Transformer Question

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brian john

Senior Member
Location
Leesburg, VA
I have an unusual transformer (see picture) This is an Allis-Chalmers 150 KVA Dry Type transformer there are two visible coils and the drawing appeas to be a T. This is 3 phase 480 VAC to 208/120 VAC. There was an issue with the output voltage and just trying to get some information. Any help would be appreciated.

We plan to megger and TTR the transformer.

From what I see A and C are derived from one coil and the neutral and B phase from a sperate coil.

Lousy picture taken with my IPhone.



 
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pete m.

Senior Member
Location
Ohio
My guess... The diagram was just badly drawn (in other words a "delta" and "wye" configuration were not correctly represented).

As for the strange voltages I'll ask the stupid question... did anyone look at the positions of the primary taps?

Pete
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
This is definitely a T connected transformer.
This construction is very common on small (i.e. <30kVA) units, I cannot say I have seen one at the 150kVA size.

I would have to confirm it, but I believe you would treat it as a wye-wye transformer during your TTR.
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Based on the location of XO it seems there is a difference in the ratios of the coils.
Yes there is a difference.

I just pulled out my TTR book and it does have a procedure for testing T-Type transformers. It looks like you apply some external jumpers between the coils and then test each coil as if it was single phase.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Thanks Jim

Interesting read

and it seems a T-T must have extra weight (copper steel) by design.

http://www.hammondpowersolutions.com/upload_files/hpsi-ta11.pdf
A Scott (Tee) connection is for going between 2-phase and 3-phase.
A standard T connection is for 3-phase to 3-phase.

They are not the same arrangements, even though they look similar. In particular there is a difference in the connection point of the teaser winding while both have a primary tap at a 86.6%, the standard T requires a tap at about 33% on the secondary (this becomes the X0).
 
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brian john

Senior Member
Location
Leesburg, VA
What I did, We ttr'd the transformer and meggered it. Then I applied 208 on the primary from a 15 amp fused safety switch, using a variac to minimize inrush current, blowing my fuses. What I found out someone a long time ago mis-marked the secondary terminals. XO was marked X3, X3 was marked XO X1 and X2 were tagged correctly.

Seems 120 VAC appliances do not appreciate 208 VAC.
 
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