3 phase voltage measurment

[udhay_cit]

I want to measure a three phase voltage. I will use three PT's for measurement (110V/3.3V). The measuring values are Vr, Vb, Vy with respect to neutral. Using the same PT i need to measure the phase to phase voltage also(Vry, Vyb, Vbr).
I got confusion to design circuit to measure phase to neutral & phase to phase voltage by using same PT. Please give some ideas for simplify my design...

Regards
udhay

 

[FvM]

You are asking about analog substraction circuits respectively differential amplifiers. https://en.wikipedia.org/wiki/Operational_amplifier_applications#Differential_amplifier

 

[udhay_cit]

Probably yes. But how can i use this circuit. I will rectify the output of three PT's to DC. without rectification i can use differential amplifier circuit but it requires dual supply for the opamp. Processing the PT output without rectification will increase hardware complexity. After rectification, i can't use differential amplifier or voltage summer circuit, because differential amplifier will give erratic output & summer will give only the average of the two output, but i need Vr+Vy to get Vry.

 

[FvM]

You want to substract the instantaneous AC voltages, not the rectified values. If you do some calculations, you'll see why.

 

[udhay_cit]

Yes. If the phase angle difference is 180°, then we can get exact voltage difference between two phases. 120° angle is giving voltage drop. How to rectify this?

 

[chuckey]

The addition of sinewaves whose phase is 120 degrees will not give 0V or 2 X Vin, but .8667 X the Vin. You can use this value with a look up table to decode the measured volts back to the phase difference. This method relies on the amplitudes being accurately known. It all depends on your application, - accuracy and how user friendly you need the system to be.
Frank

 

[udhay_cit]

The addition of sinewaves whose phase is 120 degrees will not give 0V or 2 X Vin, but .8667 X the Vin. You can use this value with a look up table to decode the measured volts back to the phase difference. This method relies on the amplitudes being accurately known. It all depends on your application, - accuracy and how user friendly you need the system to be.
Frank

But the simulation value gives different reading. Can you explain how you can get the factor 0.8667?
My PT output voltage is 3.3V
1. If the multiplication factor is 0.8667 * 3.3V =2.86V. Its not the multiplication factor match with simulation result.
2. I assume (2/180)*120= 1.333 is the factor. It is also not equal to my simulation result (3.3*1.33=1.38V)
3. The simulation result gives 5.7V output for 3.3V input which has the factor of 1.727.

I don't know what I'm doing wrong...:roll:

 

[FvM]

3. The simulation result gives 5.7V output for 3.3V input which has the factor of 1.727.

The exact factor is √3 = 1.732 = 2*sin(60). You visualize the vectorial voltage summation by sketching the respective phasors.

 

[chuckey]

Sorry, I did it in my head and my brain is getting slow!
Frank