Transformer winding resistance, balance ?

[rgrabis]

Winding resistance is one of the more common tests we perform. However, unlike insulation resistance or conducting path (contact) resistance, I am not aware of any specs. or rule of thumb for the allowable variation in resistance readings between phases. Does anyone know of any or have any thoughts on this issue? Thanks

[SecondGen]

So I did some searching and I found this from Megger:

The transformer winding resistance test is very useful in identifying and isolating the location of suspected problems. Winding Resistance Measurements are evaluated by:


Comparing to original factory measurements
Comparing to previous field measurements
Comparing one phase to another


The latter will usually suffice. The industry standard (factory) permits a maximum difference of 1/2 percent from the average of the three phase windings. Field readings may vary slightly more than this due to the many variables. If all readings are within one percent of each other, then they are acceptable.

Variation from one phase to another or inconsistent measurements can be indicative of many different problems:


Shorted turns
Open turns
Defective DETC or LTC
Poor connections (brazed or mechanical)



See the PDF attached to this post, it will pretty much explain everything you need to know about winding resistance. :cool:

[rgrabis]

Hey, that's great. Thanks for the help

[wooddy]

So I did some searching and I found this from Megger:

The transformer winding resistance test is very useful in identifying and isolating the location of suspected problems. Winding Resistance Measurements are evaluated by:


Comparing to original factory measurements
Comparing to previous field measurements
Comparing one phase to another


The latter will usually suffice. The industry standard (factory) permits a maximum difference of 1/2 percent from the average of the three phase windings. Field readings may vary slightly more than this due to the many variables. If all readings are within one percent of each other, then they are acceptable.

Variation from one phase to another or inconsistent measurements can be indicative of many different problems:


Shorted turns
Open turns
Defective DETC or LTC
Poor connections (brazed or mechanical)



See the PDF attached to this post, it will pretty much explain everything you need to know about winding resistance. :cool:

couple things that you can do to ensure that your readings are consistent, usually start with centre phase readings first, this way you have equal flux flowing between windings for all of the testing, another thing we run into with our cold weather is to be careful of how much current and how long the machine is on for each readings, we have found that if ur using say 50 amps and its taking a long time for the machine to balance and the transformer windings are really cold there is actually enuf heating provided by the winding resistance tester to progressively change the resistance of the winding as you go through the taps thus giving you readings that are out of balance compared to the other windings and or taps, obviously if you can do the testing while the transformer is still warm this is better, obviously record the winding temperature so all readings can be corrected and compared to original manufacters readings