Winding Resistance Test !

The winding resistance is going to be a function of the characteristic resistance for the kind of wire used and the total length needed for that winding.

Different size transformers (rated in KVA) have different numbers of windings and different wire diameters based on current requirements. The overall length of wire used will be a function of the KVA rating and the number of loops required to achieve the transformation.

Changing taps simply increases or decreases the overall length (number of loops) of wire used. An ohm meter on an inductor, such as the winding of a transformer, may cause damage to it when you break the measurement circuit. That is because an ohm meter provides a little bit of current into the device to be measured. When you break that circuit, you get a large voltage spike because V=L * di/dt and the value of di/dt becomes very large when you interrupt the current flow.

sir, how will i know or what are the standards on a single phase distribution transformer for the DC Winding Resistance Test?

say 3 - 500kVA distribution transformer (formerly banked) where tested individually with winding resistance test and based on the 5% phase difference with the other transformer one of the transformer go beyond the limit on the Primary side...

The winding resistance of a transformer will be dependent on the physical characteristics of the magnet wire used. The resistance will be a function of several factors like; AWG size, cross sectional area, lenght of winding(s), purity of copper and/or material used in the manufacturing and fabrication processes. It amy also be dependent where the ore was mined.. Based on past experience, copper magnet wires imported from China have different characterisitics as compared to other sources! The lower the resistance of the wire the lower the copper losses of a transformer.. less heat therefore more efficient..

Hi,

as you change your taps you are adding or subtracting turns and then your winding resistance should change accordingly. Typically you should take readings on all taps of a newly installed transformer and keep these numbers as a baseline and then during periodic testing compare these numbers to detect changes. Do you have such a baseline data? If not, a second criteria is comparing similar transformers as you are doing now. If the resistance is much lower on one transformer as compared to the other then it may be an indication of a shorted turn. Did all the rest of your testing pass? Did you do a turns ratio test?

sir james, we dont have a baseline data.

-turns ratio test on all taps were within its acceptable limits for 3 transformers . these are dc winding resistance test results of 3 transformers:

they are on primary side, we tested 3- single phase transformer

T1: (H1-H2) tap 1 -422.1 , tap 2- 412.2 , tap 3- 403.5 , tap 4- 393.5 , tap 5- 384.1

T2: tap 1 -379.1 , tap 2- 371.8 , tap 3- 363.8 , tap 4- 354.7 , tap 5- 345.8

T3: tap 1 -383.9 , tap 2- 374.9 , tap 3- 366.7 , tap 4- 356.7 , tap 5- 347.2

what are your assessments ?
TIA

I'm not sure what you are trying to determine. Keep in mind that the manufacturer can tell you what the resistance should be and what kind of equipment you should use to measure it. A simple ohm meter is not always your best choice, especially on such a large winding.

Phase shifting has nothing to do with resistance. It is a method of mitigating the effects of harmonic distortion. The winding will have most of its loops on one core and the rest on another.

Your ability to obtain a meaningful measurement will depend on the type of instrument you use. Contact resistance and thermoelectric effects (from different types of metal) can influence your reading. Remember, a thermocouple produces a small voltage because of galvanic action of the two types of metal in contact with each other. Your test leads can produce a small effect this way.

sir james, we dont have a baseline data.

-turns ratio test on all taps were within its acceptable limits for 3 transformers . these are dc winding resistance test results of 3 transformers:

they are on primary side, we tested 3- single phase transformer

T1: (H1-H2) tap 1 -422.1 , tap 2- 412.2 , tap 3- 403.5 , tap 4- 393.5 , tap 5- 384.1

T2: tap 1 -379.1 , tap 2- 371.8 , tap 3- 363.8 , tap 4- 354.7 , tap 5- 345.8

T3: tap 1 -383.9 , tap 2- 374.9 , tap 3- 366.7 , tap 4- 356.7 , tap 5- 347.2

what are your assessments ?
TIA