Amplifier for rogowski coil current sensor

Discussion in 'Homework Help' started by manjuarun, Mar 8, 2010.

  1. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    Hi guys,
    I am new to this forum,so please do bear with me.

    Let me convey my prob , which is on the design of an amplifier to use with a rogowski coil.I was asked to start the design and testing rogowski coil based current measurement. As a prototype , I thought of buying one rogowski coil from some coil manufacturers and design one amplifier, integrator , ADC and one cotroller circuit.For the design of an amplifeir I am really scared that what are all the expected constraints if the measurement would be in an industry where lots of inductance effect presents.Or any one of u pls suggest an amplifier circuit which I can readily use. What kind of analysis we should take care while doing an amplifier design for this particular application(with Op-amp).My aim is to measure current from 10A- 150A and frequency at 50Hz.

    Please gimme a solution

    Thanx in advance
    Manju
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Rogowwski coils are generally V/A. The one's I used were 0.1V/A.

    Once you identify what rogowski you're using, then the OP AMP circuit can be designed.

    And since this is homework, you might want to read the rules of the game. You must put forth the effort and we will help you. Gimmie's don't exist.
     
    manjuarun likes this.
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    My understanding of using Rogowski coils for current measurement is that their raw output is a function of the rate of change of current and as such the output signal requires integration & amplification (if necessary) to produce a signal directly proportional to the current being sensed.
     
  4. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    Thnx JoeJester & t n k....

    In order to get the continuous voltage output from the rogowski coil, the di/dt factor of the rogowski o/p [ v(t) = M di/dt ] has to be integrated. So my plan is to design like this

    rogowski coil sensor + integrator + amplifier + filter + adc + controller -> output in current format which is giving exact value like 20A or 23A.

    rogowski coil is ready. Next is integrator. or integrating amplifier.Now I am little confused to choose the opamp or any energy metering IC. My requirement is like I need that it should be of less power consumption and would give output as voltage or current not energy. For this kind of requirement which wud b best ??? either opamp or using energy metering chip. I will send a design of an integrating amplifier using opamp.Could you pls review it??
     
  5. manjuarun

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    Mar 8, 2010
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    could you pls give me ur mail id so that I can send you the design of integrating ampr for review???/
     
  6. t_n_k

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    If you can create your circuit diagram(s) as a pdf you can upload it to the site - then anyone can review it and you will receive a more representative range of comments &/or suggestions for improvement if required.
     
  7. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    Thnx again...I have uploaded my rogowski amplifier + integrator design. Anyone of you please review it and give me suggestions to modify this to make it exact suitable for a current measurement of 50-150A(which means a voltage output of .01mV onwards) at frequency 50Hz.Still I have doubts on the values of capacitors and resistors in this ckt. Offset voltage should be minimum and it be of battery powered. Is it a right way to do the amplification after integration or what will happen if integrating after amplification?
     
  8. manjuarun

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    Mar 8, 2010
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    another design for ampr + intgrtr is also uploaded.Please review this also
     
  9. manjuarun

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    Mar 8, 2010
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    sorry Attach failed in the previous reply.Now plz check.....This is another design I got from some website.will it suit for my application???
     
  10. t_n_k

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    Mar 6, 2009
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    You've not clearly stated your Rogowski output signal so it's hard to advise you on the gain issue.

    What is the 'volt-second per ampere' parameter?

    Your schematic shows three different amplifier - "integrator" sections. Not sure what that is all about.

    Each section seems to be a (x100) diff amp followed by a LPF stage.

    If the LPF is meant to be performing as an integrator at 50Hz the feedback time constant is too small.

    The parallel RC [10k || .01uF] feedback branch at 50Hz needs to have an ωRC >>1 for the stage to act as a true integrator. In your case ωRC=0.0314, which is way too small.
     
  11. t_n_k

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    Further to your Rogowski coil....

    They can actually be made to be "self integrating" which is perhaps what JoeJester was referring to w.r.t certain types he had used in the past.

    In that case you don't need an integrator stage.

    What are you Rogowski specs? Is is a commercial unit or something you've made or had made yourself?
     
