Problems with voltage on secondary XFMR

[Nsmith]

A Phase monitor was not pulling in today on a piece of equipment so I was checking the voltage. There is a transformer that knocks the Voltage down from 480 to 240 Delta Delta Config. I have attached an image of the winding setup for the transformer to this post. The 480 is wired and tests as follows.

Prim
H1 with 1 285 to Gnd
H2 with 2 265 to Gnd
H3 with 3 262 to Gnd

Sec
X1 115 to Gnd
X2 119 to Gnd
X3 191 to Gnd

X6 is not connected.

I spoke with Sola HD on the phone and they told me I need to have X6 to Gnd I told them I need 240 Phoundase to Phase and If I connect X6 that will give me 120. Speaking to another person they told me I need to have any one of the phases grounded or it will not test right.

I cant up load the pic so its on page 9 of 10 electrical connection 28

 

[gar]

110210-2154 EST

What is X6?

.

 

[ptonsparky]

If you need 240 Ph to Ph, why you measuring phase tonground?

 

[hillbilly1]

is it a phase loss monitor? or just an indicator light? Phase loss monitors do not require a neutral or a grounded phase either, so lack of these should not affect it's operation.

 

[zog]

I suggest testing the transformer. Megger, DAR, PI, TTR, and winding resistance.

 

[Nsmith]

http://www.solahd.com/products/transformers/automation/Pdfs/AutomationXfrmrs.pdf

Here is the link to the Tap arrangements its on page 9 of 10 tap arranage ment number 28

X6 is a Center tap so you get 120-0-120 I need 240-240-240

I was measureing from phase to ground to see where the problem was with the taps. They are telling me that I need to ground X6 or a another secondary tap.

Will this solve me problem of having 191 on tap X3? I am going to insulation resistance test it now. I guess some of my confusion comes from when do you want to ground one of the secondary taps? I have herd of it being done but never seen it. I know you will ground a center tap to get half the voltage but that is not needed here as there is already another controls transfromer in the panel for this.

 

[gar]

110211-0835 est

Nsmith:

I did not see any reference in your original post. So X6 is the center tap of one secondary.

I have now looked at your reference of post #6.

When you have a delta secondary you can float this off of ground, or connect any one of the secondary terminals to ground depending upon your needs.

Your test voltages of post one were
X1 115 to Gnd
X2 119 to Gnd
X3 191 to Gnd
With X6 unconnected

This implied a floating secondary. From the voltages I might have guessed that X1-X2 was the center tapped winding, but that is not what the drawing shows.

X6 is the center tap of X1-X3. A floating secondary with no external load will have voltages from the three terminals to ground (probably meaning to the transformer case) of varying values that will depend upon the leakage capacitance and resistance to the transformer case. If there was an external load, then the load's resistive and reactive components to ground would add to the transformer's and produce some different distribution of voltages to ground.

Few transformers will be designed to or by chance have leakage components that would result in equal voltages to ground.

Why not just ground any corner or X6 in your application? Do you have a need for balanced voltages to ground? Is so, then maybe choose a Y secondary. So what if you do have 120 to ground from X1 and X3 that would result from X6 being grounded, is this any problem?

Your voltages to ground do not necessarily imply that you do not have 240 across each secondary.

.

 

[Nsmith]

well measureing from phase to pahse on the secondary I do get 230-240 volts so its not really having an effect on the power. I am still confused on why the one leg is so high and I would think it would have more of an effect on the phase to phase measurement.

 

[ptonsparky]

Unless a point on the transformer secondary is grounded, there is no voltage reference to ground other than the capacitance of the secondary. Add more or less wire, a different type of load, and the voltage will change. Grounding a phase or x6 will stabilize the reference to ground from each phase. Ungrounded or grounded will not affect the ph to ph voltage.

 

[gar]

110211-1028 EST

Nsmith:

If you ground X6 and the transformer is theoretically perfect, then your voltages would be:
X1-X2 240
X2-X3 240
X3-X1 240

X1-X6 120
X3-X6 120
X2-X6 207.84

Your test voltages of post one were
X1 115 to Gnd
X2 119 to Gnd
X3 191 to Gnd
With X6 unconnected

If you relabeled your X3 of post 1 to X2, then the voltages would look similar. Thus, it is likely that internal capacitance distribution is such as to create a similar effect.

.