Elon Musk: Cooling, not power-to-weight ratio, is the challenge with AC induction motors

Tesla Model S Motor Windell Oskay copy

Elon Musk’s recent interview at the MIT AeroAstro Centennial Symposium made headlines for his warning against the careless development of artificial intelligence, which he likened to “summoning the demon.” However, while he didn’t say a lot about Tesla, he did make an interesting comment about the challenges of designing electric motors.

A questioner asked (around 41:30 in the video below) how Tesla achieves such a high power-to-weight ratio – the Model S’s motor generates 362 horsepower (according to the official specs), and only weighs 70 pounds. 

“If power-to-weight ratio is of interest to you, rocket turbo-pumps really take the cake,” said Musk. “The turbo-pump on the Merlin engine generates 10,000 horsepower and weighs 150 pounds (fuel efficiency is a separate question).”

As for electric motors, “If you have a properly designed AC induction motor, getting a high power-to-weight ratio, a great response rate, low latency, extremely low ripple current…kind of comes naturally to an AC induction motor.”

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“The bigger challenge is cooling it effectively, particularly cooling the rotor, because you’ve got this rotor going at 18,000 rpm. In the Model S, we coaxially cool the rotor in order to have high steady state. Also for an electric motor, it’s easy to get peak power for a short period of time – it’s hard to have sustained peak power, because you overheat, and it’s hard to get high efficiency over a complicated drive cycle. Those tend to be the problems we wrestle with.”


Source: MIT Aeronautics and Astronautics Department
Image:Windell Oskay/Flickr

  • Steven Alcorn

    Speaking of power to weight ratio, I always see the energy density or specific energy comparisons made between batteries and gas, but I feel it would be more appropriate to compare both the mass and volume of not just the energy sources, but the entire delivery systems. By that I mean comparing the batteries+inverters+motors against the fuel+engine+transmission+exhaust+etc. to see how the total systems compare with respect to weight and space. Has this been done for EV and ICE systems of comparable power? I am very curious to see those numbers.

    • ned_plimpton


      In terms of space, look no further than the usable space in the Model S vs. a comparable Jaguar. That’s a bit anecdotal, I’ll admit. In terms of weight, a total system comparison would certainly benefit EVs, no?

      • Steven Alcorn

        I agree that from a space perspective it seems like a slam dunk, but from a mass perspective I would like to see a Model S P85 vs. say a comparably powered RWD Panamera.

    • http://www.electric-car-insider.com electric-car-insider.com

      The Electric Car Insider EV Buyers Guide contains these numbers for every electric car sold in North America since 2010. You can get a copy at:


  • Tony Stanley

    Thats pretty much how over boost turbo engines work. There is a sustainable turbo boost, in my car that is about 8psi. It will do it all day. But raise it to 13psi, you get alot more power but the components like the turbo housing and exhaust manifold get very hot. Modern cars use electronic controlled overboost to achieve big powers from anything up to 20psi but they are NOT sustainable. There is temperature measurement which backs off the boost after 30s or so. There will always be demand for ‘over power’ for the thrill factor, if you don’t provide it, others will hack it and stretch it out. Just make sure you include sanity checks, or temperature measurement and pull back.