current transfomers

Discussion in 'General Electronics Chat' started by superway, Jan 31, 2014.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009
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    Hi,

    I have 2 current transfomers (CT rates 500A:5A) and would like to connect each other to measure Total Current of I1 and I2 ( pls see attch). Total current is measured at BNC to O'scope.
    I1 to CT1, and I2 to CT2.

    Which one is correct way to measure to Total Current? please see attch.

    Thanks for your time

    Best Regards,
     
    • CT.pdf
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  2. inwo

    Well-Known Member

    Nov 7, 2013
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  3. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
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    When connect 2 current transformers together to measure the total output current on O'scope, secondary transformer k and l of CT1 and CT2 are connected in parallel, is that right?. For example, pin k of CT1 is connected pin k of CT2... does anyone send me a link how to connect 2 CTs together?

    Thanks
     
  4. redplaya

    Member

    Jan 26, 2014
    30
    1
    (summing currents)
    Have you ever seen a double PNP Wilson current mirror when the load goes to infinity? The reference sides sums both input current sides (current summer). I am still wondering why your trying to do this . . .

    (summing magnetic flux)
    I would think an easier solution would be if you can sum the magnetic field instead by just running both wires in the same direction through one current transformer. What bandwidth of frequencies are you trying to measure (AC/DC)?
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    If you want to perform an addition of the two currents the series connection (second schematic) would be my initial choice.

    Although either configuration will give the correct answer provided you use the correct scaling factor.

    With equivalent CT's and two equal burdens (as shown in your schematics) the former parallel case would give half the output of the series case for any given I1 & I2. So the parallel case would only need one burden resistor shared between the two parallel CT secondaries. One less component is then a better outcome.

    As a general comment - This setup would produce an output corresponding to a phasor addition where there is a phase displacement between the two currents I1 & I2.

    Perhaps if you gave further details regarding your application it might help with giving the right advice.
     
    Last edited: Feb 1, 2014
  6. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
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    Thank you T_N_K. Your response is very helpful.

    The application is there is a 25KW resistor load feeding to L1 and L2 (240Vac) to DUT. I1 going to L1, and I2 going L2, so I used 2 current transformers to measure the total current of I1 and I2 on O'scope with BNC connector.
    On my schematic B is a series connection, is it correct polarity when connecting 2 CTs?

    With my application, can I use one CT (500A:5A, 15VA, 10 ohm burden) instead of using 2 CTs?

    Thanks
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    With my application, can I use one CT (500A:5A, 15VA, 10 ohm burden) instead of using 2 CTs?

    Yes, if there is only one load being driven by L1 & L2 as a single series loop.

    There is a proviso - with a 15VA rating at full 5A secondary current then the safe secondary voltage is 3V rms. This means a maximum burden resistance less than 10 ohms.

    If L1 & L2 are running separate loads then ....

    Are I1 & I2 displaced in phase?
     
    Last edited: Feb 1, 2014
  8. redplaya

    Member

    Jan 26, 2014
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    1
    Yea I was wondering why you were trying to do this because unless they are completely in phase (even a slight mismatch in complex impedance will create a mismatch) the result would be extremely hard to understand (a clash of two superimposed waves creating their own harmonics).

    You may have to convert the AC average to a DC equivalent average and then sum to get rid of the phase issue unless you want to use a phase lock loop method to match phases (unlikely and assuming they are, and only, the same frequency). This is easy with analog methods

    trying to help, ha
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    No harmonics unless there is some non-linearity in the CT behaviour - rather the linear superposition of the two signals.

    I suspect the OP is proposing driving one load off two phases of a 3-phase mains supply, although the precise nature of the system set-up has not been clearly explained. It would have been better had we been provided with a more complete schematic showing everything including the measurement part.
     
  10. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    thanks for your helps.
    Enclosed attch. is my rough draft of schematic.
    DUT is a DC / AC inverter with high power.

    Please look my drawing connection on CT1 and CT2 following with a dot on them ensure it is correct polarity.

    Thanks
     
  11. redplaya

    Member

    Jan 26, 2014
    30
    1
    Yea, to t_n_ks point, nothing in this world is linear, every system has infinite poles and zeros but you grab as many dominant factors as you need to describe inconsistencies. There's capacitance from you to the moon but its not exactly relevant in any system.

    That being said, every CT is nonlinear to an extent (even in their linear region) and if you add two fundamentals with different phases the summation is a fundamental with the differential of the Fourier components (added harmonics).

    Even though the PO still hasn't told us the phases, by the diagram now I am guessing he is pulling these from his house where L1 and L2 are from a center tap transformer (180 deg phase difference).

    I agree with the wiring in terms of in series because your measuring currents by an induced voltage. So your essentially summing induced voltages (voltages add In series). It should still work assuming the two transformers are identical in primary inductance (same turns provides same secondary inductance) and leakage. This will insure the 180 deg phase difference in currents will lag additional more the same.

    Additonal, now that I know they are exactly opposite in phase you should be able to run both wires through one current transformer and sum the magnetic flux at once. Run L2 backwards with L1 into the loop so that their magnetic flux is in phase through the same loop.
     
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