×
Follow Us
Results 1 to 6 of 6

Do you break one of the connections on a delta for winding resistance?

 Jump to latest post
    #1
  1. phasor's Avatar
    phasor is offline Junior Member Pro Subscriber
    Join Date
    Dec 2018
    Posts
    13
    Reputation

    Do you break one of the connections on a delta for 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.

  2. #2
  3. Join Date
    Aug 2018
    Posts
    7
    Reputation
    Quote Originally Posted by phasor View Post
    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.

  4. #3
  5. phasor's Avatar
    phasor is offline Junior Member Pro Subscriber
    Join Date
    Dec 2018
    Posts
    13
    Reputation
    Quote Originally Posted by baccuskt View Post
    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.

  6. #4
  7. NBatson's Avatar
    NBatson is offline
    I NETA BREAK
    Junior Member
    Join Date
    Feb 2019
    Location
    TX Gulf Coast
    Posts
    19
    Reputation

    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.

  8. #5
  9. Kalbi_Rob's Avatar
    Kalbi_Rob is offline Experienced Member Pro Subscriber
    Join Date
    Feb 2018
    Location
    Jacksonville, NC
    Posts
    290
    Reputation
    Quote Originally Posted by NBatson View Post
    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.

  10. #6
  11. SecondGen's Avatar
    SecondGen is offline
    I push buttons.
    NETA Level III Pro Subscriber
    Join Date
    Jan 2014
    Location
    United States
    Posts
    508
    Reputation
    Quote Originally Posted by phasor View Post
    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.

Subscribe

Share this thread

Related Topics

  1. Transformer winding resistance, balance ?
    By rgrabis in forum Electrical Testing Talk
    Replies: 3
    Last Post: January 19, 2018, 11:34 AM
  2. winding resistance tester and CPC 100
    By moon1958moon in forum Electrical Testing Talk
    Replies: 8
    Last Post: August 4, 2017, 12:09 PM
  3. winding resistance of power transformers
    By Sanjayo in forum NETA Level 2 Exam
    Replies: 4
    Last Post: March 27, 2016, 07:13 PM
  4. winding resistance tolerance for a transformer test
    By Larry5441 in forum Electrical Testing Talk
    Replies: 6
    Last Post: June 29, 2015, 04:33 PM

Tags for this Thread

Follow us


Explore TestGuy


NETA Certification Training


NICET Electrical Power Testing


Help and Support




You are viewing the archives. Enjoy new features and join the conversation at wiki.testguy.net