Why is my transformer a Scott-T?

Thread Starter

strantor

Joined Oct 3, 2010
5,160
I have a 6KVA sandpoxy-filled transformer I'm digging out right now (thread on the excavation) and much to my surprise when I finally unearthed the transformer, it has 2 cores.

scott t.jpg

It's a Square D model # 6T2F, 480V Delta input, 208/120V Wye output according to the generic info I can find online. The data plate was worn off mine so I didn't notice the odd connection diagram; here's what it would look like though (not my picture):

IMG_5210.JPG

It says "Type ST" on the label, there's 2 cores, and the connection diagram seems to confirm a Scott T transformer, for which the only purpose I was aware of, is to generate archaic 2-phase power from a 3-phase supply. But everything I can find online says it's a general purpose 3-phase transformer (to quote the page linked above: "General Purpose - Intended for power, heating and lighting applications").

I pulled this out of a dumpster years ago at a place I worked. I was an Electrical Maintenance Tech there, and I was familiar with nearly all of the equipment on site. There were absolutely no 2-phase machines on site that I am aware of. I am 99% sure that this transformer was never used to generate 2-phase. If memory serves, I believe I was told that it was a spare for an old industrial trash compacter that was scrapped.

I went searching for answers as to why anyone would use a Scott T transformer for uses other than generating 2-phase, and found this:

The Advantages of the Scott T configuration:
1. If desired, a three phase, two phase, or single phase load may be supplied simultaneously
2. The neutral points can be available for grounding or loading purposes

The Disadvantages when used for 3 Phase Loading:
1. This type of asymmetrical connection (3 phases, 2 coils), reconstructs three phases from 2 windings. This can cause unequal voltage drops in the windings, resulting in potentially unbalanced voltages to be applied to the load.
2. The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due to manufacturing variances of the interconnected coils.
3. This design’s neutral has to be solidly grounded. If it is not grounded solidly, the secondary voltages could become unstable.
4. Since this design will have a low impedance, special care will have to be taken on the primary protection fault current capacity. This could be an issue if the system was designed for a Delta-Wye connection.
5. The inherent single phase construction and characteristics of this connection produces a comparatively bulky and heavier transformer when compared with a normal three phase transformer of the same rating.
7. In a comparative analysis of the published weights of the one commercially available Scott T transformer (only one manufacturer still constructs a dry-type Scott T) and a number of Delta-Wye transformer manufacturers you will find that Scott T’s are, in most instances, lighter than the similar kVA copper DeltaWye units. This goes against the mathematical principals and the sizing requirements stated by “Auten Stigant & Franklin”. It suggests that the Scott T transformers do not meet and are not equivalent to their Delta-Wye equivalent kVAs.

[...]

The Scott T would not be recommended as a connection for 3 phase to 3 phase applications for the following reasons:
• The loads of modern buildings and office buildings are inherently unbalanced and contain equipment that can be sensitive to potential voltage fluctuations that may be caused by the Scott T design.
• A properly sized Scott T transformer will have to be a minimum of 7.75% larger than the equivalent Delta-Wye transformer. Properly sized, it would be a bulkier and heavier option and should not be considered a less expensive solution. Applications requiring 3 phase to 3 phase configuration are best served by a Delta–Wye configuration. This configuration is the smallest and best suited for the unbalanced loads and sensitive equipment of today’s applications that could be affected by voltage fluctuations.
It seems like this Scott-T transformer should be an odd duck, hard to find, expensive, and used only in a very small, specific niche of applications. But when I search for transformers with a 208Y/120 secondary, half of them are Scott-T types. Why is that? Why are people buying these transformers? Is there an advantage to them that my reference failed to note?
 

Kermit2

Joined Feb 5, 2010
4,163
Did you find any info on the industrial emegency uses?

Combined with switch gear, the Scott T can create balanced three phase from only two phases. It is used to keep equipment running when utility service drops a phase or otherwise have a problem with one phase of the three phase service.
Probably not the answer you wanted but I posted it antway. For some reason :)
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
Did you find any info on the industrial emegency uses?

