current transformer vs rogowski coil

Discussion in 'General Electronics Chat' started by andi1231, Jul 13, 2016.

  1. andi1231

    Thread Starter New Member

    Dec 13, 2015
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    Hello dear community,
    I've measured the current impulse of an impulse magnetization machine. In order to measure the Impulse I installed a rogowski coil and a normal current transformer with a ferromagnetic core at the supply line of the magnetization coil. They measure the same impulse curve - see attachment - but at the zero crossing you can see that with the current transformer the curve is remaining at the zero level. In contrast to it, the rogowski coil shows a negative offset. Ther was no offset at the beginning of the measurement. My question ist: Why does the rogowski coil show a negative offset at the end of the impulse? Has it got anything to do wiht the evaluation unit (integrator) of the rocowski coil?
    Thanks for your help!!!
    Andi
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Since they both seem to have the same peak output it might not be an offset. First of all, can you adjust the vertical scale and centering to see if you can make the two waveforms match at the peaks and at the asymptotes?

    If so, then what you are probably seeing is a higher low-frequency roll-off in the Rogowski coil's response. As I recall Rogowski coils need to be followed by an integrator to restore the waveform.
     
  3. andi1231

    Thread Starter New Member

    Dec 13, 2015
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    Thanks for your help. The two curves were recorded simultaneously on the same scale by an oscilloscope... accordingly they measure the same impulse and have the same scale... the peak value is nearly the same.
    Could you please explain in detail, what you mean with a higher low-freqeuncy roll-off?
    In fact the output signal of the rogowski coil is evaluated by an integrator circuit!
    Tanks
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    There is a discussion of high and low frequency roll-off at this URL:
    https://en.wikipedia.org/wiki/Roll-off

    The curve below is a high pass filter's response to a step response, which is what basic current transformers including the Rogowski coil are. Some have special features such as a hall-effect sensor to extend the low frequency response to DC.

    [​IMG]


    I am not an expert on the Rogowski coil, I just happened to have run across it a few times. The example design given in the pdf document at the URL below might give you some help.

    www.ee.bgu.ac.il/~kushnero/belles-lettres/Rogowskoil.pdf

    From the difference in timing between the two curves it looks a lot like the poor low frequency response of the Rogowski coil can explain why the waveforms show a phase difference.

    What is the low frequency specification of your particular Rogowski setup?

    (It looks like you are trying to measure a step function - which is a good way to get an idea of the roll-off frequency.

    You question is why does the output of the Rogowski coil go negative; my question is why the current transformer's output doesn't go negative. A pulse like that would be shaped such that the voltage x time product above zero is equal to the time x voltage product below zero. I also expect that if you ran the scope trace out for a much longer time you would see that output of the Rogowski coil returns to ground. There is no net DC out of a Rogowski coil so if the output doesn't eventually return to ground that would mean there is an offset in the integrator or the resistor (or other circuit) to bleed the integrator back to zero is not working.
     
  5. andi1231

    Thread Starter New Member

    Dec 13, 2015
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    sorry for my late response. i am very bussy at the moment. thanks a lot for your help.
    the frequency range of my rogowski coil is: 10Hz to 20kHz
    i measuere the impulse of a capacitor discharge impulse magnetizer circuit. (see attachment).
    if i have an offset in my integrator circuit, should it not be visible at the beginning of my records of my
    current curve?
    thanks for your help
     
  6. benta

    Member

    Dec 7, 2015
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    24
    Not offset.
    But I suspect dielectric absorption in the capacitor in your integrator.
    You need a high-quality-dielectric capacitor, my suggestion would be polystyrene or polycarbonate (air would be ideal, but normally not practical).
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    The Rogowski coil itself is just loops of wire -if the magnetic field through it is static, the voltage across the wire will eventually go to zero volts and current will eventually stop flowing. L/R circuits are like that. If the signal out of your Rogowski system never goes to zero after current stops flowing in the wire being observed then there is offset or hysteresis (forgot to mention that before) in the evaluation unit.
     
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