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# Taming a transformer's inrush.3

## Taming a transformer's inrush.

(OP)
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

### RE: Taming a transformer's inrush.

I had a similar situation at an old defense site several years ago, ended up using a 3 phase 480VAC variac and a contactor, would close the contactor, ramp up the voltage to full 480, close the breaker feeding the transformer, then open the contactor. Grand total about $600 in parts (variac and contactor came off eBay), since it didn't need to energize very often and the onsite staff were pretty sharp, it worked fine.,,May not be the solution to your problem in in that case worked for me. MikeL. ### RE: Taming a transformer's inrush. Howdy Keith, I recently had the same issue, but with transformers rated up to 12.5MVA, all with a 25kV primary. We had to do something to reduce the % voltage-drop at the PoCC with the Utility, and hence not impact other customers on the same 25kV distribution feeders. What we ended up doing was install a 25kV reactor c/w bypass contactor. This solution worked real good like. Therefore, I like your option 5 above (ie a bunch of VFD braking resistors and a bypass contactor), although I don't see why a reactor should not work as well with your LV supply. Let us know what you decide to do. GG ps You also will need to somehow interlock your secondary loads such that they can never be energized when the primary contactor is open. (Unless you like to live on the wild side). "I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931) ### RE: Taming a transformer's inrush. (OP) Thanks gents! Cat; This customer is pretty savvy since he supplies and installs about 80% of silicon valley machine tools. He's likely capable of doing a variac dial-up and the associated switching. The transformer is an odd-ball 240V to 400V to supply a single German machine. Groovy; you like it. It's probably the cheapest solution. Certainly straight forward and since it's a single machine interlocking or just making sure it's off shouldn't be too tough. In fact they generally stay off on power cycle and you have to press a start button to latch in some contactor to get things going. e.g. the load auto-turns off any power cycle. I'll definitely let you know how it shakes out. Keith Cress kcress - http://www.flaminsystems.com ### RE: Taming a transformer's inrush. 2 Dear Mr itsmoked I would like to propose the following: a) Use a [MCB with type "D " tripping characteristic]. Its magnetic/instant trip is 10-20 times the rated current. It is particularly suited for transformer switching. All major IEC CB manufacturers have them and are [not] expensive. b) Use any "left-over" wire equal or large than the phase conductor size, coil it with a diameter of 200mm approx 5-10 turns, insert these "air-core inductor" in series in each line. It is easy to make it and cost minimal. c) Use an adequately sized contactor which is [specially designed for capacitor switching] in series in the lines. All major IEC contactor manufacturer have them. This [three-pole contactor comes with three series resistors and a late-break aux contact] included. Easy installation and cost minimal. Che Kuan Yau (Singapore) ### RE: Taming a transformer's inrush. (OP) Hi che12345. I'm intrigued! This sounds easier than Variacs or braking resistors. I'm not clear on the point of the contactor. The breaker would normally be left ON with the transformer energized continuously. Can this method work with just a breaker in series with the 'air chokes' or am I missing something? Keith Cress kcress - http://www.flaminsystems.com ### RE: Taming a transformer's inrush. Dear Mr itsmoked My proposals are based on three different principles. a) Use Type "D" MCB. Making use of the 10-20 times tripping characteristic. Its cost is minimal, small in size and installation is on DIN rail. Q1. I'm not clear on the point of the contactor. The breaker would normally be left ON with the transformer energized continuously. A1. By using [capacitor switching contactor]*. Wired with CB left on/closed, then press the "on" push-button to switch on/close the contactor*. It seals-in/retain close by the NO aux contact of the contactor*. Press "off" push-button to open the contactor*. The 3-pole contactor* closed with three resistors each in series in each pole and after a short-time delay the series resistors are by-passed. The (series resistors and the short-time delay aux contact) are [inbuilt]. No external circuitry or component is required. Q2. Can this method work with just a breaker in series with the 'air chokes' or am I missing something? A2. You can try by using the existing CB and by inserting the "air-core" inductor in series. You may vary the coil diameter and the number of turns. After all it is easy to make the coil and cost is minimal. Except the conductor, no other component/wiring is required. Note: You may use any one of the three proposals or combination of any two or even three. The cost are not too high and installation are simple. Che Kuan Yau (Singapore) ### RE: Taming a transformer's inrush. #### Quote (Keith) I have a 3Ø 112kVA transformer to power a shop. #### Quote (Keith) They only need 75kVA. #### Quote (Keith) The transformer is an odd-ball 240V to 400V to supply a single German machine. 