What are transverse flux motors?

In the vast universe of electric motors, there are a number of ways to categorize different motor types; by their power source (ac or dc), how they are constructed or assembled, or by their magnetic circuit properties.

For instance, most electric motors are characterized as being either radial flux or axial flux machines, which refers to the path of magnetic flux relative to the axis of rotation. Transverse flux motors are another type with their own unique magnetic flux properties.

By far the most common type of motor is the radial flux design. Here, the magnetic flux is perpendicular to the axis of rotation. Think of the most basic rotary motor design, either an ac synchronous motor or any dc motor with the motor shaft a part of the internal motor rotor which sits inside of a stator.

Axial flux motors, on the other hand, are designed such that the magnetic flux is parallel to the axis of rotation. As a result, these types of motors tend to me thinner and less bulky than radial flux motors. The paradigm example of an axial flux motor is the so-called pancake-style motor, which is flat and thin.

In contrast to both radial and axial flux motors, a transverse flux motor features a magnetic flux path that is transverse to the axis of rotation, meaning that it flows across the axis. In practice, such motors are built with a single winding for each phase that is wound circumferentially around the stator, or the axis of rotation. This allows the magnetic flux to flow in a number of different paths, including axially across the stator, along the circumference of the rotor, and radially through the gap between them.

Because a single winding for each phase is wrapped around the circumference of the stator, this allows the number of machine poles to be decoupled from the size of the windings themselves. This means that there is higher torque density as well a higher number of poles.

This is the main advantage or benefit of transverse flux motors, their high torque density and high efficiency. This makes them suitable in direct-drive applications that eliminate the need for gearboxes.

In fact, transverse flux motors are making waves in the robotics industry as well. Recently, ETM has introduced transverse flux motors into the robotics market. The company has said that its motor technology eliminates trade-offs between efficiency and size by decoupling magnetic flux from electrical windings, enabling the torque density of high-ratio systems with the efficiency and thermal reliability of low-ratio drives. The company claimed that its patented architecture provides up to 10x greater torque density than conventional motors.

Still, transverse flux motors do have some drawbacks, including commonly cited issues with cogging and ripple torque. Other disadvantages include a more complex mechanical design, making them more complicated and expensive to design and manufacture, as well non-linear dynamics (which complicates modeling of motor behavior) and a low power factor.

For a more detailed technical account of transverse flux motors, refer to this paper from the IEEE here.

And for some recent news on some more new motor technologies, refer to this article here.