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For those wondering what a seemingly marginal difference would mean in real world examples, take a look at this video from a conference on electric aircraft design a year ago:

High Torque, Low RPM Motors – Key Enablers of Quiet Flight https://www.youtube.com/watch?v=jXd4M_pZl78

I watched about half that. Interesting bit that newer high bypass turbofans need gearing to match the turbine optimal shaft rpm to the fan's lower rpm and higher torque.

Brings up

   High torque gears -> weight
   High speed gears -> maintence hassle
If I get this talk right the hope is the difference in weight penalty for an advanced direct drive electric motor vs a gear train is nil. This guy is pushing a very high efficiency design that minimizes magnetic and resistive losses while being capable of high torque.


Random OT trivia, but the first turbofan to fly was geared, and there have been commercial geared turbofans in use at least since the early 70s.

It's always been a trade off of fewer compressor stages vs. not having gears. Either of extra compression stages and gears add weight and reduce efficiency, so it's always a tradeoff.

Transferse flux motors (from the talk) have a "too good to be true" ring to them. I wonder what will be of them in 5-10 years.

Increasing the torque density compared to the current state-of-the-art would be a big deal for robotics applications.

In the latter part of the researcher mentions using different materials for the core of the motor to limit flux losses. Some of the efficient materials (cobalt in particular) are costly and would drive up the bill of materials cost of the motor compared to conventional motors, where copper is usually the biggest expense. One of his points was that a lot of the cost is currently around designing the motor to suit a given application, unlike existing electric motor designs where you can basically look in a book to see how it should be sized.

Aviation is probably the sweet spot application for this motor because decreased weight means increased payload and money. The increased cost is less of a concern than in, say, your cell phone. If you could run the motor without a gearbox and without active cooling, that would save costs in other areas and also decrease maintenance costs.

The "too good to be true" part of aviation is if it, together with battery technology, is able to keep airplanes in the air for a significant amount of time. The high energy density of aviation fuel means that we can build a "good enough" efficiency jet engine by building it like we do today, and just throw fuel at the problem to keep an airplane in the air. With electric propulsion the energy envelope is much tighter and we basically need these miracle solutions in battery tech, motor tech, or both, to make it viable. The presenter's premise was that an airplane that currently takes 180 horsepower to stay in the air, could do the same work with 90 horsepower if the motor and fan were designed differently. Many people are looking at the battery side of the equation, but not many people are looking at the motor side of the equation and there are significant gains to be had here. He then presents evidence indicating that designing a motor and fan differently is possible, and that current turbofan designs are optimized to work with the constraints of jet engines (ie, needing gearboxes to generate enough torque to run bigger and bigger fans).

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