Delta/Wye Transformer Question....

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garrisonm1

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Ok haven't posted in awhile and scratching my head on this one. Looking for insight


I have a 120/208 3 phase delta input step up transformer with 230/380volt Y output.



Here are my current readings which looks good:

Input:

X1-G 116v
x2-G 115v
x3-G 116v
X1-X2 201v
X1-X3 201v
x2-X3 200v

Output:
H1-H0 212v
h2-H0 216v
H3-H0 213v
H1-H2 370v
H1-H3 366v
H2-H3 370v


Here is my issue, keep in mind I currently do not have any load wires landed on the transformer yet. When energizing the transformer with test leads connected to X1-X3 I get a reading of 97 volts for about 1 minute and then the voltage corrects itself. Same on X2 except the reading is about 112volts. I verified the issue is not the breaker as I took the leads off the transformer and did not experience this issue. After about a minute all reading are correct.

Any thoughts on why the input leads on the transformer have a lower voltage initially?
 
Is the Wye secondary (H0) grounded? Your input voltages are low but otherwise the numbers look pretty good.
 
190619-2116 EDT

garrisonm1:

Your 1 minute time constant implies a high resistance joint in a supply wire, and that after this time period the loose joint warms up enough to become a low resistance.

You need to go back to some main point in the supply to the transformer and check voltage.

Also, if you power the transformer for a while until voltage reaches its normal level, then remove the supply, and immediately reapply the supply it is likely that full voltage or near full voltage will reappear. If so, then turn off power for a longer time, and see how long of a cool down time is required to recreate the problem.

.
 
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