# Transformers



## All Current (Jul 31, 2011)

This seemed real crazy to me but what do I know. I figured I would throw this out there. Went on a service call today at an old auto body shop and was testing the voltage at the transformer and came up with 480 to ground, 277 to ground on two phases but between any of the two phases had 240volts. I thought I was loosing my mind so tested several times and came up with the same. Has anyone ever came across something like this?


----------



## Chris Kennedy (Nov 19, 2007)

How about a more detailed description?


----------



## All Current (Jul 31, 2011)

That's really the whole situation. They had a few smaller transformers around to get 120 volts. Never saw anything like this before. Has anyone ever come across anything like this. The trans was so old I was unable o make out the nameplate.


----------



## wildleg (Apr 12, 2009)

sounds like a 240 delta that was ungrounded, and your readings to ground were just floating measurements that didn't mean anything, but that's just a guess.


----------



## All Current (Jul 31, 2011)

Definitely weren't floating measurements. I checked multiple times just to be sure I wasn't crazy.


----------



## socalelect (Nov 14, 2011)

All Current said:


> This seemed real crazy to me but what do I know. I figured I would throw this out there. Went on a service call today at an old auto body shop and was testing the voltage at the transformer and came up with 480 to ground, 277 to ground on two phases but between any of the two phases had 240volts. I thought I was loosing my mind so tested several times and came up with the same. Has anyone ever came across something like this?


480 to ground on one and 277 kind of sounds like a corner grounded setup


----------



## etb (Sep 8, 2010)

wildleg said:


> sounds like a 240 delta that was ungrounded, and your readings to ground were just floating measurements that didn't mean anything, but that's just a guess.


Sounds like a good guess. 240 between all phases indicates those connections seem solid and either 3W/4W delta. If ungrounded 3W the phase-grounds would be erratic. But the voltages he gets matches the _ratios_ he should get for 4W delta, so there must be some sort of connection to the CT of one xfmr. Bizarre that they happen to be right at 480 & 277 though.



All Current said:


> That's really the whole situation.


It's not the whole situation unless you explain exactly how the xfmrs are connected, how they're bonded, and where the neutral (if exists) connects, and more measurements. You said you're measuring to ground at the xfmrs, but what are the phase-neutral voltages at xfmrs? And what are the phase-neutrals at the panel. What was the problem the body shop called you for?



All Current said:


> Definitely weren't floating measurements. I checked multiple times just to be sure I wasn't crazy.


You mean you simply re-measured several times? What did you do to prove the system isn't floating? What is your voltage from neutral in at the panel to building steel?


----------



## Chris Kennedy (Nov 19, 2007)

All Current said:


> 480 to ground, 277 to ground on two phases but between any of the two phases had 240volts.


Phase converter???


----------



## brian john (Mar 11, 2007)

All Current said:


> Definitely weren't floating measurements. I checked multiple times just to be sure I wasn't crazy.


How do you know this?

ETB is spot on in regards to this.


----------



## retiredsparktech (Mar 8, 2011)

socalelect said:


> 480 to ground on one and 277 kind of sounds like a corner grounded setup


Not a corner grounded setup, but it seems that the tranformer is a three phase, four wire setup. Simular to the 120/240, three phase delta setup, only all readings are doubled. The winding from A and C phase is center tapped. Readings would be as follows:
A to B, 480 V.
C to B , 480 V. 
A to C, 480 V.
A to CT, 240 V.
C to CT, 240 V.
B to CT, 277 V. ,Wild leg.


----------



## socalelect (Nov 14, 2011)

retiredsparktech said:


> Not a corner grounded setup, but it seems that the tranformer is a three phase, four wire setup. Simular to the 120/240, three phase delta setup, only all readings are doubled. The winding from A and C phase is center tapped. Readings would be as follows:
> A to B, 480 V.
> C to B , 480 V.
> A to C, 480 V.
> ...


