What’s in a motor?

Behold: the motor generator unit that is (probably) powering Formula E.

A 26kg, 120kW electric motor has appeared on the McLaren Electronics Systems online catalogue. It bears remarkable resemblance to the one featured in The Engineer magazine last year, which was earmarked for both the P1 supercar and the Formula E Spark-Renault. And the vital statistics match those quoted by the publication too.

However, the motor that’s available to buy is rated at little more than half that of the Spark-Renault’s claimed peak power of 200kW; it is also less powerful than the unit in the P1, which produces around 130kW. Could it really be the driving force behind Formula E?

Yes – based on the circumstantial evidence, reckons Chris Vagg, an electric drivetrain expert and former Brawn and Mercedes AMG F1 engineer. But in order to realise nearly double the power, there must be more to the story. Here’s how McLaren may be using the same motor for all three applications.

Option one: beef up the motor

“Of the factors that limit the power output of an electric motor, three are significant here,” Vagg explains. “Firstly, thermal considerations: you can push through as much current as you want to get more out of the motor, but at some point you have to get rid of all that heat.”

So is the Spark-Renault running this motor with increased current and bigger radiators? Vagg doesn’t think so. “If cooling was no issue, we’d see the curves for continuous power and peak power sitting on top of each other on the graph,” he says. “They’re actually pretty close, which suggests that the motor is very well cooled when producing these figures. It’s not uncommon to see peak power twice that of continuous power on these sorts of charts.”

Electric motor performance courtesy McLaren Electronic Systems

The other two factors to consider are the stator and rotor components. “Increasing the amount of iron in the stator or the size of the magnets on the rotor could yield an increase in power, if you’ve allowed enough space to change these,” Vagg says. “The rotor would be the easier of the two to swap out.”

That’s a contender for what’s behind the McLaren magic, according to Vagg: “This has been a very fast development cycle, so it would make sense for the designers to have built in some allowances which would allow for the motor to be tuned and tweaked, depending on the application.”

Could that mean that we’ll be seeing a radically faster P1 thanks to upgraded electric motors? Probably not, says Vagg: “What you’d put in a race car, which will be constantly maintained and regularly stripped down and rebuilt, is not that same as what you’d put in a road car. In a motorsports application, McLaren may be willing to extend the operating window to tune the motor up – but I don’t think we’d see that appearing in the P1.”

Option two: twin motors

The other way to make up Formula E’s 200kW power is simply to use two motors.

“This might be the easiest route,” Vagg says. In fact, early technical specifications allowed for two motor generator units to power the Formula E racing car.

Electric motor courtesy McLaren Electronic Systems

But images from December’s track testing appear to show just one motor installed: “With the motor in a longitudinal configuration, which is what the picture shows, it would be no small task to add another one at this point,” he says.

Of course, we’re only seeing a partial picture. “There could be two motors housed in a larger, bespoke casing,” Vagg goes on. “They could be stacked one atop the other, essentially in series. That might require two inverters though.”

Coincidentally, a motor control unit that looks similar to the one used in the Spark-Renault has also popped up on the McLaren website. In the description, McLaren says that “bespoke variants can be provided to suit customer requirements” – so the Formula E car could well be running two motors controlled by one customised inverter unit.

Keeping us guessing

Using an upgraded rotor or deploying twin motors are both viable options, but there’s one last thing worth pointing out: the motor in the catalogue produces a continuous power of 110kW. Two of those together would produce 220kW – or 300hp, which is what development driver Lucas di Grassi said that the car produced in a video last month.

However it has been done, the consortium partners building the Spark-Renault have kept mum on the specifics of the car, and we probably won’t know any more about the details until at least May. In the meantime, enjoy the pictures.

 

Images courtesy of McLaren Electronic Systems.

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