Regenerative Elevator Drive Back-feeding a Generator

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Tainted

Tainted

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I'm doing as much research about it and there seems to be not much on the internet about it.
In newer model elevators when an elevator is accelerating downwards by gravity, it will generate the energy to miscellaneous loads by using regenerative drives rather than dissipating it as heat traditionally.

The issue is when you connect the elevator to generator, the energy will go back to the generator and it will cause damage to it over time. One known way to counter this would be to install a load bank... but even if I did, how will that even work? How will the energy know to go to the load bank rather than the generator?

I found this article which was actually a little helpful:
https://cdn2.hubspot.net/hubfs/2004...s-of-regenerative-elevators-on-generators.pdf
 
Tainted said:
I'm doing as much research about it and there seems to be not much on the internet about it.
In newer model elevators when an elevator is accelerating downwards by gravity, it will generate the energy to miscellaneous loads by using regenerative drives rather than dissipating it as heat traditionally.

The issue is when you connect the elevator to generator, the energy will go back to the generator and it will cause damage to it over time. One known way to counter this would be to install a load bank... but even if I did, how will that even work? How will the energy know to go to the load bank rather than the generator?

I found this article which was actually a little helpful:
https://cdn2.hubspot.net/hubfs/2004...s-of-regenerative-elevators-on-generators.pdf
Click to expand...
Interesting ill check that out.

Wait a sec, so an elevator can sync with the grid and backfeed the AC system in a building?
So if I ride down an elevator at 3am when the building has little to no load, could it export a little burp of energy onto the grid?
 
The energy doesn't 'know' to flow to the load bank rather than the generator, but this doesn't matter.

The generator only cares that the total energy flow is out of the generator.
 
tortuga said:
Interesting ill check that out.

Wait a sec, so an elevator can sync with the grid and backfeed the AC system in a building?
So if I ride down an elevator at 3am when the building has little to no load, could it export a little burp of energy onto the grid?
Click to expand...
it exports the load generated by gravity to the loads on the entire system as a whole, not sure how it works. Kinda like a cogen but even with cogens I scratch my head on how we could prevent damage from cogens due to the back-feed
 
You can’t push energy back into a generator. What is it going to do, turn the electricity back into fuel and pump it back into the tank? The energy from regen drives must be used within the facility. It can only go back on the grid if the user has an NEM contract and to get that, the drive must be UL listed under UL-1471 as “line interactive” that will disable the regen if the utility line is dead (to avoid killing line workers) and most VFDs do not have that, the liability insurance is too much for most mfrs with a lot to lose.

If you have regen drives on an elevator that is powered by an emergency generator, you must have have either a load bank to dissipate that energy, or a backup set of braking resistors on the drive. Most of them just have the braking resistors, but it usually means the elevator will have an emergency routine that limits its use when on the generator because the resistors have limitations on duty cycle.
 
Jraef said:
You can’t push energy back into a generator. What is it going to do, turn the electricity back into fuel and pump it back into the tank? The energy from regen drives must be used within the facility. It can only go back on the grid if the user has an NEM contract and to get that, the drive must be UL listed under UL-1471 as “line interactive” that will disable the regen if the utility line is dead (to avoid killing line workers) and most VFDs do not have that, the liability insurance is too much for most mfrs with a lot to lose.

If you have regen drives on an elevator that is powered by an emergency generator, you must have have either a load bank to dissipate that energy, or a backup set of braking resistors on the drive. Most of them just have the braking resistors, but it usually means the elevator will have an emergency routine that limits its use when on the generator because the resistors have limitations on duty cycle.
Click to expand...
I'm currently trying to look for a drive that has a braking resistor with the ability to regenerate on command. Once the utility goes out and the generator is on, it would send a signal to the drive to disable regeneration. But I don't even know products out there to do that if you have any suggestions let me know. I'm not even sure yet how to tie into the VFD because I don't know the products available.
 
If the elevator regen tries to backfeed into a generator the generator becomes a “motor” and can over speed the engine causing damage
 
A backfed induction machine might reverse torque and speed up a little, but it won't exceed its synchronous speed by more than its slip, which is seldom more than a few percent. An engine rated for, say, 3600 rev/min is not going to be damaged by turning at 3700, 3800, or even 4000 rev/min.

But it's pretty doubtful that a standby generator is going to be an induction machine. It's almost certainly going to be a self-excited synchronous machine, which isn't going to speed up, no matter how much 60-Hz power you backfeed into it.

Which is not to say that it won't be damaged, and might be why Kuether's whitepaper says, "Most generator manufacturers indicate that their generators can handle up to 10% of regenerative loads ... ".

Elevator manufacturers who offer regenerative-descent mode have almost certainly encountered this by now an figured it out. We should ask one of their application engineers.
 
In the solar bidness we never connect systems such that the PV and a generator can be on the same AC bus at the same time. I believe that the control circuitry in the generator is the vulnerable component.
 
I may be dismissing the issue without enough thought, but it looks like something that could theoretically be an issue but probably isn't an issue in real application. As was mentioned above, the generator only cares about the total power it sees. Any regeneration would only be seen as a reduction in load unless the regenerated power exceeded the power being used by the rest of the system. I don't see that being too likely during a power outage, but I suppose it is possible. As was mentioned in the white paper linked above, the more likely scenario where the regenerative power could be an issue would be a generator test that transferred only the ATS with the elevator(s) on it and then had a car descend. Staff training could be used to mitigate the likelihood of that happening.
 
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