Australian team advances EV motor design

RESEARCHERS at the University of NSW have developed an electric vehicle motor design that promises to tick a lot of boxes in terms of size, cost of production, reliability, safety, power output and efficiency.

The potentially game-changing electric motor design is being developed by a team headed by associate professor Rukmi Dutta and Dr Guoyu Chu at UNSW’s School of Electrical Engineering and Telecommunications.

Said to have the potential to increase the range of electric vehicles, the prototype motor has has achieved speeds of 100,000rpm

It is an evolution of the Interior Permanent Magnet Synchronous Motor (IPMSM), a design predominantly used in traction drive of electric vehicles.

Maximum power and speed achieved by this novel motor have successfully exceeded and doubled the existing high-speed record of laminated IPMSMs (Interior Permanent Magnet Synchronous Motor), making it the world’s fastest IPMSM ever built with commercialised lamination materials.

Importantly, the motor is able to produce a very high power density, beneficial for electric vehicles in reducing overall weight and therefore increasing range for any given level of charge.

A regular IPMSM type motor has magnets embedded within its rotors to create strong torque for an extended speed range but can suffer from low mechanical strength due to thin iron bridges in the rotors, which limits maximum speed.

The UNSW team has patented a new rotor topology that significantly improves robustness while also reducing the amount of rare-earth materials per unit power production.

Its design is based on the engineering properties of the Gyopo rail bridge, a double-tied arch structure in South Korea, as well as a compound-curve-based mechanical stress distribution technique.

And the motor’s impressive power density potentially offers improved performance for electric vehicles where weight is extremely important.

“One of the trends for electric vehicles is for them to have motors which rotate at higher speeds,” said Dr Chu.

“Every EV manufacturer is trying to develop high-speed motors and the reason is that the nature of the law of physics then allows you to shrink the size of that machine. And with a smaller machine, it weighs less and consumes less energy and therefore that gives the vehicle a longer range.

“With this research project we have tried to achieve the absolute maximum speed, and we have recorded over 100,000 revolutions per minute and the peak power density is around 7 kW per kilogram.

“For an electric vehicle motor we would actually reduce the speed somewhat, but that also increases its power. We can scale and optimise to provide power and speed in a given range - for example, a 200kW motor with a maximum speed of around 18,000rpm that perfectly suits EV applications.

“If an electric vehicle manufacturer, like Tesla, wanted to use this motor then I believe it would only take around six to 12 months to modify it based on their specifications.

“We have our own machine design software package where we can input the requirements of speed, or power density and run the system for a couple of weeks and it gives us the optimum design that satisfies those needs.”

The new IPMSM prototype motor was developed using the UNSW team’s own AI-assisted optimisation program which evaluated a series of designs for a range of different physical aspects – namely electrical, magnetic, mechanical and thermal.

It evaluates 90 potential designs, then selects the best 50 per cent of options to generate a new range of designs and repeats the process, until the optimum is achieved. The final motor is the 120th iteration analysed by the program.

Apart from electric vehicles, the motor has many other potential applications including large heating, ventilation, and air conditioning (HVAC) systems that require high-speed compressors to use a new form of refrigerant which significantly reduces the impact on global warming.

It can also be utilised in high-precision Computer Numerical Control (CNC) machines that are highly demanded by the aviation and robot industries. The UNSW high-speed motor technology can allow such high-precision CNC machines to mill or drill with minimal diameters.

Another application is as an IDG (Integrated Drive Generator) inside an aircraft engine to provide electrical power for more-electric aircraft systems.

The UNSW team’s new motor also offers a significant cost advantage over existing technology and uses fewer rare earth materials such as neodymium.

“Most high-speed motors use a sleeve to strengthen the rotors and that sleeve is usually made of high-cost material such as titanium or carbon-fibre. The sleeve itself is very expensive and also needs to be precisely fitted and that increases the manufacturing cost of the motor,” explained Dr Chu.

“Our rotors have very good mechanical robustness, so we don’t need that sleeve, which reduces the manufacturing cost. And we only use around 30 per cent of rare earth materials, which includes a big reduction in the material cost – thus making our high-performance motors more environmentally friendly and affordable.”

Car-makers are likely viewing these Australian developments with keen interest and a view to adopting such a motor in production EVs but choices are already available to car-makers relating to what sort of electric traction motor can be utilised.

Ferrari and AMG for example have been looking for an electric edge sharp enough to satisfy “production” supercar customers, which means high-end axial flux electric motors fulfil their requirements. 

Axial flux motors have the capability to deliver extra power out of a battery pack when a driver pushes the accelerator. It’s as simple as that. The motors are much smaller than predominantly used radial motors yet pack a more powerful punch.

By way of explanation, a conventional radial EV motor works by pushing hundreds, in some cases thousands of amps of electric current to copper coils. When these coils are energised, they become electromagnets with attractive and repulsive forces. The magnetic force created by a stationary stator surrounding a rotating rotor produces the torque that turns the wheels of the vehicle. 

On the other hand, in axial motors, rather than have a rotor spin inside a stator, disc-shaped rotors spin alongside a central stator. This leads the flow of current – the flux – to travel axially through the machine, rather than radially out from the centre. 

Since the motor generates torque at a bigger diameter, less material is needed thereby facilitating a smaller unit size.