Boost 208 to 480???

[chris kennedy]

I have a set of plans that calls for an auto-transformer to boost 208 to 480. 3Ø 10hp 11.2A. I am well aware if this was possible it would take 2 trannies but none of my charts show this as being remotely doable.

Before I fire off a RFI, am I missing something here?

Thanks

 

[GoldDigger]

I have a set of plans that calls for an auto-transformer to boost 208 to 480. 3Ø 10hp 11.2A. I am well aware if this was possible it would take 2 trannies but none of my charts show this as being remotely doable.

Before I fire off a RFI, am I missing something here?

Thanks

Assumptions:

1. The input is 208Y/120.
2. The desired output is 480 delta.

Any autotransformer use on a wye supply would be most understandable if each autotransformer were wired line to neutral on its input.
You would need 3 transformers for this configuration, since a "open wye" cannot drive a true three phase load.
Each autotransfomer would raise 120 to 277, producing a 480Y/277 output.
A delta load would not have to make use of the neutral, but the output would necessarily be grounded wye since the windings do not provide isolation.

Another alternative would be to use two 208 to 480 autotransformers, which would produce an open delta where the "neutral", if used, would not be a true neutral and the resulting delta would not be symmetrical with respect to ground.

Or you could use three such transformers to produce a symmetrical-to-ground delta output which would be a triangle rotated with respect to the original delta triangle (in the schematic, not the phasor diagram). Not sure how the code would treat that one.

It is not clear to me that using an autotransformer for a step-up ratio greater than 2 would be a good engineering and cost solution.

 

[Smart $]

Unless a make & model part number is provided an RFI is in order.

 

[Smart $]

...
Any autotransformer use on a wye supply would be most understandable if each autotransformer were wired line to neutral on its input.
...

IIRC, for grounded systems, a connection to the grounded conductor is required for autotransformers.

 

[GeorgeB]

The company which employed me in 1980 manufactured machinery for customers having 208, 240, 480, and 600 v site power. We were moderately small, and had 208 for the plant.

As we tested all equipment before shipping, we had to have the ability to source all of these voltages. We had (I'm sure it was a special, but it had been there long before I was) an auto-transformer with 208 in which could handle (I'm guessing here, I wasn't involved with the tests) 15 kVA. The shop (this was WAY before 70E was talked about) used SO cable from the transformer to the control panel. We used no motor over 15 HP, so each could be run independently. All control wiring could be tested as well.

All, I THINK, was WYE, so the common was grounded.

 

[cadpoint]

I was punching in some fake numbers.

http://www.buckboostcalculator.com/index.cfm

I don't know if you can do it with your applied voltage of 208

note

http://www.schneider-electric.us/en...-and-boost-transformers?N=3775874915#tabs-top

 

[JFletcher]

Why not just a step up transformer or reverse wire a 480/208? RFI on the autotransformer...

 

[Besoeker]

I have a set of plans that calls for an auto-transformer to boost 208 to 480. 3Ø 10hp 11.2A. I am well aware if this was possible it would take 2 trannies but none of my charts show this as being remotely doable.

Before I fire off a RFI, am I missing something here?

Thanks

I don't know why you would need two.
Coming at it from a different perspective, autotransformers are usually a cost saving exercise where the input to ourput voltage ratio is fairly close. That way, it can effective be sized for the difference in voltage. But in your example the voltages are not close.

If it were my choice, I'd go for a double wound. That way you can ground primary, secondary, or both to suit requirements/regulations you have to comply with. And you can have whatever configuration best suits the application. It isn't a very big transformer so the gain in going for an autowound unit wouldn't be all that great anyway.

 

[winnie]

A 3 phase autotransformer setup can be built with 2 or 3 single phase transformers.

I think that Chris' comment about 2 trannies comes from using an 'open delta boost' configuration. As smart$ notes this might not be allowed because the supply is a grounded system.

There are two reasons that I can think of for using an autotransformer arrangement. The most common is that with small % voltage changes (say 208 to 240) you can greatly reduce the size and cost of the transformer versus a dual winding transformer.

The second reason is that an autotransformer is _not_ an SDS and may be easier to ground.

IMHO the designer had a brain fart and said 'autotransformer' when 'step up transformer' was intended.

-Jon

 

[Besoeker]

A 3 phase autotransformer setup can be built with 2 or 3 single phase transformers.

I think that Chris' comment about 2 trannies comes from using an 'open delta boost' configuration. As smart$ notes this might not be allowed because the supply is a grounded system.