The thing that made me think. Corner ground was his first statement of voltages which he said 480 to ground


----------



## socalelect (Nov 14, 2011)

socalelect said:


> The thing that made me think. Corner ground was his first statement of voltages which he said 480 to ground


And 277 to ground on the other 2 phases


----------



## wwilson174 (Apr 25, 2008)

retiredsparktech said:


> Not a corner grounded setup, but it seems that the tranformer is a three phase, four wire setup. Simular to the 120/240, three phase delta setup, only all readings are doubled. The winding from A and C phase is center tapped. Readings would be as follows:
> A to B, 480 V.
> C to B , 480 V.
> A to C, 480 V.
> ...


It sounds like a Scott tapped transformer secondary, BillW (Google it)


----------



## hardworkingstiff (Jan 22, 2007)

retiredsparktech said:


> Not a corner grounded setup, but it seems that the tranformer is a three phase, four wire setup. Simular to the 120/240, three phase delta setup, only all readings are doubled. The winding from A and C phase is center tapped. Readings would be as follows:
> A to B, 480 V.
> C to B , 480 V.
> A to C, 480 V.
> ...


I would think the wild leg would be 415 not 277.


----------



## retiredsparktech (Mar 8, 2011)

socalelect said:


> And 277 to ground on the other 2 phases


IIRC, you would see 277 volts from A/C CT to B phase. :whistling2:


----------



## socalelect (Nov 14, 2011)

at an old auto body shop and was testing the voltage at the transformer and came up with 480 to ground, 277 to ground on two phases but between any of the two phases had 240volts. I thought I was loosing my mind so tested several times and came up with the same. 

480 to ground and 277 between 2 phases he didn't mention anything about ct s here. Last I checked if u had 480 to ground that was a corner grounded setup


----------



## freeagnt54 (Aug 6, 2008)

socalelect said:


> at an old auto body shop and was testing the voltage at the transformer and came up with 480 to ground, 277 to ground on two phases but between any of the two phases had 240volts. I thought I was loosing my mind so tested several times and came up with the same.
> 
> 480 to ground and 277 between 2 phases he didn't mention anything about ct s here. Last I checked if u had 480 to ground that was a corner grounded setup


With 240v between phases?


----------



## etb (Sep 8, 2010)

wwilson174 said:


> It sounds like a Scott tapped transformer secondary, BillW (Google it)


The Scott-t I'm familiar with is for 3-phase to 2-phase. In 3W 2-phase he would have got 240 L-N and 340 L-L. Is there something else you're meaning?



hardworkingstiff said:


> I would think the wild leg would be 415 not 277.


Correct, the math is all linear: 480/240 = 2, so 208*2 = 416.


----------



## socalelect (Nov 14, 2011)

freeagnt54 said:


> With 240v between phases?


How would he have a 480 and a 277 and a 240. Did he get the primaries ans secs?


----------



## Big John (May 23, 2010)

All Current said:


> Definitely weren't floating measurements. I checked multiple times just to be sure I wasn't crazy.


 What meter did you check with? Was it a numeric multimeter, or was it a tester sort of like this:









-John


----------



## All Current (Jul 31, 2011)

Big John said:


> What meter did you check with? Was it a numeric multimeter, or was it a tester sort of like this:
> 
> -John


It was a fluke multi meter


----------



## All Current (Jul 31, 2011)

The readings were dead on. There was no fluctuation.


----------



## Chris Kennedy (Nov 19, 2007)

Its been 3 days, have you been back to the facility? Got any pics?


----------



## wwilson174 (Apr 25, 2008)

etb said:


> The Scott-t I'm familiar with is for 3-phase to 2-phase. In 3W 2-phase he would have got 240 L-N and 340 L-L. Is there something else you're meaning?
> 
> 
> The Scott-T transformer connection may be also be used in a back to back T to T arrangement for a three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0 Neutral tap is part way up on the secondary teaser transformer (see below). The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned.
> ...


The Scott-T transformer connection may be also be used in a back to back T to T arrangement for a three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0 Neutral tap is part way up on the secondary teaser transformer (see below). The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned.


----------

