How does a two dimensional elevator work?

Modern VFDs can feed energy back to the grid when braking a motor. Some energy will be lost, but 70-90% may be recovered, depending on sizing and available loads.

The alternative to feeding power back to grid is to burn it off in braking resistors. This is a common practice in applications where feeding it back to grid is not feasible.

You could even do it without a VFD or active braking. The grid is rather stiff; it wants to stay at whatever frequency it is. If you power up a standard asynchronous three phase motor, it will rotate at the same frequency as the grid, minus some slip. If you however power the axle, so it want's to move faster than grid, the slip will become slightly positive.

But as the grid provides a large inertia, it will not change the frequency of the grid. Hoists using normal three phase motors without further braking will go slightly faster down than up, but will hold load perfectly fine.

Further to vidarlo's answer, these lifts will be controlled by some sort of variable frequency drives (VFD).

In a typical VFD deceleration causes regeneration and current is fed back onto the DC bus. This will cause the bus voltage to rise and at some point the voltage will reach the safe bus limit and the energy will have to be burnt off in a braking resistor.

Where multiple motors are accelerating, running and decelerating asynchronously it is common to use a common DC bus for a group of drives. This is relatively simple compared with feeding back into the grid which requires an inverter built into the drive. The idea is that the regenerated power is consumed by the driving motors and the overall effect is to reduce overall power consumption.

It also has to do it without risk of runaway/freefall.

Correct. This implies the use of brakes or motors with zero-speed holding torque. The least-wear / least power option would be to decelerate to zero speed and then apply the mechanical brake. The safety brake is, presumably, a mechanical centrifugal or standard lift / elevator brake.

And, it shouldn't just burn off all that energy as heat (terribly inefficient). So my question is, how is it done?

I think we've covered that.

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Thanks for the video link. It all looks horrendously complicated with a lot of maintenance.

I tried to think of an application for the system. A couple come to mind:

• With a simple two-car / two-shaft system one could have the cars travelling in a loop. The left door is always going up. The problem is that there is some lost time while the car traverses horizontally.
• It may be possible, as I think is shown in the video, to have multiple cars in each shaft. If the shafts operate as one-way streets with linking cross-roads every so-many floors it seems that a much higher capacity system could be created for a given pair of lift / elevator shafts. In a hundred-story building each lift shaft saved could create another 100 rooms. The economics then start to look rather different!