Currently reading: Under the skin: why hybrid makers love a biscuit tin
Axial flux motors are being developed as a new means of hybridising combustion engines
3 mins read
6 May 2019

If last week’s column was anything to go by, EV motors are far more interesting than they look. Once people get their heads around them, they may hold the same fascination that engines have enjoyed since they were invented. 

The electric machines (the correct name for motor-generators) in EVs today take various forms electrically, but the one thing they have in common is that they are all radial flux machines. Flux is the scientific name for the direction taken by the magnetic fields but, in simple terms, it means they are cylindrical in shape with a rotor spinning inside a cylindrical stator. 

There is, however, another type of electric machine emerging, and it’s one that some car makers are looking at intently, especially for integrating with a combustion engine to hybridise it, or tacking onto a small engine to make a range-extender generator. It’s the axial flux ‘biscuit tin’ motor. 

If biscuit tin sounds derogatory, it isn’t (and come on, who doesn’t like a Hobnob?). It’s the simplest way to describe the shape: short in length and large in diameter. This is useful because it means that in some applications they can use space more efficiently than a radial flux motor, such as when sandwiched between an engine and gearbox on a transverse engine. They have other important advantages, such as the ability to generate more torque than a radial flux equivalent. 

Whereas the rotor (which rotates) of a conventional radial flux machine is relatively small in diameter because it is housed inside the stator (which remains static), the rotor and stator of the axial flux machine are like two large dinner plates facing one another. So both plates’ magnets, the fields of which interact to generate torque and spin the rotor, are set much further away from the motor’s driveshaft. Because of that, the force they produce has more leverage on the main shaft. That means more torque, or the same torque for less power consumption. 

A good way to visualise that is to think of the steering wheel on a car. Imagine taking the wheel off and grasping the steering column with your hand. It would be pretty difficult to turn. Moving the effort you’re applying further away from the column – by using a wheel – gives you more leverage, or turning force. The same thing happens with an axial flux motor. 


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More companies are developing the technology, and one of those in the forefront is the Oxford-based firm Yasa. Its P400 electric machine can be sandwiched between an engine and transmission in a ‘P2’ hybrid configuration or be used standalone. At just 80mm thick, this sliver of a machine weighs only 24kg, develops peak power of 215bhp (160kW) and 273lb ft peak torque. Yasa also has a complete electric drive unit (EDU) concept comprising a motor, controller and two-speed powershift automatic transmission. The motor itself produces peaks of 402bhp and 368lb ft and yet it weighs in at just 85kg. That compares pretty favourably with the average four-cylinder engine, which weighs around 150kg without the transmission. 

The hub of the matter

It’s only a matter of time before electric machines move to the wheels, freeing up space and possibly changing the way cars look. The axial flux design lends itself perfectly to this. US firm Protean Electric has yet another design, its ‘inside out’ permanent magnet motor, with the stator on the inside and rotor on the outside.

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6 May 2019

Ferdinand Porsche would approve, although he might smile a little that it took so long to revisit his invention.

6 May 2019

 Certainly reads like the next step, and might satisfy drivers more.

6 May 2019

aren't they an unspring mass disaster?

7 May 2019
russ13b wrote:

aren't they an unspring mass disaster?

Absolutely.  Handling won't be a priority for the electric commuter boxes for which this technology might be used.

6 May 2019

Superficially seems interesting but ... where do the brakes go, cos inboard brakes are orrible to service, I know this !

6 May 2019

If you move the electric machine closer to or into the wheels there'll be an increase in unsprung weight and a penalty inthe ride/handling department. Engineering is always a compromise.

6 May 2019

It's nice that so many British firms are apparently right at the front of this technology...but how long before they're bought out by Chinese or German companies?

6 May 2019
Bazzer wrote:

It's nice that so many British firms are apparently right at the front of this technology...but how long before they're bought out by Chinese or German companies?

No strong opinions either way re Brexit buts its interesting China a fully independent country won't allow takeovers of key companies and if car companies want to sell in that market then they must go into joint ventures with Chinese companies... they protect their own industrial base .

6 May 2019

"Quick Buck" UK PLC and Government abandoned this policy a long time ago!

This country punches well above it's weight in the patents stakes, but instead of long term investment they will sell out to the highest overseas bidder.  Aerospace, Electronics, Consumer appliances, Pharma. We have pioneered and sold out time and time again.

Hate mentioning Brexit but maybe once we leave we "may" be allowed to support UK innovation with taxpayer subsidies, doubt it though as the shower we have running the place have proved they couldn't organise a piss up in a brewery let alone a shag in a brothel.

6 May 2019

'twas a lovely article. Much enjoyed the sophisticated mechanics & engineering Grazi editors! 


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