3D flow of magnetic flux occurs axially through the stator, radially through the air gap between them, and circumferentially through the rotor. The rotor has multiple permanent magnets axially in a ring around the motor’s shaft. The stator arrangement involves laminated U-shaped structures securing a ring-shaped coil around the motor.
Transverse flux motors are even more complicated than axial flux motors. There’s still only one moving part—the rotor—but more magnets, wire, and structures to hold them precisely in intricate positions. Transverse flux motors offer the highest torque and power density. Mercury Marine’s Avator outboards are transverse flux motors.