  12. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    Thank you so much ..
    The three sections are for 3 phases.In each section the first part is Amplifier and the second part is supposed to be Integrator. Our selected rogowski coil output would be in the range 1mV to 100mV.Is *100 gain enough???

    Anyway in this site(Electronics Forum) I saw one post like for a fast response and battery powered applications with rogowski usually opamp of Linear Technologies LT1630 is the best (I am not sure about this...) .I searched its data sheet and came to know that its good for battery powered applications. And I designed a circuit using this LT1630 and attached the same(in png format). Could u plz review it and give me corrections.Actually I am overshooting my schedule for designing the signal processing system for coil o/p.
     
  13. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    Dear t n k,

    I didn't find any rogowski coil which is of type self integrating.Initially we ourselves wound a coil and it didn't work well. And also we outsourced to somebody for winding the same.But that was also failed. Finally we planned to purchase a commercial one like LEM's RT500. I contacted LEM's dealers for technical queries and they didn't even mention anything regarding this "Self Integration". I don't have any idea about this.

    JoeJester,
    If you have experience on this plz share with me....


    Thanx
    Manju
     
  14. t_n_k

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    Mar 6, 2009
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    You mentioned your current is 50-150A (RMS?) at 50Hz.

    Using the data sheet your minimum Rogowski peak output would be {at 50A rms}

    Vp=50*1.414*314*0.064e-6=1.42mV

    A x100 pre-amp gain therefore gives 142mV [pk] @50A RMS to 426mV [pk] at 150A RMS.

    Your integrator has a gain of about 0.32 at 50Hz so the minimum output would be ~45mV peak at 50A RMS.

    Provided that is sufficient for the overall design requirement the circuit concept looks OK.

    Oh and don't forget to include the x2 gain from the output LPF stage after the integrator.

    Self integrating Rogowski coils would be a special design case so I wouldn't bother going down that path.
     
    Last edited: Mar 10, 2010
  15. manjuarun

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    Mar 8, 2010
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    Thanks t n k,

    Our current range we defined is not rms..its just load current we meant.

    Yesterday our team designed one another ckt for ampr & integrator and I uploaded the same with this.Could you pls review it and give us corrections on it as we are planning to go with this circuit.After approval of this part we would be moving to filter,clamper and then adc ckts.Please review the below attached ckt

    Thnx
    Manju
     
  16. manjuarun

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    Mar 8, 2010
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    t n k,

    I didn't understand why should we put x2 gain from the output LPF stage after the integrator? could u plz clarify?
     
  17. t_n_k

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    Your schematic in post #12 is a two stage design comprising an integrator stage plus a low pass filter [LPF] stage.

    Presumably this was your design so you should have a clear understanding of it's characteristics. In any case the second stage [LPF] gain at 50Hz would be ~x2. If you don't know why then you should put some thought into analysing the circuit.

    With respect to the more recent proposed schematic I would think you have again missed the point of the integrator stage design by having a 2.2pF capacitor as the integrator feedback value.

    I suggest you at least attempt to do a proper simulation of any proposed design before you attempt to build anything.
     
  18. JoeJester

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    Apr 26, 2005
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  19. t_n_k

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    It's also interesting that the second stage your most recent design behaves as a very good differentiator at 50Hz - rather than as an integrator. Do you agree? Can you explain why? Are you able to synthesize an adequate integrator using the same circuit topology with other component values?

    BTW if you are not measuring the current as an RMS value then how would you interpret the output signal - or is that all done in the ADC? Presumably since it's a 50Hz AC signal you ultimately have to interpret it as a peak-to-peak, peak, RMS, ...etc value.

    My thanks to JoeJester for a nice link.
     
  20. manjuarun

    Thread Starter New Member

    Mar 8, 2010
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    How do we calculate integrator gain if it has a feedback branch of R (220K) and C (1uF) and Rin of 10K? I want to get a unity gain output.And from theory I see that the phase shift would be 180 degree. But I got only 90 degree phase shift.How do we obtain a phase shift of 180 degree?

    Thanks in advance
    Manju
     
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