100 A x 240 V x 1.73 = 41.6 KVA 1000 CODE 112 KVA x 1000 = 270 Amps 240 x 1.73 Breaker; Not more than 300% = 810 Amps, When the secondary is protected by an overcurrent device rated or set at not more than 125% of rated secondary current. (162 A x 1.25 = 202 Amps @ 400 volts) Canadian Code. Please check the NEC. ELSE 125% of 270 A = 337.5 Amps. Conductor Ampacity; = 125% of 270 A = 337.5 Amps. That aside, a few years back, we had a thread on methods to reduce the residual magnetism of a transformer. That's worth a search of this site. OR #### Quote (Keith) They have a a grand opening next week and are asking if there's anything that can be done. #### Quote (Keith) No Bill, it's not on a generator. YET A rental may get them through the grand opening. grin Bill -------------------- "Why not the best?" Jimmy Carter ### RE: Taming a transformer's inrush. I would try 2, light bulbs and a contactor or switch. If they have enough wattage to energize the TX it should work. ### RE: Taming a transformer's inrush. (OP) Thanks pwrngrnd. Thanks for those numbers Bill! Hadn't done that yet. Wow. He said 75kVA and only has 100A.. I just pointed that out and it caused a long pregnant pause. LOL. Knocking themselves out to get a transformer up while not having enough power to actually run the tool is not logical. They're getting me a picture of the machine plate so I can cut-to-the-chase. Half the time they're bonkers about what is really needed. Keith Cress kcress - http://www.flaminsystems.com ### RE: Taming a transformer's inrush. The "B curve" MCB issue can't work, MCBs don't come in sizes that big. Same for the variac; a 3 phase variac capable of 270A is NOT going to cost$600..., more like $10,000! But to Bill's point, you probably have too small of a circuit breaker on the 240V side (you didn't say what it is). Max out the size per the NEC and use one with adjustable magnetic trips, cranked up to the maximum allowable. Also keep in mind that people often rush to buy a "400V transformer" simply because a machine came in for the EU, but fail to realize that a 400V 50Hz motor will run BETTER at 480V 60Hz. It's going to get 60Hz here no matter what, so they need to make sure the motors get the correct V/Hz ratio (withing reason). They might be better off using 480V. " We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden ### RE: Taming a transformer's inrush. (OP) Thanks Jeff. Not seeing 100A "D" breakers here they peter out at about 63A. I think you missed the point on the Variac as it was just to 'start the transformer' unloaded. It was to be bypassed once the transformer was started. Same with the braking resistor gambit. This is a modern high-tech machine tool and so nothing runs at line frequency except maybe a control transformer that's rated 50/60Hz. All the motors are VFD'd. So, people adjust the voltage and the hertzes take care of themselves. The biggest problem I deal with is that these machines are beasts often stating maybe 60 or 70A at 480V (60kVA) but then when you clamp on ammeters while the machine is doing the absolute maximum production load that it will do day-in-out you see readings like 7A, (6kVA). No way will anyone shell out for 70A service and all it represents when the system will only ever draw less than 10A. This causes lots of these oddball problems with too large transformers in too low current chains. Keith Cress kcress - http://www.flaminsystems.com ### RE: Taming a transformer's inrush. Dear Mr itsmoked 1. NEC and CSA documents are useless. They impose limitations but without providing any clue at all on how to solve the problem. 2. Renting a portable gen (just for the grand opening) would cost [100 times!] the cost of any other alternative (simple?) solutions. 3. ..." on methods to reduce the residual magnetism of a transformer... " is but academic. These information/solution will take you time to do a research, costly measuring instrumentation/equipment and time consuming to carry out. The grand opening would be long over by then. 4. There are MCCBs [with adjustable thermal and adjustable magnetic trip I3 = 10Ir]. Adjust the Ir to the max value permitted by regulation in your area, and adjust the adjustable magnetic trip I3 to 10x Ir or Max (or "OFF" if? provided). Note: Attention! Most MCCBs magnetic trip I3=3Ir or 5Ir. These are [not] suitable for this case. Che Kuan Yau (Singapore) ### RE: Taming a transformer's inrush. The transformer we had to energize was 250kVA, we didn't carry any secondary load with the variac, just got it energized, closed the "normal" source , removed the variac from the input and then loaded it. Works pretty much like when we would do large rental projects, first unit online in a field of 20 units would come up with breaker closed, and ramp up slowly, then bring on the other units in parallel, figured if a 2MW generator could energize a 125MVA transformer then a 9 amp 480 variac can do the same for a smaller transformer, it did and worked fine for about 5 years until the site upgrades service and distribution. I'm sure Keith will find a suitable solution for the application in question. MikeL. ### RE: Taming a transformer's inrush. Dear Mr. che12345 1. NEC is not just a good idea, it is the law. The NEC is not intended to solve problems or give instructions. The NEC is a legal document specifying safe practices and safe minimums and maximums. 