Control panel testing

[nebiter 0]

The control panel is wired, the PLC and HMI far enough along to get the thing energized and start debugging. The primary power is 480VAC/3Ø and there is a 1.5KVA transformer for control circuits, chemical pumps, sensors, etc. The usual method is to open the fuse to the transformer primary and use a suicide cord to energize the 120VAC circuit and get everything working. There are no motor loads connected to the panel, it is still in the fab shop. If one were to close the transformer primary fuse and backfeed the transformer, one could generate a bit of 1Ø/480VAC to turn on the VFD's and include them in the configuration and final programming of things without bring in 480VAC/3Ø of primary power, eh?

Good, bad or indifferent?

[Mendon Systems 22]

The way you described it you would be backfeeding the 1.5kva transformer through the secondary windings. Don't you have separate protection to isolate the devices on the 120 volt secondary?

[nebiter 0]

" Don't you have separate protection to isolate the devices on the 120 volt secondary? "

Yes, the secondary is fused, that would be backfed too. In the other direction, the zero wire is fused, it being fed in the forward direction to energize the control components. The idea is to just tickle the drives enough to allow the PLC to see them without bringing in several amps of 480VAC/3Ø. 

[Mendon Systems 22]

OK ... I see what you are after.

Whether that will work or not depends on how much inrush current those VFD's draw. I would not be at all surprised if you wiped out some fuses. Is there a way that you could power up the VFD's one at a time?

[nebiter 0]

Yes they are individually fused. I am pretty sure that I would not be able to put any of them to run mode, but just to see the e-net work with them all and the settings are all good would be great before applying 480 to the box. I think I'll put a fuse on my test line cord going in and also my clamp amp to see what is going in. Might be a useful technique.

Edited 6 Feb 2018 by nebiter

[Joe E. 259]

Without any loads, you should get single-phase 480 on the phases that are connected to the transformer and nothing on the 3rd phase. Once there's a load, I would expect something weird to happen, depending on the load. I would also expect the drives to fault as soon as you power them up due to the missing phase. I wouldn't expect it to damage the drives, but I guess anything is possible. You might reach out to the drive manufacturer to see what will happen.

If you want to try it, I would isolate all 480V loads and then energize the transformer. Measure voltages with the 480V loads disconnected and then apply them one at a time.

Some drives have separate control power supply terminals, especially bigger/fancier ones. This allows the main power feed to be turned off while leaving the controls on and working. Do your drives have that option?

[nebiter 0]

No these drives do not have separate control power, the manual says:

"Three-phase input provides full rating. Single-phase input provides 35% rating on
three-phase drives"

So single phase should be no problem for the drive and there is not an input phase loss parameter to enable/disable.

I'll give it a go and report back.

[Joe E. 259]

Cool. What drives are you using?

[nebiter 0]

Well, it seems to be OK so far. I energized the panel without any drive fuses closed and the panel control circuits drew about 0.4A of 120VAC. Then I de-energized, closed the fuses to drive#1 and re-energized and it came up and I was drawing 2.5A. I then powered down and closed the fuses to another drive and current draw went DOWN to 1.9A! I powered down again and closed the rest of the fuseholders, powered up and current draw is 3.75A. I don't have any sensors attached yet, just the 1.5KVA xfmr, my 240W 24VDC supply, Compactlogix PLC, 15 inch Panelview (on 24VDC), e-net switch and the eight (!) A-B 525 drives. The drives are all 5HP and less. CLX IO light is steady so it must see the drives. Time to fire up RSLogix and take a look. This is definitely more tech-friendly than bringing 480/3Ø into the shop that someone might trip over or drop something sharp on. Current at 3.68A now and fairly steady. Time to get to work.