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NEED HELP FOR CURRENT MEASUREMENT...

Discussion in 'Microcontrollers, Programming and IoT' started by BHARGAV SHANKHALPARA, Dec 9, 2013.

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  1. BHARGAV SHANKHALPARA

    BHARGAV SHANKHALPARA

    35
    0
    Nov 6, 2013
    Hello everyone i need help in my project...

    Actually i made one control circuit which control one 1HP motor. its working fine but i added one extra function which is over current protection. for current measurement i used current transformer. i used ATTINY88 microcontroller for whole project. i decide to use ADC of microcontroller for current measurement.

    My current transformer ratings are as below...

    Primary winding turns : 1 turn of 2 wire (17 SWG wire).
    Primary Resistance : 0.01 ohm
    Primary inductance : 3.72 uH

    Secondary Winding Turns : 350 turn of 36 SWG Wire.
    Secondary Resistance : 32 ohm
    Secondary Inductance : 380 MH

    after obtaining output from secondary i used signal conditioning circuit link attached below...

    https://www.crmagnetics.com/Product...tifier Circuit for CT Signal Conditioning.pdf

    in above document i used burdan resistor R1=100 ohm, zener diodes of 5.1 volt, as mention in document R2 should be at least 10 times so R2= 10 * R1

    R2=1K

    also mention in doc that R2=R3 Should be equal so i used both R2=R3=1K.

    For amplification i used R4=R5=22K.

    Diode at output of each op amp i used 1N4148.

    and at last FOR Filtering of output R=100 ohm and capacitor C=10uF.

    When i test my microcontroller circuit in simulator for particular value of ADC its work fine but when i do it in real circuit at particular value of ADC circuit not give exceat response(of course i connect AVCC AND GND WITH VCC AND GROUND IN PRACTICAL CIRCUIT RESPECTIVELY). also another problem is that when i pass 5.5 amp current from primary i get some value after signal condition but when i double, mean connect 10 amp load there is minor change in value (in some points only). means CT is not giving LINEAR response...!!

    i dont know whats going on...is there any problem in my value of signal conditioning circuit or any other thing is wrong...!!

    please help me to solve my problem...!!

    Thanks in advance...!!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Can you detail what actual output you're getting at 5.5A and 10A.

    What happens at lower currents (1A, 2.5A, etc)?

    Have you viewed what's happening on an oscilloscope?
     
  3. BHARGAV SHANKHALPARA

    BHARGAV SHANKHALPARA

    35
    0
    Nov 6, 2013
    various value of CT outputs...

    When current is 5.5 amp through primary i am getting 1.40 DC volt after signal conditioning which i directly apply to ADC pin of microcontroller.

    in another test at 8.30 amp load i get 1.30 DC volt.

    At 1 amp load i get 0.395 DC volt.

    and i have no oscilloscope is there any other method to see any fluctuation...?

    Thanks...
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    Please measure AC volts (millivolts perhaps) across the current transformer. Leave everything else attached (100 ohm load resistor, zeners, etc.)

    What do you use to get a particular load current? Are you certain that it has a good power factor (using a bar heater or light bulbs would be my preference)
     
  5. BHARGAV SHANKHALPARA

    BHARGAV SHANKHALPARA

    35
    0
    Nov 6, 2013
    i measure direct AC voltage across burdan resistor of 100 ohm and take reading which is as below...

    Reading at BURDAN RESISTOR

    AT LOAD 5.5 AMP = 0.858 VOLT (AC)
    AT LOAD 8.3 AMP = 0.936 VOLT (AC)
    AT LOAD 1.0 AMP = 0.216 VOLT (AC)

    OUTPUT AFTER FIRST OP-AMP (RECTIFIED)

    FOR 5.5 AMP = 0.772 VOLT (DC)
    FOR 8.3 AMP = 0.808 VOLT (DC)
    FOR 1.0 AMP = 0.195 VOLT (DC)

    One question i want to ask is if i remove all other signal conditioning circuit than there is only AC signal output is there from CT but i want to give it to microcontroller for measurement so rectifier is must needed than how can i remove it and amplification circuit...???

    Thanks...
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    What do you use to get a particular load current? Are you certain that it has a good power factor (using a bar heater or light bulbs would be my preference)

    Can you remove R2 and see if that makes any difference t the voltage at the burden resistor
     
  7. BHARGAV SHANKHALPARA

    BHARGAV SHANKHALPARA

    35
    0
    Nov 6, 2013
    I measure load current using multimeter. actually i want to give protraction to single phase induction motor which is 5.5 amp. but i have also another load which is water heater (have a good power factor) of same current rating. and i have tested it with my circuit but result is same.

    Of course i can remove R2 but in article which i attached in first post written that

    "R2 should be chosen at least 10 times greater than R1 for proper accuracy"

    so in my case i used 100 ohm resistance but when i measure resistance across burden resistance (reading affected by CT s its own secondary winding resistance and R2 as you say) is 9.6 ohm.

    Thanks...
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    When I say remove R2, I mean disconnect it. I want to see if you are loading down the output from your current transformer.

    You would be best off using a purely resistive load to set up the current transformer (or at least one where you know that your meter is reading consistently).
     
  9. BHARGAV SHANKHALPARA

    BHARGAV SHANKHALPARA

    35
    0
    Nov 6, 2013
    ok i remove R2 from my circuit and than measure voltage across burden resistance and reading is now...0.815 AC Voltage for my heater (resistive load) have a load current 5.5 amp.

    and i measure also resistance across burden resistance which is 10 ohm now.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I would recommend you retain the conditioning circuit. The second op-amp and the output filtering allow you to scale and filter the output so it is more convenient to measure from a uC
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    It is interesting that the voltages you read has fallen.

    It seems like you're just getting a non-linear response. That's not too much of a concern if you're comparing currents against some sort of reference (as opposed to trying to measure an absolute value).

    Of note, your circuit will tend to report the peak current, not the average or RMS value. This means that some load which draws 1A RMX, but has peaks to 10A will read higher than a purely resistive load drawing 5A RMS (it has peaks of 7.07A).
     
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