I started out by just wanting to make a very small drone. But I quickly realized that there was a limiting factor in just how small and light I could make any design: the motors. Even small motors are still discrete packages that have to be attached to all the other electronic and structural elements. So I began wondering if there was a way to merge these elements and save some mass.

I drew inspiration from how some radio systems used antennas made from the copper traces on a printed circuit board (PCB). Could I use something similar to create a magnetic field strong enough to drive a motor? I decided to see if I could build a motor of the axial flux type using electromagnetic coils fashioned from a PCB’s traces. In an axial flux motor, the electromagnetic coils forming the motor’s stator are mounted parallel to a disk-shaped rotor. Permanent magnets are embedded in the disk of the rotor. Driving the stator coils with alternating current causes the rotor to spin.

The first challenge was making sure I could create enough magnetic flux to turn the rotor. It’s simple enough to pattern a flat spiral coil trace and run current through it, but I limited my motor to a diameter of 16 millimeters, so that the overall motor diameter was comparable to that of the smallest off-the-shelf brushless motors. Sixteen millimeters meant I could fit only about 10 turns per spiral and 6 coils in total, arranged under the disk of the rotor. Ten turns just isn’t enough to produce a sufficient magnetic field. But the nice thing about PCBs is that it’s pretty easy today to make one with multiple layers. By printing stacks of coils, with coils on each of four layers, I was able to get 40 turns per coil, enough to turn a rotor.

A bigger problem emerged as the design progressed. In order to keep a motor spinning, the dynamically changing magnetic field between the rotor and stator must be synchronized. In a typical motor that’s driven by alternating current, this synchronization arises naturally due to the arrangement of the brushes that electrically bridge the stator and rotor. In a brushless motor, control electronics implementing a feedback system are required.