Plant Nominal Line Voltage Question

On Sat, 28 Oct 2006 18:51:39 -0700, Steve Cothran proclaimed to the world:

I have ran into this several times which has not seemed strange until now. Perhaps it is because a lot of my customers are factories in rural areas.

One factory I have done a lot of work with makes it standard policy to provide a separate transformer for each major piece of equipment. They run a medium voltage buss in the ceiling made from aluminum bar and tap off of this line. I am not for sure what their voltage is, but they reduce it to equipment voltage with a transformer mounted on top of the main disconnect for the equipment. The transformer is multi tapped so they can adjust the voltage to each application.

So a solution that comes to mind is to put a multi tap buck/boost transformer between your main disconnect/breaker and the equipment. Then set the taps to give you the o/p voltage you require. You can get transformers that drop the voltage from 480 to 460 without additional step down and have a +10% and -10% O/P tap. I have never had to order them but have seen them in use. If you have trouble finding one, give me a yell and I will ask the guys at the factory where they get them.

Here is a good description of Buck/Boost Transformers

This link goes into a little more detail

He is a place that sells surplus transformers that should have what you need.

I would insist that the input voltage be within some parameters in the contract with your customer. Make the liable for damage, not you.

Are you sure? That would be 18,000 amperes at 480 volts!

That is a good point, discouraging though it may be.

I don't know what part of the world you are located in, but 503 is not an atypical voltage for a US 480 volt system. Actually, in many, if not most parts of the US, the voltage specification for electric service provided primarily for power applications is plus and minus

10%, which would give you a window of 432 to 528 volts. The tolerance for primarily lighting and commercial loads is generally tighter.

My suspicion, and it is nothing more than that, is that your drives are of european design, and as such were really intended for 380 - 400 volt service, but were "stretched" by the manufacturer to tolerate

460. Their luck may run out when the tolerance is in the wrong direction.

Do your drives have pre-charge circuitry? Is it working properly? I have on rare occasions had a situation where a stuck precharge contactor causes havoc when power is applied. Connecting the bus caps directly to the power line through a 6% reactor will undoubtedly cause all kinds of transient voltages at the moment of turn-on.

You aren't starting and stopping the drives with the line contactor, are you?

That would be nice to have here...in this situation distribution panels for equipment are wired to a 15 MVA transformer. The transformer has a neat auto-tap which no one but Jesus is allowed to touch.

Someone has to buy and install these, and what I get from the plant engineer is "all our other stuff works ok". Might be worth mentioning that this is a shiny,brand-new plant.

That was my plan but was nixed by the plant EE for cost reasons. He's an EE, I'm not. So I lose.

I wasn't a party to the contract/specs and will not be liable for any damage...I'm a sub of the sub of the sub that put all this stuff together. I just hate to have stuff out there in the world that I think is gonna fail, that has my name on it.

Thanks for the info.

!!!!!!!! Isolation transformer between your drives and the XFRMR. Or maybe a seperate line reactor for each drive. You haven't stated all the specifics of the installation upstream of your tiny drives, but it really looks like a seriously low impedance path bewteen the drives and the large transformer that feeds them. I'd involve reps from the drive manufacturer and ask their opinion on whether or not an isolation XFRMR is needed. Usually I see these kinds of things result in inverter damage. I'm surprised that isn't the case...

On Sun, 29 Oct 2006 13:17:01 -0800, Steve Cothran proclaimed to the world:

I agree with BF, 503 volts is not that high for a typical plant. All the big hp stuff runs better at the higher end of the supply voltage range, which costs the most and so high supply voltage most likely is a better situation to have overall than low supply voltage.

Is all the stuff you are worried about fractional hp? I always use 240 or 120 volt open frame drives for the small stuff and supply a separate transformer to use off the 480 feed to the equipment. The transformer is small and inexpensive. It provides protection for all the smaller electronic from the spikes that come from the big stuff. It makes a huge difference in the life of the equipment.

I understand wanting to see your stuff last. Even if you do warn everyone, when the equipment fails, they remember the failure and the company that supplied the "junk", not the warning. Best you can do is make a recommendation in writing to condition the supply power. Pull a copy out and show them when it fails.

I still do not see why you are having breaker problems. Something is not right there. What is on the circuit other than the fractional hp drives?

On Sun, 29 Oct 2006 16:13:21 -0800, Steve Cothran proclaimed to the world:

Then in has to be the combined reactance/capacitance of the drives. The higher voltage increases the effects on charge current through the breaker. You need a precharge circuit added. I know you don't want to suggest doing this, particularly when it worked on the bench at the lower voltage. Get the drive rep involved. There may be a solution I don't know of. With Leeson, you might have to contact the factory rep.

It sounds like this will get fixed when some drives start burning up. The money and time suddenly become available when the line is down.

I'll look at the plate again when I go back...It's the size of a Budget Rent-A-Truck. My eyes are 50,along with the rest of me. :)

Sucks to be me some days.

I'm in TN, and have yet to see voltages above about 485V. But I only have about 35 regular customers.

That may be it...I'll see if anyone will own up to it.

The drives are a low-end Leeson (spec'ed being the low bidder, and not by me,btw.) . The maximum output current is less than 2A. About a dozen of them on the bus, give or take 3. (Multiples of the same design). They run 1/8hp gearmotors. We had absolutley no problems with them during the build, (at~470-480V) where the power was cycled 50 times or more, yet when we installed them onsite at 503V the breaker tripped on the first power-up, on all the units. If you immediately reset the breaker (before the drives discharge) it holds fine.

Don't think so...they cost $185 or so.

No,sir. The contactor never opens unless the emergency loop goes open (rare) or of course power is turned off.

Thanks for the info-could be that I'm fretting over nothing-none of the drives have failed (yet) but I keep thinking about that slam at power-on.

Nuttin'. :) There are a dozen or so little work stations (can't be too specific) with a box that contains a profi io block, and a drive and breaker. Between 12 and 15 boxes on a bus.

My first thought was that the breaker itself sucked, but when the

30A-Rated contactor welded, I got a clue. :)