Special commendation: Electric avenue

A team with a holistic view towards vehicle installation has created an efficient and low-cost direct-drive motor

Judges’ special commendation for environmental technology winner/500Nm modular YASA motor/delta motorsport/ oxford university

It is becoming increasingly obvious that electric cars are going to be a part of our future. Whether powered by hydrogen fuel cells or charged up as part of a smart-grid system, all of these cars need electric drive-trains. The form of these drives is far from settled, though, and Delta Motorsport, a Silverstone-based company working on a four-seat electric coupe, turned to a research team at Oxford University to find a solution.

The Oxford researchers, at the Electric Power Group (EPG) in the engineering department, have an interesting track record in this area. Led by Prof Malcolm McCulloch, the researchers specialise in lightweight motors that produce high torque, based around a concept first suggested by environmental engineers at the Rocky Mountain Institute in Colorado: that electric cars must be as light as possible.

McCulloch’s projects include the Hyrban city car – recently launched by Riversimple in a local manufacture and leasing programme – and Morgan’s eye-catching LIFEcar, a retro two-seater electric sports coupe.

Both of these projects used direct-drive motors mounted on each wheel, but they were prototypes and therefore rather expensive to produce. Delta Motorsport approached the EPG to develop a high-torque, low-weight direct-drive motor that would be easy and cheap to produce in high volumes.

McCulloch’s research focuses on designing the electromagnetics that make the motor work; his team has developed a type of motor known as yokeless and segmented armature (YASA). The stator of these motors – the part that stays static but that generates the moving magnetic field that spins the rotor – is a ring made up of trapezoidal magnetic segments, each carrying its own winding, fixed together with a strong bonding material.

To make each of the segments, the team used a sintered magnetic metal powder and subjected it to high pressure to bond the powder together. The team came up with a motor weighing only 25kg but that can develop 500Nm of torque. Delta’s mechanical design then came into play, with a bearing arrangement that allows the 500Nm unit to be stacked together in parallel, with each unit acting as a ‘slice’ of the whole. The partners then developed a direct liquid cooling system for the stator, which ensures even cooling and allows the motor to run continuously at its peak torque capacity for long periods.

“Even fairly small increases in volume have yielded 20-fold reductions in costs”

Refining the YASA design has boosted the torque output over previous versions – the LIFEcar motor, for example, developed 120Nm. This means that four motors, one on each wheel, are powerful enough to drive a car directly, without the need for a step-down gearbox, which would make the car more complex mechanically and, crucially, heavier. It is also a very scalable design, thanks to Delta’s bearings – although 500Nm motors on each wheel would be sufficient, a high-performance car might have 1,000Nm or even 1,500Nm per wheel.

The modular design of the motor means that it can be broken down into subassemblies that can be manufactured together. Moreover, according to Nick Carpenter, technical director at Delta, further refinement, especially in the bearing design, has reduced the number of parts. ‘As well as needing fewer parts, the parts that are required can be smaller and lighter and involve fewer tight tolerances,’ he said. ‘This all comes together to give a significant decrease in weight, cost and complexity, while also improving the reliability.’

Carpenter claims that the system is one of the most efficient drive-trains ever produced, and work is now under way to reduce its cost even further. The cost of motors, he explained, is a major barrier to the take-up of plug-in hybrid cars and range-extended electric vehicles, where a petrol engine charges up a battery that runs electric motors rather than providing the torque to drive the car. Delta has already identified a number of components where cost reductions might be possible. Carpenter said: ‘Even fairly small increases in volume – 1,000 pieces – have yielded 20-fold reductions in manufacturing costs.

‘The project is a fantastic blend of skills: the deep understanding of the electromagnetic behaviour of axial flux electric machines held by the team at Oxford, alongside the motorsport-honed mechanical engineering experience and wider “holistic” vehicle installation view of Delta Motorsport.’