+ Post New Thread
Results 1 to 8 of 8
  1. #1
    Member level 4
    Points: 1,561, Level: 9

    Join Date
    Sep 2007
    Location
    California, USA
    Posts
    70
    Helped
    3 / 3
    Points
    1,561
    Level
    9

    Ratio error of isolation transformer

    Not having a lot of experience with isolation xfmrs I'm puzzled with the performance.

    I have 3 different 115/115 transformers and with about a 50% load I'm getting about 10-15% higher output voltage between primary and secondary.

    Measuring the resistance of the coils I get a significant difference too (5 to 7 ohms)

    Am I missing something or is this normal ?

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 20,466, Level: 34
    Achievements:
    7 years registered

    Join Date
    Apr 2008
    Location
    EU
    Posts
    2,332
    Helped
    329 / 329
    Points
    20,466
    Level
    34

    Ratio error of isolation transformer

    Hi freddybaby,
    I belief this is normal, because your 2 windings are probably all over the other, this is the reason for 2 different resistances too.
    The measured good 10% higher Uout with 50 % load is OK: (usually)you should apply 100% load for nominal specifications...
    Remember: what is your main voltage if you do chek the transformer_maybe it was some higher too
    K.


    1 members found this post helpful.

  3. #3
    Super Moderator
    Points: 261,177, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,631
    Helped
    13878 / 13878
    Points
    261,177
    Level
    100

    Ratio error of isolation transformer

    Yes. The coils resistances would be different anyway for most transformers due to the different mean winding diameter
    between primary (usually inner) and secondary (outer) winding.

    But most transformers are designed to achieve nominal output voltage at nominal (full) load , so the unloaded or partially
    loaded output voltage can be expected higher. 10-15% difference at 50% load is a lot, however. There should be a
    specification, I think.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  4. #4
    Member level 4
    Points: 1,561, Level: 9

    Join Date
    Sep 2007
    Location
    California, USA
    Posts
    70
    Helped
    3 / 3
    Points
    1,561
    Level
    9

    Ratio error of isolation transformer

    Thank you for the replies..

    My nominal V is whatever is coming out of the power line. My problem was my line voltage runs about 123 and the extra 12-15V on the secondary is causing my contactor to heat up (+100 C)

    I would expect the windings to be fairly similar in a 1:1. An extra 2-5 ohms might mean more windings on one side. (Using a 4 wire kelvin sense btw)

    Would they really need or use different wire diameters for an isolation xfmr ?

    Added after 5 minutes:

    Oh ya, in reference to specifications, I scoured a bunch of data sheets and they only spec regulation and output impedance. No specification for ratio error (except on instrument xfmrs)



  5. #5
    Advanced Member level 5
    Points: 20,466, Level: 34
    Achievements:
    7 years registered

    Join Date
    Apr 2008
    Location
    EU
    Posts
    2,332
    Helped
    329 / 329
    Points
    20,466
    Level
    34

    Ratio error of isolation transformer

    It must have an short in primary: this can be the reason for your practically 50% delta between winding resistances...
    I belief, that 100C is too much, this can not be coming from +10-12% higher main voltages_ this is the other resultat of an short in primary winding.
    K.



  6. #6
    Super Moderator
    Points: 261,177, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,631
    Helped
    13878 / 13878
    Points
    261,177
    Level
    100

    Ratio error of isolation transformer

    If the transformer itself isn't heating up, it should be O.K.
    My nominal V is whatever is coming out of the power line.
    The usual specification is different. Standard transformers don't have 1:1 windings ratio for 1:1 input to output ratio.
    They have an extra on the secondary to compensate for the voltage drop with full load. This can be of course a problem in
    some cases. You need a variable or tapped transformer to adjust the output voltage in this case. As a workaround, you
    can reverse primary and secondary of the transformer, then it should sligthly step down the input voltage.



    •   AltAdvertisement

        
       

  7. #7
    Member level 4
    Points: 1,561, Level: 9

    Join Date
    Sep 2007
    Location
    California, USA
    Posts
    70
    Helped
    3 / 3
    Points
    1,561
    Level
    9

    Re: Ratio error of isolation transformer

    Quote Originally Posted by FvM
    If the transformer itself isn't heating up, it should be O.K.
    My nominal V is whatever is coming out of the power line.
    The usual specification is different. Standard transformers don't have 1:1 windings ratio for 1:1 input to output ratio.
    They have an extra on the secondary to compensate for the voltage drop with full load. This can be of course a problem in
    some cases. You need a variable or tapped transformer to adjust the output voltage in this case. As a workaround, you
    can reverse primary and secondary of the transformer, then it should sligthly step down the input voltage.
    This is what I ended up doing and now my contactor will not pull in below 111VAC. Ok for my area but If the line drops I'm sunk.. I think the low spec for utility is 105 ??? Guess I'll keep my fingers crossed. Should have went with a DC coil and step down+rectifier...

    Thanks again for all the comments and help.



  8. #8
    Super Moderator
    Points: 261,177, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,631
    Helped
    13878 / 13878
    Points
    261,177
    Level
    100

    Ratio error of isolation transformer

    Isolation transformators used for industrial automation mostly have additional taps for +5% and -5% output, they can handle most of these cases.



--[[ ]]--