Can this be used as an Autotransformer or will it be an SDS

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busman

busman

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I have a manufacturer providing equipment that runs on 240V single phase. Supply voltage will be standard 120V. They are recommending the transformer shown below. Can this be made into an autotransformer by connecting H1 (line side neutral) to X2/X3, or does it need to be an SDS with X2/X3 connected to the GES and the load side EGC's?

Thanks,

Mark

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It can be made into a non-SDS in more or less the way you describe.
Any isolation transformer can be made into an autotransformer, but that requires putting the primary and secondary windings in series to drive the load.

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GoldDigger said:
It can be made into a non-SDS in more or less the way you describe.
Any isolation transformer can be made into an autotransformer, but that requires putting the primary and secondary windings in series to drive the load.

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Click to expand...
Thanks. Sorry, yes, I had forgotten that auto-transformers share a part of the windings. Thanks for the reminder.
 
210519-1513 EDT

busman:

The power handling capability will be roughly 1/2 of the transformer rating unless all windings are used, actually less because you are limited by winding current rating, and a lot of window space was consumed to provide the primary winding combinations.

You can use X1 thru X4 as an auto transformer alone, or H1 thru H4 alone. Either of these provide less than 1/2 power capability.

If there is very little difference in the separate windings, then you might parallel the windings without too much circulating current, but still not full power capability.

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Say you want to use that transformer as an autotransformer. You can connect X2 to X3; that makes X1-X4 a 1:1 turns ratio with H4-H1.

So with a 120V supply L-N, connect N to X1, and L to X4. Then L/X4 to H4. Does that make H1 240V relative to N, or have I got it backwards, and it will be about 0V?

Cheers, Wayne
 
210519-2013 EDT

whitney:

Unfortunately there are no phasing dots on the diagram, but based on the way transformers are usually labeled one can assume ---
X1, X3, and H1 are in phase. Thus, when X2 and X3 are connected together, then X1 to X4 can tolerate 240 and just start into core saturation.

Then H4 to H1 is approximately equal in voltage to X4 to X1, and the two are in phase if H1 is connected to X1. If voltages are equal enough, then H2 could connect to X2, and H4 to X4.

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So you expect based on numbering that with X2 and X3 connected, the voltage vector X1 -> X4 matches the voltage vector H1 -> H4? (That would mean I had it backwards, not being familiar with the typical terminological conventions). In which case to double a voltage source N -> L you could connect N to X1, L to X4 and H1, and the resulting N -> H4 would be double N -> L? Did I get that right?

Cheers, Wayne
 
To me, there's no puzzle. 120v to H1-H2 (cap 3,4,5), and 120/240v out from X1-X23-X4.

The only question is how to ground bond the secondary neutral, as asked in the OP.

If it's compliant to tie it to the source grounded conductor, then by all means do so.
 
LarryFine said:
To me, there's no puzzle. 120v to H1-H2 (cap 3,4,5), and 120/240v out from X1-X23-X4.
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Sure, you could do that, but it would be an isolation transformer. And you'd be using only half of the H side coil, so I think the capacity would go down to 0.5 kVA. If you wanted to use it as an isolation transformer at full capacity, you'd supply 120V power to the "low voltage" side configured for 120V, and take your 240V from the "high voltage" side at H1 and H4.

Cheers, Wayne
 
210519-2121 EDT

LarryFine:

The question presented in the title of this thread was --- "Can this be used as an Autotransformer".

The answer is yes.

.
 
Yes, it can. Sorry.

Secondaries in parallel, placed in series with source at neutral.

You may have to experiment with polarity to get 240v.
 
wwhitney said:
Sure, you could do that, but it would be an isolation transformer. And you'd be using only half of the H side coil....
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No. You would be using the only coil on the H side, so you would be able to use it's full rating. Taps on windings are full capacity unless notedo on the label.

The only reason to use this as an autotransformer would be an approximate doubling of it's nameplate capacity.
 
jim dungar said:
No. You would be using the only coil on the H side, so you would be able to use it's full rating. Taps on windings are full capacity unless notedo on the label.
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OK, please correct me if I'm making the wrong inferences here:

The transformer has a KVA rating, but the coils themselves have both a voltage rating (insulation) and current rating (ampacity). If the transformer had only two connections on the H side, for 240V, then for 1000 VA, the H current would be limited to 4.17A and the coil would be sized appropriately. But by having the 120V tap on the H side, the H current could be up to twice that, and so the H coil size has to be increased to handle 8.33A.

In which case, why wouldn't the transformer maker just split the H coil at what is currently H2? One of the H coils could still have the 208/240/277 taps for use when the H coils are in serires; and the H coils could be used in parallel (with the 240V tap) for 120V on the H side. And now the H coils would only have to be sized for 1000/208 = 4.81A instead of 8.33A.

Cheers, Wayne
 
The wiring diagram shows how the transformer was built, not how it could have been built. What you are describing would require two separate reconnect able windings.

The full kVA rating is available on the primary 120V tap, therefore it is wound with 8.33A worth of conductors.
It is possible, but unlikely, the manufacturer used smaller conductors for each of the H3, H4, and H5 taps. The savings in material probably does not cover the hassel of doing this.
 
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210520-1310 EDT

Consider a 1 to 1 isolation transformer, with two identical windings. Suppose the input rating is 120 V 10 A, and the transformer is used and rated as an isolation transformer.. Assuming an ideal transformer the output is also 120 V 10 A.

Now connect this transformer as a step up autotransformer. The output is now 240 V and you know that the upper half of the transformer can handle 10 A. Thus, output power is double or 240*10 instead of 120*10. No change in the transformer size, weight, or power loss.

Then the question is about the lower half of the transformer. Will that winding overheat? In other words what is the current in the lower winding. Note the current going into the lower half of the winding via the center tap is double the current coming out of the top winding.

The current in the lower winding is the same as the current in the upper winding. Thus, no increase in the power dissipation in the lower winding compared to use as an isolation transformer.

This means that using the transformer as an autotransformer doubled its power output capability compared to use as an isolation transformer. If I do not need isolation then a less costly device can do the job.

Had the transformer been wound as an isolation transformer, physical size and primary the same, and to produce 240 V from the secondary, then its secondary current capability would have been cut in half.

.
 
The wording of 210.9 says "Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer."
So if you derive 240V from a 120V supply, that's saying you must have a grounded conductor provided in that derived branch circuit. But it doesn't say you have to connect any loads to that grounded conductor (neutral).
 
synchro said:
So if you derive 240V from a 120V supply, that's saying you must have a grounded conductor provided in that derived branch circuit. But it doesn't say you have to connect any loads to that grounded conductor (neutral).
Click to expand...

I'm not following. How are you going to complete the circuit?
 
david luchini said:
Still not following. If the input circuit is 120V (L-N), then the output circuit is 240V (L-N).
Click to expand...

I guess I was thinking about the following post of Larry's, where I assumed that he was proposing using the transformer to derive an output 180 degrees from the 120V supply input, and it would provide the full transformer kVA rating for that phase since both of the secondary windings are in parallel. So the derived circuit would be 120V/240V, with one of the 120V output lines directly connected to the 120V input.

LarryFine said:
Yes, it can. Sorry.

Secondaries in parallel, placed in series with source at neutral.

You may have to experiment with polarity to get 240v.
Click to expand...
The primary and secondary windings would be connected in series as in an autotransformer, but with that connection point also tied to the neutral. Not what is traditionally done in an autotransformer where one end of the series windings is grounded.

How to properly provide overcurrent protection for this arrangement would need some consideration.
 
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