Taming a transformer's inrush. 3

[itsmoked]

I have a 3Ø 112kVA transformer to power a shop.
The shop has 100A available for the circuit.
Every time they try to close on the transformer the breaker checks-out. I don't have any trouble believing this.

They have a a grand opening next week and are asking if there's anything that can be done.

They only need 75kVA.

No Bill, it's not on a generator.

1) Do those units that automatically close at the peak of the sine wave have a chance of working here? (If so who makes them?)
2) Would a bunch of light bulbs and a bypass contactor work?
3) A choke would work but would probably take longer to get and cost the same as a transformer.
4) Soft starter for the primary? (dang that would be a big one)
5) A bunch of VFD braking resistors and a bypass contactor?
6) ?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

99.9% of all 400V rated VFDs are exactly the same as 480V rated VFDs; the ones we use as 480V are in fact often referred to as "400V Class" (as opposed to 300V Class) and they accept any input from 380 to 480V. The VFD would not know the difference and you could program the output of a 480V VFD to be limited to 400V. The same is true for 99.9% of 400V servo amplifiers. You will still need a transformer of course because you said they only have 240V available and I suppose they have already bought and installed it, hence your problem. I'm just pointing out that you should convince them to consult you next time BEFORE they try to solve a problem that could have been done easier.

Back to your problem at hand, I have used NTC thermistors as Inrush Current Limiters (ICLs). The largest I have ever seen are 50A so I don't know if they go larger or not, but you may be able to use even a 50A if the transformer is unloaded at start and the ICL is immediately bypassed. I have also on 3 occasions used marge MV soft starters as ICLs for big transformers. Major overkill, but in those cases there was no other solution that worked, and they had tried several.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Good to know that NTCs get up to 50A!! I never would've imagined.

Same with the VFD classes. I realize there are 200V and 400V classes. A lot of these are indeed servo drives. Often it's very hard to get to the parameters or to even get enough info to understand how to set something like a voltage limit in the drive. It would definitely help to have them call me first.

OK. Well, I've been text message informed that "200A breakers solved the problem". Plural because apparently 2 are in series.

So, use a 200A breaker on wiring that is good for maybe half that and probably the insane line drop of the inrush and the bigger breaker gets the transformer launched.

So simple!

Why didn't we all think of that?

They're numskulls. (And the future Ghost-Ship of Silicon Valley)


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Probably one breker to energize the transformer and then close the second breaker in parallel to carry the load. I feel your pain.

Bill
--------------------
"Why not the best?"
Jimmy Carter