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I'm in the process of designing a new facility with about 50-60 VFDs in the 5-10 HP range, another 5 VFDs in the 100-225 HP range, and then one burly beast at 1000 HP. I've been looking into various technologies for that. One that a colleague had mentioned was a "regenerative drive VFD" for energy savings.

I understand how a regenerative drive works, in the sense of an electric vehicle, but I can't quite see how it works in a facilities sense. Especially in this sense where 1000 HP can very well be more than 50% of the active load at a given time.

  1. When the large motor would be commanded off, the VFD would back-feed the plant so would the "battery" in this sense be the load on the remaining plant?
  2. What if there isn't enough load online at the time that the VFD 'regenerates'?
  3. How do you factor this into the design?
  4. Does a regen-drive force you into a bi-directional meter?
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When the large motor would be commanded off, the VFD would back-feed the plant so would the "battery" in this sense be the load on the remaining plant?

Correct. If the plant was drawing 10kW from the mains during that time and the VFD was generating 5kW, then the draw from the mains would be about 5kW.

What if there isn't enough load online at the time that the VFD 'regenerates'?

Then it boosts the AC voltage up to some limit set by the VFD, and/or slows the motor deceleration, and/or shunts the excess current to a braking resistor network to prevent further AC voltage rise.

How do you factor this into the design?

Envisioning and planning all events that ever could happen. Including what happens when the power goes out (regen drives like to become generators!) If a condition exists like the above (shift change; all machinery shuts down including the 1000HP motor = large surplus of electricity) then this has to be considered and addressed. Really, a specialist should do this, as there are lots of details and gotchas.

Does a regen-drive force you into a bi-directional meter?

If doing something that could potentially back-feed into the mains grid, then maybe. But most facilities are not going to do this; the big motor stops in a few seconds and plant draw is always higher than regen. The only time a bidirectional meter would be needed is when it is known that regen will exceed demand. In that case the electric company may require the bidirectional meter (along with synchronization equipment.) Again, a specialist would cover all the bases.

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A regenerative drive only makes sense if it is going to be started and stopped frequently or if it is driving a high inertia that needs to be braked to a stop rather than allowed to coast.

  1. Yes, a regenerative drive would "back feed" the plant.
  2. If the VFD returns more power to the plant distribution system than the other plant loads are using, the excess would be returned to the utility.
  3. The regenerative power probably should not influence the plant distribution system design very much. When the large VFD is braking, the plant voltage may rise more than it would if the load was shut off and allowed to coast.
  4. A standard meter would probably accommodate the regeneration, but the utility might like to be advised of the nature of the load.
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