State of flux

Racecar investigates the differences between the axial flux and radial flux motors currently used in motor racing

- LAWRENCE BUTCHER

Electrified motorsport is here to stay, whether one is talking hybrids in Formula 1, the World Endurance Championship (and soon the World Rally Championship) or pure electric, pioneered by Formula E. This will soon be joined by series such as Extreme E, electric Touring Cars and Rallycross.

However, much like traditional internal combusion (IC) racing, there is no such thing as a standard architecture for electric motors, and in this article we will take a look at the two main topologies, axial flux and radial flux, exploring how and why they are different and their various pros and cons.

All rotating electric motors share two common components, a rotor and a stator. The stator is fixed and the rotor, as its name suggests, rotates. Commonly, the rotor sits inside the stator, though there are also external rotor motors, such as those found on some in-wheel applications.

Permanent magnets

For the purposes of racing, it is only worth considering permanent magnet machines, so induction motors will be left for production cars. In a permanent magnet motor, the rotor is constructed from permanent magnets, and these can be either surface mounted or internal. Surface mounting is more efficient, but creates a more complex problem when it comes to securing them at high rpm.

The principle of operation is based on the interaction between the rotating magnetic field of the stator (made up of multiple coils, which are energised) and the constant magnetic field of the rotor. According to Ampere’s law, this interaction creates a torque reaction, causing the motor to rotate. The speed of the motor is controlled by changing the magnetic field of the stator and, in the case of traction motors, via a variable frequency drive.