Oxford University graduate students have been working on a yokeless and segmented armature (YASA) motor for the last several years. The unique construction simplifies the motor and provides a lightweight (11 kilogram – 23.2 pound) unit that, as shown, puts out 40 kilowatts (53.6 horsepower). Computer simulations show the motor could put out up to 150 horsepower, over six horsepower per pound, although the current version is being held to no more than 75 kW.
Originally intended for the Morgan Lifecar, a hydrogen fuel cell powered retro-styled disguise for futuristic high technology, a pair have been mounted to drive the rear wheels of a Westfield Lotus 7 replica. This extremely light sports car is also a retro ride, having been featured as Patrick McGoohan’s homebuilt car in the 1960’s TV series, “The Prisoner.” Even prisoner Number Six’s car couldn’t perform like this, and its ability to perform four-wheel drifts would be the pride of any dirt-track racer.
YASA’s web site explains the motor this way:
“The Yokeless And Segmented Armature (YASA™) topology is a new type of axial flux motor that shows a step change improvement in torque density when compared to other axial flux motors. The topology is based around a series of magnetically separated segments that form the stator of the machine. The step change in the specific torque of the motor (20Nm/kg which is typically at least 2 times better than the best alternatives) comes from the combination of patented improvements in the magnetics, the cooling and the packaging of the motor.”The 500Nm YASA™ motor has been developed specifically for electric and hybrid vehicles. The motors are compact (34cm in diameter and 7cm wide), and fit within the space of the front or rear differential of a typical vehicle (the differential is no longer required). The motors output 500Nm per ‘slice’, with a peak power of around 75kW. This means 2 motor slices are enough for most vehicles. For example the Westfield iRacer uses two motors mounted at the rear of the vehicle and is targeting a sub 6s 0-60 time.”
The motor is being produced by a spin-off company using the Oxford University design, and as detailed in The Engineer, “Oxford YASA Motors, recently received government and private investment totalling approximately £3.4m to redesign its motors for the wider automotive and industrial market.” Besides its Lifecar use, it also powers the Riversimple, a thoroughly modern fuel-cell powered electric car with a motor on each wheel; and is being incorporated in the Delta Project, another leading edge automotive development.
The Engineer quotes the YASA head, “”With the Morgan LIFEcar, we were told the car was required to do an emergency stop from 70mph to 0mph in less than 75m,’ he said. ‘When you do the sums you find you need 0.7G worth of acceleration just to stop the vehicle at that speed, and you need 350 Newton-metres per wheel to do that. We were given a budget of just 20kg per motor.’ In the end [Tim] Woolmer (Head of Technology) and [Malcolm] McCulloch (Head of Oxford’s Electric Power Group) were able to achieve a 13kg motor and 7kg gearbox that could deliver 360 Newton-metres. While good enough for the LIFEcar, Woolmer said they knew a greater power density could be achieved.”
In an added note of cheer for readers of this blog, “[Woolmer] added the company plans to adapt the motor for aerospace and industrial applications.”