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# Delta winding resistance

1. ## Delta winding resistance

Does anybody break one of the connections on a delta winding when doing winding resistance? If you're injecting current and measuring voltage drop across H1-H3 for example you are still sending current up through H2, essentially putting 2 additional windings in parallel and not measuring the true resistance of any one winding. On our forms we do record the readings as H1-H2 etc but just wondering if anyone does it different.

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Originally Posted by phasor
Does anybody break one of the connections on a delta winding when doing winding resistance? If you're injecting current and measuring voltage drop across H1-H3 for example you are still sending current up through H2, essentially putting 2 additional windings in parallel and not measuring the true resistance of any one winding. On our forms we do record the readings as H1-H2 etc but just wondering if anyone does it different.
Most transformer do not allow disconnecting of individual phases. You are using DC current so it shouldn't make a difference because the current is mostly flowing through the path of least resistance i.e. the single winding rather than having to travel through two windings. I thought about this before also but I have never heard of anyone disconnecting the other phases.

3. Originally Posted by baccuskt
Most transformer do not allow disconnecting of individual phases. You are using DC current so it shouldn't make a difference because the current is mostly flowing through the path of least resistance i.e. the single winding rather than having to travel through two windings. I thought about this before also but I have never heard of anyone disconnecting the other phases.
That's not gonna cut it if your tolerance is 3%. You're right about not being able to break connections on many xfmrs but on larger dry types you can and usually will break some connections if you're PF testing.

4. ## multiply

Take your reading and multiply by 1.5, for instance h1-h2 is 22.5 milli ohms times 1.5 is 33.75. Therefore your h1 winding resistance is 33.75 milli ohms. I'm headed to the engineers office now for a in depth explanation.

5. Originally Posted by NBatson
Take your reading and multiply by 1.5, for instance h1-h2 is 22.5 milli ohms times 1.5 is 33.75. Therefore your h1 winding resistance is 33.75 milli ohms. I'm headed to the engineers office now for a in depth explanation.
First, your calculation is for contact resistance and not winding resistance.

----------------------------------------------------------------------------------

--On small Dry transformers, winding resistance is not a required or optional test.

--On large Dry Transformers, winding resistance is an optional test and the passing requirement is:

Temperature-corrected winding-resistance values shall compare within one percent of
previously obtained results.

--On Oil Filled Transformers, Measure the resistance of each high-voltage winding in each de-energized tap-changer
position. Measure the resistance of each low-voltage winding in each de-energized tapchanger
position.

And the Passing requirement is:
Consult the manufacturer if winding-resistance test values vary by more than two percent
from factory test values or between adjacent phases.

------------------------------------------------------------------------

So, multiplying by 1.5 will give you incorrect parameters. I find most technicians don't realize there is a difference between the different types of transformers and their parameters.

6. Originally Posted by phasor
Does anybody break one of the connections on a delta winding when doing winding resistance? If you're injecting current and measuring voltage drop across H1-H3 for example you are still sending current up through H2, essentially putting 2 additional windings in parallel and not measuring the true resistance of any one winding. On our forms we do record the readings as H1-H2 etc but just wondering if anyone does it different.
We don't do this at my shop and I've never heard of breaking delta connections for winding resistance but I find it interesting because its something I haven't thought about before. I think you are correct that current is passing through all of the windings simultaneously, however for this test, current doesn't really matter as long as it stays constant - its from what points you measure the voltage drop that gives you the resistance value.

Correct me if I'm wrong, but it appears that you could simplify winding resistance by applying current to H1-H3 and only move the voltage leads between phases after each measurement? By taking voltage drop measurements H1-H2, H2-H3, H3-H1 - you essentially measure each side of the delta symmetrically.

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