A miniature DC brushless axial motor showing the integration with PCB construction techniques. The rotor shown to the right is magnetized axially with alternating polarity.

An axial flux motor (also known as an axial gap motor, or pancake motor) is a geometry of electric motor construction where the gap between the rotor and stator, and therefore the direction of magnetic flux between the two, is aligned parallel with the axis of rotation, rather than radially as with the concentric cylindrical geometry of the more common radial gap motor.[1]

Although this geometry has been used since the first electromagnetic motors were developed, its usage was rare until the widespread availability of strong permanent magnets and the development of brushless DC motors, which could better exploit some of the advantages of the axial geometry. The axial geometry can be applied to almost any operating principle (e.g. brushed DC, induction, stepper, reluctance) that can be used in a radial motor, and can allow some topologies that would not be practical in a radial geometry, but even for the same operating principle there are considerations in the application and design that would cause one geometry to be more suitable than the other. Axial motors are typically shorter and wider than an equivalent radial motor.

The rotor construction of a Lynch motor, a type of axial brushed DC motor. The brushes and connections are seen in the centre, the alternating permanent magnets (not shown) align with the segments of the rotor in which current is flowing.

Yokeless axial flux motors provide excellent torque densities. Axial motors have been used for some time for low-power low-cost brushless DC motors, since the motor can easily be built directly upon a PCB, or even using PCB traces as the stator windings, but recently there have been more efforts to design high-power brushless motors in an axial geometry.[2] A successful brushed DC axial motor is the Lynch motor, where the rotor is almost entirely composed of flat copper strips with small iron cores inserted allowing very power-dense operation.

Benefits

  • A motor can be built upon any flat structure, such as a PCB, with only the addition of coils and a bearing.
  • The coil winding process may be significantly simpler, as well as the process of joining the coil and core.
  • Since the coils are flat, rectangular copper strips can more easily be used allowing high-current windings to be simplified.
  • It is often possible to make the rotor significantly lighter.
  • The rotor-stator gap can be smaller since it is not affected by centrifugal forces and can be adjusted after construction.
  • Potentially shorter magnetic path length.
  • Most structural components are flat and can be produced without specialised casting or stamping tooling.
  • Since the magnetic path through the windings is straight, grain-oriented electrical steel can be easily used, having higher permeability and lower core losses.

Detriments

  • The rotor is typically much wider, causing increased rotational inertia, and centrifugal forces which can reduce the maximum rotational speed.
  • Uneven flux distribution due to wedge-shaped segments.
  • Since the segments narrow towards the centre there is less room to arrange windings and connections there.

References