There are two reasons that I can think of for using an autotransformer arrangement. The most common is that with small % voltage changes (say 208 to 240) you can greatly reduce the size and cost of the transformer versus a dual winding transformer.

The second reason is that an autotransformer is _not_ an SDS and may be easier to ground.

IMHO the designer had a brain fart and said 'autotransformer' when 'step up transformer' was intended.

-Jon

You may be right about the designer.
I agree with your point about 208 to 240. It's about size and cost for close ratios. I thought I had made that point.
But 208 to 480......hmm...

What is an SDS?

 

[jumper]

What is an SDS?

Separately Derived System

From our NEC definitions:

An electrical source, other than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those established by grounding and bonding connections.

 

[wwhitney]

Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those established by grounding and bonding connections.

So, can one choose to install a transformer not as an SDS? E.g with a delta/wye transformer fed from a wye system, bring the primary neutral to the transformer and directly connect it to the secondary neutral.

If this configuration is allowed, does it have any benefits over an SDS?

Cheers, Wayne

 

[jumper]

So, can one choose to install a transformer not as an SDS? E.g with a delta/wye transformer fed from a wye system, bring the primary neutral to the transformer and directly connect it to the secondary neutral.

If this configuration is allowed, does it have any benefits over an SDS?

Cheers, Wayne

Trannies are not a big strong point of mine, I just do simple normal set ups (SDS), but I am pretty sure that you do not want to do the above. IIRC, screws up the output voltages or something like that.

I know about gennies set up as either SDS or not a little better.

 

[ActionDave]

So, can one choose to install a transformer not as an SDS? E.g with a delta/wye transformer fed from a wye system, bring the primary neutral to the transformer and directly connect it to the secondary neutral.

If this configuration is allowed, does it have any benefits over an SDS?

Cheers, Wayne

I'd have to think too hard about the legality of it, but there is no advantage.

 

[wwhitney]

I'd have to think too hard about the legality of it, but there is no advantage.

So I don't know much about transformers, but there is one possible small advantage that occurs to me:

I assume if you configured a transformer as a non-SDS as I suggested, then you'd just use the primary side EGC as your secondary side EGC, and there'd be no neutral-ground bond on the secondary side. It would save you having to run a GEC for the secondary side, at the expense of running a primary side neutral. So if the primary side neutral happens to be available much closer than a grounding electrode, it would be less wire to run.

Cheers, Wayne

 

[wwhitney]

Trannies are not a big strong point of mine, I just do simple normal set ups (SDS), but I am pretty sure that you do not want to do the above. IIRC, screws up the output voltages or something like that.

I don't think there'd be a voltage issue like you get when using a transformer with a wye configuration on the primary side. In an SDS where the primary and secondary sides are grounded wye systems (even if the transformer primary side is delta wound as usual), the primary and secondary neutrals are electrically connected by both being attached to the grounding electrode system. Electrically, my proposed configuration is the same in that regard, it's just that by making that primary neutral to secondary neutral connection directly at the transformer, instead of relying on the grounding connections, the transformer is no longer an SDS. So the grounding on the secondary side is presumably handled a little differently.

I know about gennies set up as either SDS or not a little better.

Yes, exactly, I was wondering if the same thing applies to transformers, or if the NEC requires installing an isolation transformer as an SDS.

Cheers, Wayne

 

[Besoeker]

Separately Derived System

From our NEC definitions:

An electrical source, other than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those established by grounding and bonding connections.

Thanks Del Boy.

 

[ActionDave]

So I don't know much about transformers, but there is one possible small advantage that occurs to me:

I assume if you configured a transformer as a non-SDS as I suggested, then you'd just use the primary side EGC as your secondary side EGC, and there'd be no neutral-ground bond on the secondary side. It would save you having to run a GEC for the secondary side, at the expense of running a primary side neutral. So if the primary side neutral happens to be available much closer than a grounding electrode, it would be less wire to run.

Cheers, Wayne

I thought about that but left it out of my post because I was headed out the door.

So other than a weird way to connect to the GES I don't see any advantage and still not sure if it's legal.

 

[kwired]

IIRC, for grounded systems, a connection to the grounded conductor is required for autotransformers.

If connecting to the grounded conductor I believe the grounded conductor needs to be the "common" conductor.

One of the most common uses of auto transformers is to buck/boost 240 to 208/208 to 240 - no grounded conductor is utilized for those applications.

 

[winnie]

You may be right about the designer.
I agree with your point about 208 to 240. It's about size and cost for close ratios. I thought I had made that point.

You did make the point. I was being wordy and repeated it

-Jon