Combined with switch gear, the Scott T can create balanced three phase from only two phases. It is used to keep equipment running when utility service drops a phase or otherwise have a problem with one phase of the three phase service.
Probably not the answer you wanted but I posted it antway. For some reason :)
huh? When you lose a phase from a 3-phase supply, you're down to single phase. You saying a Scott T can generate 3 phase from single phase?
I'm curious now because that document I link to alluded to a similar thing:

At one time, when deliveries were long and locations were remote, it was possible to install three single phase transformers connected as a Delta-Delta or Delta-Wye. If one winding failed the remaining two units could be reconnected in Vee-Vee to replace the Delta-Delta or in a Scott T configuration to replace a Delta-Wye as a de-rated unit. This would get the system back up and running even if it was in a limited capacity. This was a very practical solution for remote locations such as marine applications where replacement units are not readily available for the Scott T design
I simply did not understand this blurb. When I read it the first time I thought it applied to "a long, long time ago, in a galaxy far far away" back when 2-phase was used. But now that I re-read it, it seems to be talking about 3-phase. Now I'm totally confused. I thought it was impossible to generate 3 phase from single phase without a RPC or static unit employing capacitors.
 

Kermit2

Joined Feb 5, 2010
4,163
The paper I just scanned through suggested industrial power utility connection often sees a problem with 1 of the 3 phases. Loss of two phases was not even talked about(in that paper)
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
The paper I just scanned through suggested industrial power utility connection often sees a problem with 1 of the 3 phases. Loss of two phases was not even talked about(in that paper)
loss of 2 phases would be total power loss
loss of 1 phase would leave you with single phase, same as your home service.
When you lose one phase, you don't drop down to 2-phase, you drop down to single phase.
 

Kermit2

Joined Feb 5, 2010
4,163
Okay. I went back and re-read the paper.
It clearly stated "with the loss of one phase, or more often, two phases..."
:)
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
Okay. I went back and re-read the paper.
It clearly stated "with the loss of one phase, or more often, two phases..."
:)
Well i guess i have one more thing to add to the list of ways in which i don't understand this transformer. In order for the transformer to do anything at all with only 1 hot leg (loss of 2 phases ), it would require a 4 wire supply with a neutral. And my understanding (was) that one of the only reasons for this transformer was to make a 4 wire output with a usable neutral from a 3 wire supply.
 

Kermit2

Joined Feb 5, 2010
4,163
Okay. I'm seeing a pattern.
There are two kinds of Scott T.
One is a connection and uses TWO individual transformers. Another is a transformer like yours that uses the aforementioned connection(3ph-3ph version) AND is mounted on a special shared magnetic core.
Using the one you have for anything other than 3 to 3 duty is frowned upon due to large neutral return currents and disruption in magnetic Flux through the central leg of xformer. I fully expect some of the above is not right, but I think it is going the right direction

Found the paper too. Seems my memory is getting worse every year, but I had the spirit of the text correct. :)
 

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Thread Starter

strantor

Joined Oct 3, 2010
5,160
I guess now i need to decide whether to try to make this into a static phase converter or continue on with my plans to make a welder out of it.

  • I've always wanted (but not needed, really) a static phase converter.
  • It's not every day you just "discover" you have a Scott T transformer, half the recipe for a SPC.
  • I need a good welder, now. I'm saving up for a nice one, but my needs are more immediate than my budget.
  • If i chop up the windings that are in it, I'll never get them back and I'll probably never see another scott t.
  • If i chop them up, I'll have two really nice big cores for my welder.
  • If i make a welder out of it, 5 years from now it will sit unused while I weld away with the fancy TIG outfit i should have by then. I'll look at it sitting lonely, no chance of ever being sold, and kick myself for destroying that scott t.
  • But i need a welder and i don't have money.
... I'm pretty sure I'm going to murder this bitch and make a stick welder. I can't afford any more crap sitting around waiting for its divine purpose to be realized in the future.
 
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