dumb MechEngr voltage-power question

[tygerdawg]

A client has received a machine rated at 480Vac 3Ph 60Hz 15KVA max, 3KVA running. They want to install the machine in a temporary location that only has 208Vac 3Ph 60Hz available.

I had to reach 'way back behind the cobwebs and vaguely remember the phrase "step-up transformer", but associated with that phrase was a cold chill of dread.

What can be done? What's the equations behind this decision?

TygerDawg
Blue Technik LLC
Advanced Robotics & Automation Engineering

http://www.bluetechnik.com

 

[Skogsgurra]

Dust off the cob-web and overcome that chilly feeling. Just do it!

If you go for an autotransformer, and make it big enough, it should just work. Contact an electric engineer (there are professional consulting engineers) to work out the details. That's a cheap insurance.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[tygerdawg]

Thanks Guys, there may be hope for this GearHead yet.

TygerDawg
Blue Technik LLC
Advanced Robotics & Automation Engineering

http://www.bluetechnik.com

 

[itsmoked]

Skogs; Isn't there a code rule about using boost transformers to boost more that twice the voltage?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

It is possible. But I am rather weak in the US NEC. I hope that the consultant can answer that question. Is that rule also for full transformers? I have gone from 500 V to 11 kV in a recent application. And that's a lot more than twice the voltage.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

If the 15kVA is just starting current you sure don't need 18A @ 480V. Things need a closer look.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

I think that Keith is correct about code regarding an autotransformer for this application, but it may be moot anyway as I never saw an autotransformer for those voltages, in either direction.

One big word of caution on this. While you need a step up transformer, and yes transformers don't care which side you connect to your source and which to your load, DON'T use a 480-208Y/120 step down transformer for this application. Buy a 208 delta-480Y/277V transformer designed for this application. You really want a 480 wye winding, not the 480 delta winding you would get with the standard 480-208Y/120V transformer. Grounding of a 480V delta is rather more complicated than the grounding of a 480V wye. It can be done, but it will likely be outside the experience of the folks who have to work on your equipment and might have undesired consequences. You needn't carry the neutral to the equipment, you just need to ground it at the transformer to establish the system grounding.

 

[itsmoked]

Good point David! A buck-boost for 200V! Not something you will ever see on the shelf.

Gunnar I don't remember if it is in the boost direction or only the buck. If you go too far and of course get an open winding you can subject your low side to the highside voltages. Not really a boost problem but certainly a buck problem.

David has another excellent point about wanting that center-point for grounding.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[tygerdawg]

Thanks to all for the excellent insight & advice.

TygerDawg
Blue Technik LLC
Advanced Robotics & Automation Engineering

http://www.bluetechnik.com

 

[Skogsgurra]

Yes. I was thinking autotransformer mostly because of their low impedance. But, when I think about it, more than 2:1 will probably not be any better than a full transformer.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Here's a suggestion;
SquareD
1S46F TRANSFORMER DRY 1PH 1KVA 120X240V-16/32V
Connected as a 120/272V auto transformer on each phase, phase to neutral will give a high side phase to phase voltage of
272 x 1.73 = 471 volts. This is less than 2% low.
The 120 volt windings will handle 1000VA /240 volts = 4.2 amps.
4.2 x 272 = 1133 VA. Three of these in star will give you a capacity of 3.4 KVA.
This will run your 3 KVA load. If you describe your actual load we may suggest the proper size for your application.
respectfully

 

[davidbeach]

waross, help me out here, how the heck would you connect one of those to get those voltages? You could connect the high side at 120 and add the 32V winding to get 152V, but could you really connect to one of the 120V windings and have an additional 120V induced in the other 120V winding? I don't believe that the two halves of the primary are wound in such a manner as to allow that to happen. Sure, you'll get something, but an awful lot of the flux from one half of the primary isn't going to link the other half, is it? Have you actually made this connection work?

On the other hand, I did look at the code and if the appropriate transformer can be found it appears to be permissible, but I think a conventional two winding transformer will be easier.

 

[ScottyUK]

Hi David,

Why would the flux linkage be so poor? I guess that this transformer will be a single bobbin design with concentric primaries nearest the core and the secondary as the outermost winding. I wouldn't expect massive flux leakage on that winding configuration. Things might be a little worse with a two bobbin configuration with separated primary and secondary windings, but even then most flux should be mutually coupled.

It's certainly a novel way of applying an isolating transformer.


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If we learn from our mistakes I'm getting a great education!

 

[HCBFlash]

Hey "gearhead", cut out the self deprecatory stuff, we're peers here, and not uppity either!

Heck, gearhead is no insult in my book anyhow! .... and uh, I don't think tygerdawg is either!

davidbeach's 1st posting is probably one of the best options you'll get. Your mention of a temporary installation would be another factor in favor of 208Delta x 277/480Wye configuration. Simplicity can be priceless.

Constructing the installation of 1 transformer as opposed to 3 isn't too hard to defend, what with all the raceways, cabling, terminations, racking for it all, clock hours, project scheduling,,, including stupid stuff like back-ordered seal-tite fittings,,, etc.. I'm sure your experiences in your field of expertise have made you wary of these things as well.

Well, that's my 2-cents. Best of luck in any case!



Flash

Me wrong? I'm just fine-tuning my sarcasm!

 

[davidbeach]

ScottyUK,

I don't know. I picture two bobbins on a single core, with one higher voltage and one lower voltage winding on each bobbin. I also picture a certain amount of the flux directly linking the two windings on a bobbin without linking the core. If it does work I would whole heartedly agree with you that it would be a novel way of applying the transformer.

 

[Digger425]

Acme Transformer makes an array of autotransformers (buck/boost). I found their Tech Service Dept to be quite responsive and helpful with my need to go from from 208v single phase to 230v single phase.

I ordered the transformers they suggested.

 

[ietech]

tygerdawg,

All of the folks responding to your question are far more knowledgable than I in this area so my suggestion may be way off base.

The machines I work with have the option of being setup for 3 phase low voltage wiring. Is this possibly an option for your machine? You probably already thought of this but I thought I would put it out there just in case.

 

[rbulsara]

Folks:
OP never mentioned autotransformer! Why not just use a two-winding 208:480V transformer and call it a day? Its is only 15kVA unit, off the shelf item. It costs a fraction of the value of the time all of us have collectively spent here so far.

 

[tygerdawg]

OK, guys, enough already. Problem has been solved by the customer agreeing to install a step up transformer.

rbulsara, autotransformer was mentioned. But the posters/respondees got deleted for some reason.

Time for us all to get on with other things. Thanks for everyone's comments, suggestions, and help.

TygerDawg
Blue Technik LLC
Advanced Robotics & Automation Engineering

http://www.bluetechnik.com