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Comment by jbay808 | Hacker News Reader

Motor torque capability generally scales in proportion to volume, but shaft speed capability typically increases as the motor shrinks, so the motor power will usually fall at a smaller rate. A motor with 1/64 volume will probably output between 1/30th and 1/16th of its power. So 20 horsepower might barely be achievable. It all really depends on the driver though.

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Why don't you see a lot of very long thin, high revving motors for higher power with smaller size than large motors? This one has completely the opposite scale of dimensions even.

Heat dissipation mostly. Square-cubed laws are brutal for getting the heat out.

Long and thin - volume and area increase proportionally with length. Think of 10 small motors put together end to end.

So, mainly two reasons that I am aware of. First, with a long spindly motor running at high speeds, eventually shaft whip becomes an issue; the distance between the bearings ends up being really far. Second, designers usually want low-speed torque more than super-high-speed power, and at high diameters you get a bit more torque than small long motors of equal volume, because torque scales more like the volume of a somewhat hollow cylinder rather than a fully solid cylinder.

For most brushless motors--though not axial flux motors like the one under consideration here--it's much easier to get heat out by passing air axially across the stator. This means that while the surface area is increasing proportionally with length, your ability to flow air across that surface area is not keeping up. For that same motor volume, a shorter and fatter motor will have better airflow and cooling than a long skinny motor.

Moving coils at faster surface speeds require faster switching at less efficiency. Extra diameter for same volume buys extra performance for free.

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