Q: I’ve got a question about the Tesla automobile. I understand the drive motor is on the order of 250 horsepower and only weighs 70 pounds which is multiple horsepower per pound. I’ve worked in the transit industry and I’ve never seen a motor other than one that weighs multiple pounds per horsepower, the opposite way, so you have an advantage of an order of magnitude. Some of it can be explained by high speed, can you explain how you achieve that?
A: Actually if power to weight ratio is of interest to you, rocket turbo pumps really take the cake. The turbo pump on the Merlin engine generates 10,000 horsepower and weighs 150 pounds. Well, fuel efficiency is a separate question <laughs>. But power to weight, it’s at the ragged edge of pulling those molecules apart. It’s kind of amazing you can get 10,000 horsepower in this thing you can basically pick up.
For electric motors, if you have a properly designed AC induction motor, getting a high power to weight ratio and a really great response rate, low latency and all that, extremely low ripple current, it just kind of comes naturally to an AC induction motor. The bigger challenge is actually cooling it effectively, and then particularly cooling the rotor. You’ve got this rotor going at 18,000 rpm, so 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, but it’s hard to have sustained peak power, because you overheat. It’s hard to get high efficiency over a complicated drive cycle. Those tend to be the problems we wrestle with more than the peak power. We can get peak power pretty easily, but sustained power and efficiency over the drive cycle are hard.
