measur v-i parameters

[sush]

hi

i want to make a circuit which measures: ac voltage upt 1000v...current upto 100amp...of three phase supply and also to measure power factor... i want to display these on lcd using microcontroller 89c51. what basic circuit is required before adc?

thankx

 

[amrstaha]

you have to step down the voltages using a step transformer or with a resistive potential divider. If you intend to invest some money, perhaps you can buy a voltage transducer. If your voltage is sinusoidal, you can then simply use a precision rectifier and then low pass filter. the resultant signal is then to be fed to the ADC. The RMS vlaue of the voltage is just the measured voltage (peak vlaue) divided by √2. for non-sinoidal signal, you will have to sample the voltage signal with a fast enough ADC.
to measure the current you will need a current trnasducer. this can be a simple resistor in series with the load and measure the volatge drop across it. Hall effect sensors are more expensive but are easier and safer to work with.

 

[Kral]

Sush,

The techniques mentioned by amrstaha will work, with the following caution:
~
The output of the precision rectifier/filter combination will be equal to the average value if an active precision rectifier circuit is used. You then multiply the average value by PI /(Sqrt(2) X 2) (approximately 1.11) to get the RMS value.
~
If the waveforms are sinusoidal, for each phase you can measure the phase shift between the line voltage and line current by measuring the time between zero crossings. The phase shift (Theta), in degrees, is then equal to:
360 X tzc/ Period.
Where:
~ tzc = time between zero crossings.
The power factor = Cos (Theta).
~
The power per phase will be V X I X Cos(Theta)
Where:
~ V = line RMS Voltage
~ I = line RMS Current
If the 3 phase load is balanced, then you can get by with measuring the voltage and current for the load on one phase only, and multiply by Sqrt(3) to get total power. (This works for either a "Wye" or a "Delta" connection). Otherwise, you must make the measurements for all 3 load phases.
~
If the waveforms are non-sinusoidal, re-post your question and I'll provide details for this situation (it's a lot more complicated).

 

[amrstaha]

Sorry such, I meant that you feed the output of the precision rectifier to a capacitor to get the peak value of the sine wave. the RMS is then is what you got divided by √2.
If your current is not sinusiodal, and this is case with non-linear loads such as switching supplies, you will have to sample the current signal at a constant rate. the sampling frequncy should higher than twice the highest frequncy present in your signal. Standard practice dectates the use of an anti-aliasing filter before sampling the signal. The rms vlaue of the current signal is then calculated from
Irms=√(Ξi(k)/n)
where n is the number of samples, i(k) is the current sample number k, 1≤k≤n