What Is an Axial Gap Motor?
An axial gap motor is a motor that rotates by generating an attractive or repulsive force between a permanent magnet attached to a disk, which is fixed to a rotating shaft, and a coil facing it. The disk rotates like a record, creating motion.
The greatest advantage of this PM (permanent magnet) synchronous motor, which uses a permanent magnet as the rotor, is that torque loss is minimal, even when the motor is thinner than traditional radial gap motors.
Uses of Axial Gap Motors
Axial gap motors are used in robots, electrically assisted bicycles, drones, and various industrial equipment. Research and development are also underway to use them as in-wheel motors for electric vehicles and motors that drive electric airplane propellers.
Compared to radial gap motors, the greatest advantage of axial gap motors is that they maintain torque even when their thickness is reduced. Motors typically need to increase current to achieve greater torque, but axial gap motors can be thinner and still produce the same torque with less current than radial gap motors.
This feature has expanded their use in applications where thin motors are necessary due to space constraints, but power consumption must also be minimized.
Principle of Axial Gap Motors
Axial gap motors rotate a shaft using the magnetic flux generated by an electric current passing through a coil and the magnetic flux from a permanent magnet, which attracts or repels each other depending on the flux direction.
In conventional radial gap motors, a permanent magnet is attached along the axis of rotation, and several coils are wound in the same direction as the axis, arranged around the magnet in a cylindrical shape. When an electric current is applied, a magnetic flux is generated perpendicular to the axis, and the interaction between this flux and the magnet’s flux causes the shaft to rotate.
1. Radial Gap Motors
In radial gap motors, reducing the motor’s thickness decreases the height of the coil and magnet, which lowers the torque.
2. Axial Gap Motors
In axial gap motors, multiple magnets are concentrically arranged on a disk attached to the rotating shaft. Thin coils are arranged in the same concentric manner on a doughnut-shaped substrate opposite the disk. The coils are wound perpendicular to the axis of rotation, and the substrate has no contact with the rotating axis.
When an electric current is applied to the coils, a magnetic flux is generated parallel to the axis of rotation. This flux creates an attractive or repulsive force with the disk-shaped magnets, causing the disk and the axis of rotation to move.
By adjusting the disk’s diameter, or modifying the size, shape, and arrangement of the magnet and coil, it is possible to create a thinner, more energy-efficient motor with higher torque than radial gap motors. Research and development are ongoing to create designs that surpass radial gap motors in these areas.
Axial gap motors can achieve higher torque than radial gap motors, especially when the aspect ratio (motor thickness divided by the outer diameter) is less than 0.75.
How to Select an Axial Gap Motor
When selecting an axial gap motor, ensure it offers clear advantages over radial gap motors for the intended application. Compared to radial motors with the same output, axial gap motors tend to be more expensive and fewer pre-made models are available. Custom development by a manufacturer can be even more costly.
The advantages of axial gap motors include:
- They are structurally better suited for thinner motor designs than radial gap motors.
- They provide higher torque compared to radial gap motors of the same thickness.
- They can directly drive vehicle wheels and airplane propellers without requiring reduction gears, due to their compact yet powerful design.