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When a fully loaded elevator is moving down its potential energy is decreased and I guess is somehow converted by the elevator traction motor into heat.

How exactly does it happen? What exact elements of the motor or other parts do the conversion and how?

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A typical three-phase induction motor has a synchronous speed and direction at which applied voltage will be precisely canceled by back EMF, and can operate in several modes:

  1. When the shaft is turning at precisely the synchronous speed (and direction) for a supplied voltage, as noted, no current will flow.
  2. When the shaft is turning faster than synchronous speed (but same direction) for a supplied voltage, the phase of voltage and current will be such that the motor feeds power back to the supply.
  3. When the shaft is turning slower than synchronous speed (but same direction) for a supplied voltage, the phase of the voltage and current will be such that the motor takes power from the supply. A stalled motor is a special case of this.
  4. If the shaft is turning in a direction opposite the applied voltage phases, the motor will turn all of the supplied electrical power and mechanical energy into heat, a condition known as "plugging". A motor which is plugging consumes more power and current, but generates higher torque, than one which is merely stalled. Plugging is generally bad, since it not only wastes energy, but it subjects the systems involved to very high mechanical and electrical stresses. While plugging might sometimes be useful in an emergency-stop scenario, in most cases a mechanical brake would be better.
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First, elevators have counterweights, so for some mid range load there is no net potential energy change in the elevator going up or down. However, a maximally loaded elevator probably has more potential energy than the fixed counterweights, so your question still stands.

In any case, the friction in the pulleys, motor losses, etc, all have to be overcome before there is any net gain. I don't know if this is typically the case when the elevator is fully loaded or not, but let's say it is.

At this point, it's at least theoretically possible to run the motor in such a way that it returns power (acts as a generator) while regulating the elevator speed. Whether this is actually done, I don't know. The few elevator systems I am familiar with don't do this, but these are older. If this is done, I expect it to be more prevelant recently where energy costs are higher and advanced motor control more accessible.

One system I know of works on hydraulics. It is only a 3 story building. There is a hole in the ground below the elevator that the piston fits into. I know that system is counter weighted so there is always net down weight on the piston. To go up, a electric motor drives a pump to raise the piston. To go down, it simply opens a valve to let the hydraulic fluid in the piston back onto the reservoir tank. In that case, the potential energy of the elevator heats the hydraulic fluid a bit as it passes thru the valve.

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  • \$\begingroup\$ Are you really sure about counterweight on hydraulic actuated elevators? Every elevator of this kind I've seen was not counterbalanced. \$\endgroup\$ – Axeman Nov 16 '11 at 15:56
  • \$\begingroup\$ @Axeman: Now that you mention it, I'm not sure. I thought there was a iron weight going up and down on a rack against the wall, but maybe I'm remembering a different installation. You are right, in hydraulic systems the counterwieght is not necessarily used. In any case, there is always positive down pressure on the piston, whether some weight is counterweighted or not. \$\endgroup\$ – Olin Lathrop Nov 16 '11 at 17:15
  • \$\begingroup\$ Getting electrical energy back from an elevator isn't done commercially as the energy you get back from potential energy is really small (that is, even assuming 100% perfect energy conversion). Mind you, that doesn't mean the energy isn't recovered...it is, in heat. If your building requires heat during the winter time, that potental energy returns as a minute decrease in your heating bill (not that far fetched, we actually use heat energy from lighting and computers when calculating a building's heating requirements...). \$\endgroup\$ – Faken Nov 16 '11 at 23:38

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