2. What will it cost to buy a 100 Amp breaker that will supply 270 Amps? 3. Residual magnetism: Please take the time to search this site and find the thread on reducing residual magnetism. 4. In North America, thermal magnetic circuit breakers with adjustable magnetic trips with settings from 5X to 10X are very common. That may well be what is in place now. 100 Amps is not enough. Hopefully Keith will find that the actual load is much less than 75 KVA. Keith, is this a case of the customer wanting to use an existing breaker rather than spending money on the proper size breaker? I have seen and installed so many 25 KVA transformers matched to 100 Amp, 240 Volt panels on industrial sites that when I see 100 Amps, 240 Volts I immediately think 25 KVA (single phase) 112 KVA was just too much. I had to do the math. As for the auto-transformer. Be sure that the auto-transformer output voltage matches the line voltage very closely before closing the bypass switch. Any difference in voltage will cause large circulating currents until the auto-transformer is disconnected. I found that out the hard way years ago. Bill -------------------- "Why not the best?" Jimmy Carter ### RE: Taming a transformer's inrush. Rather than light bulbs I would consider cheap and dirty strip heaters from Chromalox or similar. (If the actual demand is low enough. But then if you end up with a 37.5 KVA Xformer it may get online with the 100 Amp breaker.) Bill -------------------- "Why not the best?" Jimmy Carter ### RE: Taming a transformer's inrush. Dear Mr Dear Mr itsmoked 1. If a correct current rated MCCB with the highest Ir permitted by NEC or CSA and with the magnetic trip set to [10x Ir] failed to overcome the problem, you may have to submit your problem to the attention of the ( NEC or CSA Board ) to look into possible [ "exceptions" ] to be included/legalized as "law" during their [next revision]; in order [not to break the law]. Attention: A "note" that is added at the foot of the standard is for [informative purposes]. These do [not] have the legal status equivalent to the standards. These "notes" are therefore [not] "law" . 2. Inserting "left-over" light bulbs of any wattage [two in series per phase] during switching on and then bypass them with a 3-pole contactor. The cost would be minimal. You may have plenty of these "left-over" lamp bulbs from other projects. They can be used later even you have to purchase them, which are not costly. 3. Strip heaters of any brand with the correct [voltage] (or may be two in series) would cost 20x the cost of lamp bulbs. You may not need these heaters in any of your future projects. Yours sincerely Che Kuan Yau (Singapore) ### RE: Taming a transformer's inrush. Light bulbs? What wattage are you thinking about? I have a fixture with such creatures for low level load testing. There are nine 300 W bulbs, that is 2.7 kW in total. They will not pass enough current to make a 112 kVA transformer build enough flux to avoid inrush. Depending on core quality and saturation level during operation, I would expect at least 5% of rated kVA. That is 5 or 6 kW. The 300 W bulbs were not exactly cheap. And more than doubling that isn't either. Add to that that there's no guarantee that they will allow sufficient flux to build in the core. Gunnar Englund www.gke.org -------------------------------------- Half full - Half empty? I don't mind. It's what in it that counts. ### RE: Taming a transformer's inrush. (OP) Hi Bill. I asked for a picture of the machine's plate so I can start with a logical plan. It turns out they sell the machines so it's strictly a demo that they want to show off and sell. After asking for the machine's plate, hours later I got a picture of the plate.....the transformer's plate. 100A => 25kVA good to know! I haven't don't very many. As for your breaker size/cost question. Other response today: Them: "I could just put in a 200A breakers instead". Me: "Um no. You realize the wire and everything beyond the breaker has to be sized correctly?" Them: "Ahh, the wires pretty big number 4 or 3 maybe 2." Me: "Not going to fly. How about you send me a picture of the machine's plate." Keith Cress kcress - http://www.flaminsystems.com ### RE: Taming a transformer's inrush. EBay, 2500 Watts, 240 Volts,$99.99 each.
EBay again, A case of 48 x 100 Watt incandescent lamps, $24.95. x 2 =$49.90
Then add 60 sockets (for 6 kW) at about $2.50 ea. for another$150.
Plus labour to wire 60 lamp holders.
Are we doing this to code? Add 60 junction boxes and at least one splitter.
For a one-time demo?
With a rental gen set-you won't need the transformer. grin
Remember, the 25 KVA and 100 Amps is single phase, but it is easy to remember and to extrapolate to other sizes.

#### Quote:

Them: "Ahh, the wires pretty big number 4 or 3 maybe 2."
How about 250 or 300 MCM?
Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Taming a transformer's inrush.

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

### RE: Taming a transformer's inrush.

(OP)
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

### RE: Taming a transformer's inrush.

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

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