Transformer ratio question

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jow

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Can somebody help me with a transformer question? I have a 3 phase 250 KVA 13200 Y - 120/240 delta tx. When I try a 2VM ratio test I inject 240vac on the secondary and read (pp) 12400 A/B, 8460 B/C, and 12400 A/C. the nameplate is a bit old and I suspect there is some crucial information I am missing. Could this be "T" connected? Do i need some factor (i.e 1.73,etc)when going from y to delta? Can i "backfeed" without ramifications? My question is, Can anyone help explain these values to me? :confused:
 
Re: Transformer ratio question

jow, are you driving this xfrmr with single phase 240 on just one phase of the delta? If so you are applying half voltage to the other two secondaries of the xfrmr, and you will not see the normal 3-phase voltages. Vac and Vab are roughly correct for this scenario, but Vbc ought to be zero. Can you measure the the line to neutral voltages on the primary?
 
Re: Transformer ratio question

I wish I could see a schematic of it. I don't deal with three phase often.

I notice the ratio between the two voltages on the tested side are close to .707.

I'm curious what the purpose of a "Y" configuration on a primary is.
 
Re: Transformer ratio question

jow,
Are you using a single phase Turns-Ratio Tester, or is it one designed to test the three phases at once?

You are using an approved Turns-Ratio Tester, right?
Should we be concerned about your words "I inject 240vac on the secondary"? Hope you are still OK.

Ed
 
Re: Transformer ratio question

o.k... i used a standard low voltage 3 phase ttr and was able to obtain the correct ratio based on its vector group configuration (y - d), by the way it is 55 (calculated) 55.7 (measured). The next test i usually perform is with rated three phase voltage. This is where i inject, yes Ed i'm fine, secondary voltage (all 3 phases) and read back on the primary. Thats where I obtained all of these weird numbers(yes i checked out all of my equipment and it is fully functional and accurate). I cannot measure line to neutral on the primary (equip. constraints). Sam.... We call these ignitor Tx's used in our generation plants. I've read that backfeeding a Y - D is not the thing to do, but have yet to see an adequate explanation as to why. This just further sparked my curiosity as to why my test board gave me those strange voltages. If someone can tell me how to post a diagram, I would be happy to try to draw it.
 
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