Step up transformer
- Thread starter craigo321
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kingpb
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- SE USA as far as you can go
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- Engineer, Registered
Not knowing anything more about it, the simple answer is a 208 - 240V transformer around 25KVA in size.
What is the rating and configuration of the equipment being fed? I'd have thought that would pretty useful for determining transformer rating.
With the limited information provided I would probably have selected an auto-transformer (buck boost) given that 208V to 240V is not a large ration. That would keep size and cost down compared to a double wound unit.
I can't comment on any potential code issues - I'm in the UK and I presume that you are in the USA.
Different rules apply.
I agree with Besoeker in using a buck/boost as it is the most cost effective approach.
However should isolation be required, that is if you need to establish a new system then you need an actual step up transformer as buck/boosts can no provide as such.
Also, your question is somewhat vague in that you have not identified your application as a 1ph of 3 ph transition. If it is 3ph then you are dealing with a 240delta which creates a grounding concern. There are transformers designed which are commonly available for drives with a 230/133 secondary if 230v would work for you which provides you with the opportunity to ground the neutral at a much lower voltage of 133v than you would have when you corner ground a 240v delta where you end up with 240v to ground.
If the 240v is 1ph then I would suggest you use a 124/240 secondary even though 120 is not required you have the opportunity to ground the neutral at 120v.
As a former applications engineer my concern has always been to make sure that all of the issues are considered. I remember when I questioned someone with regard to a 1ph transformer which they wanted to order with a 240 2w secondary and I new that what they really needed was a 120/240. They insisted that their electrical engineer specified a 240 specifically which I had to supply. The installing electrician asked me where the neutral was when they were setting the pad mounded transformer at an airport. They then had the privilege of buying a new core and coil where I had to have my engineers design a special load termination assembly to fit in the same cutout as the original. The application was a step-up step-down arrangement to power the 120/240v avionics on the end of main runway.
The important thing is to think the job through so that you will end up with a reliable and safe installation.
However should isolation be required, that is if you need to establish a new system then you need an actual step up transformer as buck/boosts can no provide as such.
Also, your question is somewhat vague in that you have not identified your application as a 1ph of 3 ph transition. If it is 3ph then you are dealing with a 240delta which creates a grounding concern. There are transformers designed which are commonly available for drives with a 230/133 secondary if 230v would work for you which provides you with the opportunity to ground the neutral at a much lower voltage of 133v than you would have when you corner ground a 240v delta where you end up with 240v to ground.
If the 240v is 1ph then I would suggest you use a 124/240 secondary even though 120 is not required you have the opportunity to ground the neutral at 120v.
As a former applications engineer my concern has always been to make sure that all of the issues are considered. I remember when I questioned someone with regard to a 1ph transformer which they wanted to order with a 240 2w secondary and I new that what they really needed was a 120/240. They insisted that their electrical engineer specified a 240 specifically which I had to supply. The installing electrician asked me where the neutral was when they were setting the pad mounded transformer at an airport. They then had the privilege of buying a new core and coil where I had to have my engineers design a special load termination assembly to fit in the same cutout as the original. The application was a step-up step-down arrangement to power the 120/240v avionics on the end of main runway.
The important thing is to think the job through so that you will end up with a reliable and safe installation.
kingpb
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- SE USA as far as you can go
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The OP does specifically state the service is supplied with 120/208V single phase, and as such the voltage is stated properly.
Joethemechanic
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
So from his original post he needs a boost buck . The thing is with a boost buck is only the secondary will be carrying the 100 amps.
Probably the easiest choice would be a 240V primary 32V secondary boost buck. You would end up with about 236 volts.
32 volts at 100 amps be 3.2KVA, so a 3.25 0r 3.50KVA boost buck would work
Something like in the picture would work but you might find something less expensive elsewhere. You might want to split your secondary boost to 14 volts on each phase wire depending on the load's requirements
Probably the easiest choice would be a 240V primary 32V secondary boost buck. You would end up with about 236 volts.
32 volts at 100 amps be 3.2KVA, so a 3.25 0r 3.50KVA boost buck would work
Something like in the picture would work but you might find something less expensive elsewhere. You might want to split your secondary boost to 14 volts on each phase wire depending on the load's requirements
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I'm curious about that.
Wouldn't 120V be the line to neutral voltage of a three phase 208V line to line voltage.
The line to neutral and line to line voltages would not be in phase so I'm not sure how that cold be configured as single phase.
I'm curious about the 120/208 single phase aso. I've been in this industry for 30 years and have never seen 120/208 single phase refered to. I have been involved with Electrical distribution protection and control as well as directly involved in the sale and manufacture of transformers and to be quite frank this is not doable.
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It's two hots and a neutral fed from a wye secondary instead of three. What would you call it- Three phase wye, less one, plus a grounded?
Joethemechanic
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
I've seen plenty of it, just not as a service as far as I remember. Like Dave said. it's just a single phase taken from a 120/208 WYE. What's so strange about that?
Where I've seen it is, for example, a guard shack on an industrial site. You run a single phase feeder from the plant to the shack. You use the 208 for the heating and the 120 for the lights and the electronics.
Where I've seen it is, for example, a guard shack on an industrial site. You run a single phase feeder from the plant to the shack. You use the 208 for the heating and the 120 for the lights and the electronics.
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PetrosA
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Common in apartment buildings with Wye 3 phase split off to single phase panels for individual customers IIRC.
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- Massachusetts
Exactly
The 208V and the 120V sources would not be in phase though if they are derived from a 3-phase 208/120 system.
Line to line and line to neutral.
That's why I was curious about it being called single phase.
Joethemechanic
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Joethemechanic
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- Location
- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Joe, I hope you mind that I've cut down you image to the basics.
So what do we have?
V bc, Vbn, Vcn
All at different phase angles.
Difficult for me to construe that as single phase.
It does makes sense guys. Of course.
But I do find it strange that I have never had the opportunity to provide a load center of panelboard that would be ordered as such knowing that one labeled for 120/240 could be applied as such.
For some reason I have never had any oportutities for inquiries regarding 120/208 1ph3w.
Thanks for pointed that out.
But I do find it strange that I have never had the opportunity to provide a load center of panelboard that would be ordered as such knowing that one labeled for 120/240 could be applied as such.
For some reason I have never had any oportutities for inquiries regarding 120/208 1ph3w.
Thanks for pointed that out.
Joethemechanic
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- Location
- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Joe, I hope you mind that I've cut down you image to the basics.
So what do we have?
V bc, Vbn, Vcn
All at different phase angles.
Difficult for me to construe that as single phase.
I know it seems funny to look at if you are not use to seeing it as single phase. And I guess in a way you could be right. I have seen 3 phase motors incorrectly connected to it, and they do start and run, although performance is very poor. But here in the US we look at it as single phase because as a supply, it is commonly used interchangeably with true single phase. AND in practice, it is never used for 3 phase
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Joethemechanic
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
I see what you are saying. In effect it is sort of an open delta
kingpb
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its considered an open wye
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