An axial flux permanent magnet synchronous motor (AF-PMSM) is a motor in which the magnetic flux is oriented axially in reference to the shaft rotation direction. The motor is constituted of at least one rotor and one stator, where normally the rotor has surface-mounted magnets attached to a rotor yoke, and the stator is made of coils placed on a ferromagnetic or non-ferromagnetic core.
Axial flux motor technology is not new; in fact, it has been around since the first electromagnetic motors were created some 200 years ago. The topology has been explored in different applications, starting with permanent magnets in the 1970s. Since then, axial flux motors have faced manufacturing challenges for a high-volume production, making it an unpopular solution.
Historically, this motor technology was used in stationary applications such as elevators or agricultural equipment. Recent improvements on magnetic materials and manufacturing processes, in conjunction with new electrification trends have brought AF motors back as an attractive market option.
The favourable performance characteristic of relatively high-toque in a compact envelope, caused by the big diameter and low saturation led the comeback to applications where this performance characteristic is required, such as low-speed/high-torque traction, automotive and e-mobility related, as well as laundry appliances. Their compact and lightweight properties, combined with a high torque to weight ratio makes them ideal for electric airplanes and EVs.
Most radial motors are cylindrical in shape, while an axial motor, or 'pancake' motor has a rotor and stator which are flat, resembling a stubby, disc-shaped motor, much like a biscuit tin. Flux is produced in an axial, rather than a radial, airgap. For this reason, an axial motor is ideal for applications with limited space constraints.
Axial flux motors can produce a higher torque than conventional motors of an equivalent size, so they are ideal for use in applications requiring high torque density. It is easy to assume that high power density is only achieved with fast speed, and not through torque density. At the core of a high-speed motor will be an increase in torque density, and an axial flux motor is a great solution here.
With an impressive list of benefits, this type of motor could be considered superior to some other applications. There are four reasons why an axial flux motor will supply more power while having a lower weight density. They are:
Whilst these motors are a fantastic choice for many applications, there are always points to note or consider when discussing or potentially using axial flux motors. There can be a higher cost involved, compared to a radial equivalent, along with added design and production challenges thrown into the mix. For example, it can be challenging to keep a uniform air gap between the rotor and the stator, due to the strong magnetic forces between them. There can also be bigger cooling problems in some types of axial flux motor, due to the location of the windings being deep within the stator and between the rotor discs.
Axial flux motors have a long history and were perhaps ahead of their time in terms of the technology available during their prime. Now, with improved technology and materials, the axial flux has made a comeback and is considered suitable for many applications today, specifically those requiring rapid speed changes or high torque densities. With major companies performing R&D, this tried-and-true motor technology may just be the go-to choice for new applications including new iterations of electric vehicles and aviation in decades to come.
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