YASA P400 series electric motors and generators are small and light, capable of delivering up to 370 N·m of torque and 160 kW of power from  an axial length of 80.4 mm.

Axial-flux technology gives EVs a boost

Motor developments for electric vehicles (EVs) often are shaded by the emphasis placed on battery capability. But the enduring need to deliver improved packaging, power, torque and range from EVs brings significant opportunity for new approaches to motor design and production.

The United Kingdom company YASA (the initials stand for yokeless and segmented armature), which focuses on axial-flux technology and associated controllers, is moving from specialist, low-volume EV/hybrid applications to series production in a new manufacturing facility, with projected ramp-up reaching 100,000 units annually by 2021.

A spin-off from Oxford University, YASA has received a tick in the box from the UK government with the official opening of the new facility by Business Secretary Greg Clark, who said: “YASA is a brilliant example of what can be achieved when government, academia and industry come together to turn the best ideas from the best minds into scale-up companies.”

Aston Martin, Jaguar, Nissan and Williams took prototype/research vehicles to the event, each of which incorporates YASA electric motors.

The magnetic path

Unlike regular electric motors, the axial-flux motor (also known as "pancake" motor), presents a different magnetic flux path, flowing parallel to the axle of the motor instead of radially. The company’s axial-flux motors use less copper, iron and permanent magnet materials than conventional radial motors, resulting in a smaller, lighter package offering greater power to meet driver demands.

Although axial-flux technology has been in existence since the 1950s, the challenges facing its application to volume production for vehicles has been considerable. The company’s founder (in 2009) Dr. Tim Woolmer, now its Chief Technical Officer, and his team tackled several technical hurdles, including developing low-cost volume-manufacturing processes for the windings in the stator and assembly of the rotors to the stators.

“YASA has overcome these challenges and offers motors with very high torque and power density,” says Chief Executive Officer Chris Harris. “The company has delivered axial-flux motors with up to 6.7kW/kg capability, these demonstrated in several high-performance concept cars.”

The concepts include the Jaguar C-X75 and the ultra-high performance hybrid Koenigsegg Regera, a low-volume production model revealed in 2015 that uses three YASA motors in combination with a 5.0-L twin-turbo V8. The motors in total produce 520 kW (697 hp) and 900 N·m (664 lb·ft). One motor of 160 kW (214 hp) operates as a starter-generator on the crankshaft and for torque infill; two 180-kW (241 hp) torque motors are positioned within the rear wheels, providing both drive and torque vectoring capability. Koenigsegg also claims the car's battery pack is the most power-dense ever created for a production vehicle. The Regera is a limited-edition model of 80 units.

In 2013 a Lola B12 69/EV Le Mans prototype converted by Drayson Engineering and using YASA’s axial-flux technology, set a lightweight electric car world speed record of 326.6km/h—and that year saw YASA’s axial-flux motor enter low-volume production.

The company’s axial-flux motors also were used for the Drive eO PPO3, which became the first electric car to become the overall winner of the Pikes Peak international hill climb in 2015.  It used six YASA-400 axial-flux electric motors delivering a combined 1000 kW (1340 hp).

Details of the company’s technology from a recent interview with CEO Chris Harris:

The EV technology spotlight is very much on battery development—but motor advances are essential, too; what are the salient benefits of axial-flux?

What’s unique about our axial-flux motor is that it is smaller and lighter than conventional radial motors. YASA’s axial-flux motor also offers significantly greater efficiency when integrated with a two-speed gearbox, providing vehicle manufacturers and their customers with the choice of either extended range on a single charge or reduced cost for the same range, as the greater efficiency reduces battery size requirements for a given range.

Can you provide some examples of its efficacy in terms of weight, cost, torque delivery and power output compared with alternative automotive electric motor technologies?

YASA’s motors are lower-cost than the competition for a given power and torque due to the topology of our motor, which results in the use of less copper and iron than conventional radial motors.  Our motors typically are half the size and a third of the weight of conventional radial motors and manufacturing and integration processes also are simpler than conventional radial motors, e.g. coil manufacture and integration resulting in lower processing costs in manufacture.

Do you see axial-flux having both pure EV and hybrid-electric roles, as well as for range extenders?

Our axial-flux motors are suited to pure EV, hybrid and range extender applications and we are currently working with customers in all three areas. The compact size of our motors (axial length less than 100 mm) makes them ideal for integrating into hybrid applications between the engine and transmission/gearbox where there is typically very limited space. 

The speed of our motors matches that of internal-combustion engines and combined with the small package sizes makes YASA’s axial-flux technology very well-suited to integration into a range-extender application.

The theory behind axial-flux solutions has been known for six decades; what have been the major challenges to prepare a convincing solution for high-volume production?

What makes YASA’s motor different from those that came before it is the topology. Instead of using a yoke for flux concentration, we use segmented pole pieces. We also directly cool the coils in the motor and mount the rotors off a central bearing that forms part of the stator. 

We use conventional manufacturing processes that have been proven in volume automotive applications (e.g. laser welding) but which have never been applied to electric motors previously. In this way we are able to offer the power and torque density benefits of our axial-flux motor using manufacturing processes that are robust and reliable and can scale to volume.

You recently opened a production facility with a capacity of 100,000 units. is it on stream now, and if so in what numbers?

We are initially introducing a production capacity of 20,000 per annum on a single shift. This will be in place by the summer of 2018 to meet customer commitments.  We anticipate reaching full capacity of 100,000 by 2021. We recently announced signing multiple long-term development and supply agreements with automotive OEM customers. We are not able to share the name of our customers due to confidentiality agreements, but we can say that they will go into series production.

As a spin-off business from Oxford University, does YASA maintain a close R&D association with Oxford's relevant departments?

YASA was founded eight years ago as a spin-out from Oxford University to develop and exploit novel electric motor technology based on the research that our CTO Tim Woolmer undertook at the university. Oxford was an early investor and has participated in every funding round to date. While we currently have a close cooperation with a number of academic institutes, including Coventry University and Oxford Brookes University, all our R&D is conducted at our new production facility in Yarnton, Oxford.

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