Radial and axial flux motors

In a recent issue of Race Engine Technology magazine, there was an article on the propulsion motor being used for the Drayson Racing electric car project. It is short in length and low in mass, and if you have read the article and seen the pictures of this ‘yokeless and segmented armature’ machine you will have noted that the motor, while axially compact, is not so compact radially.

The motor’s architecture is a variation on what is known as an axial flux, or axial gap, machine. Such motors have a notably small ratio of axial length to diameter, and are sometimes described as ‘flat’ or ‘pancake’ motors because of this aspect ratio. They develop high torque, but their speeds are limited owing to the rotor’s construction. This is not a particular problem though for ‘wheel motors’ that provide propulsion directly to each driven wheel, as they don’t need a large speed reduction. Axial flux motors also have a higher proportion of electromagnetically active material, so the percentage of winding mass which develops no torque, but which still generates losses, is lower in axial flux machines.

Most motors for hybrid systems are of the radial flux type, where (usually) a rotor fitted with permanent magnets spins within a wound stator. There is a small gap between the outer surface of the rotor and the inner diameter of the stator, so these motors are known as radial gap or radial flux machines, and can develop high power owing to their ability to run to high speeds. The output of the new Formula One engines for 2014 will be augmented by the use of sophisticated energy recovery systems; one of the electric motor/generator units will spin at the same speed as the turbocharger shaft, which might be as high as 125,000 rpm.

So, even though the casual observer might think that all electric motors used in motorsport might be very similar, there are two distinct ‘branches’ – axial flux motors that drive the wheels directly or with only a small speed reduction, as we might find in pure electric racing vehicles; and radial flux motors, which will be capable of higher speeds and which lend themselves to hybrid applications such as KERS, where they drive and take power from the engine or the transmission. Of course, dividing lines are not always absolute –  there are, for example, high-torque, relatively low-speed radial flux machines that have been used for pure electric propulsion, although these are in a minority.

It is likely that these two branches will continue to co-exist, as they use many of the same design strategies in high-performance applications. For instance, liquid-cooled stators, which allow high current densities in the motor windings, have been used in both axial and radial flux motors. Material choices are again similar – high-strength permanent magnets, high-conductivity coppers, and stator materials that saturate at high levels of magnetic flux, are required for both kinds of motor if optimum performance is to be achieved.

Written by Wayne Ward