# How to measure Current?

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##### Banned
Hello!

I have a power sytem whose AV voltage is 220V 50 Hz 1 phase. The Voltage is a pure sine wave but the current has 3rd and 5th Harmonics.
I know that the frequencies of 3rd and 5th Harmonics are 150 Hz and 250 Hz. I want to know how to measure instantaneous current and rms current due to the 3rd and 5th Harmonics. I need to calculate displacement power factor for non sinusoidal current. I am using PIC to measure the voltage and currents.

Thanks
Jayanth D

#### goldsmith

I have a power sytem whose AV voltage is 220V 50 Hz 1 phase. The Voltage is a pure sine wave but the current has 3rd and 5th Harmonics.
I know that the frequencies of 3rd and 5th Harmonics are 150 Hz and 250 Hz. I want to know how to measure instantaneous current and rms current due to the 3rd and 5th Harmonics. I need to calculate displacement power factor for non sinusoidal current. I am using PIC to measure the voltage and currents.
Hi Jayanth
one of the simplest ways is converting a sample of your out put current to the DC signal (or perhaps pulsed DC signal ) and then measuring it easily .
Best Wishes
Goldsmith

#### rohitkhanna

##### Banned
My guess is you would have to implement some sort of ADC followed by doing an FFT to get your answers. Or maybe just a digital filter (IIR ? FIR?) in software, since you already know your exact harmonic frequencies.

cheers!

##### Banned
@ goldsmith and rohitkhanna

I wanto calculate the displacement power factor. I know there is 3rd and 5th Harmonics. I want to get the value of current when there is only 3rd Harmonic by eliminating 5th Harmonic and I also want to get the current by eliminating 3rd Harmonic and when there is 5th Harmonic. I have to get the values of Io ie., fundamental current, I3 ie., current due to 3rd Harmonic, and I5 i.e., current due o 5th Harmonic.

#### goldsmith

Hi again
current when there is only 3rd Harmonic
That's so simple ! just try to use a filter to pass the desired harmonic and then measure difference between current and voltage ( with considering the phase shift of that filter too ) .
On the other hands : filtering ===> sampling =====> measurement
Good Luck
Goldsmith

##### Banned
My voltage is a pure sine wave and it is scaled down to dc 5 v to get the voltage readings. My current has 3rd and 4th harmonic and used with non linear loads. So the displacement power factor is not just cos(phi). Can you show how a filter can be used to eliminate 3rd harmonic and 5th harmonic.

#### goldsmith

My voltage is a pure sine wave
What do you mean by the word Pure ? a pure sine wave means a sine wave without any unwanted harmonic ! it means a single harmonic ! just fundamental .
My current has 3rd and 4th harmonic and used with non linear loads
where this current is came from ? from that pure voltage source ? if yes , how it is possible a voltage source ( sine source ) prepare a pure voltage and also deliver a distorted current ??! a question : are you referring to the PFC circuit ?? i think it is a good idea that you send a schematic that you're referring to ! thus it is easier to help you out .

Best Luck
Goldsmith

#### rohitkhanna

##### Banned
@ goldsmith and rohitkhanna

I wanto calculate the displacement power factor. I know there is 3rd and 5th Harmonics. I want to get the value of current when there is only 3rd Harmonic by eliminating 5th Harmonic and I also want to get the current by eliminating 3rd Harmonic and when there is 5th Harmonic. I have to get the values of Io ie., fundamental current, I3 ie., current due to 3rd Harmonic, and I5 i.e., current due o 5th Harmonic.

My guess is you would have to implement an ADC followed by doing an FFT to get your answers. Or maybe just a digital filter (IIR ? FIR?) in software, since you already know your exact harmonic frequencies.

I doubt you can implement an analog filter for these low & narrowband frequencies.

You will HAVE to digitise both the voltage & current simultaneously for your purposes. To digitise the current readings, some excellent current transformers are available for such purposes.

Once you have this data in your uC, you can calculate all sorts of details.

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