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Comment by 0xFFFE | Hacker News Reader

For vehicle of that size to have a controlled descent using only the magnetic field (from the generator) as the braking force, it has to be one massive magnet, no? In which case, it is also adding to the overall weight of the vehicle.

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It needs the same size motor as the one used to drive it to the top. The forces are the same, just in the opposite direction.

It weighs a lot more coming down, since it's full.

See my reply to the sister comment.

Ah, I see what you mean.

Apologies if it sounds too dumb, but on the descent you have additional mass of your cargo that needs to be opposed by equal amount of braking force (magnetic or otherwise), on the ascent you have an unladen truck which needs lesser energy to move. So essentially the magnet has to be sized for the descent not for the ascent. Or am I completely missing something?

It's not dumb at all.

But this truck would need to be able to back up, make u-turns, etc, while fully loaded.

So the motors must be strong enough to drive it uphill even when fully loaded, if only simply for the sake of positioning and maneuvers.

Since size isn't much of a constraint it probably uses electromagnets instead of rare earth magnets as fitted on cars.

Also trains have been doing it for decades.

Look into self-exciting generators as a way to (mostly) avoid big permanent magnets. Tldr:

> Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils.

Electric motors, like just about everything else, can get mechanical advantage through gearing. I.e. you can use a smaller magnet if it's geared to spin, say, 10 times versus 2 times per meter traveled.

Ever tried to open a magnetic lock?

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