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Purely Capasitive Load Not Equal 90 Degrees...


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#1 fancy102

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Posted 08 March 2008 - 12:23 AM

hi,

i am currently carrying out power factor measurement system. If my load is pure resistive, i could get unity power factor.
However, when my load is pure capasitive, i cant get phase shift of 90 degrees. My phase shift will change to different
value depends on my capasitive value. For 1micro Farad, i get 80 degree,power factor =0.19. For 6 Micro FArad, i get 33 degree, power factor 0.83

Is it the capasitive value will change my power factor? But the theory stated that if pure capasitive or pure inductive load, there will be phase shift of 90 degrees.Is it practical and theory have difference?I am confused about this. Thanks



#2 marke

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Posted 08 March 2008 - 12:43 AM

Hello fancy102

It depends on what you are measuring and where.
If you have a pure capacitor and compare the voltage across it against the current through it, you will have exactly 90 degrees.

If you are testing small values of capacitor at low frequencies, the impedance of your measuring device can alter the phase angle.

Best regards,

#3 fancy102

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Posted 08 March 2008 - 03:00 AM

Hi marke,

i do have a pure capasitor as my load...i do compare the voltage and current against it..but i cant get the phase of 90 degrees...

ya..i use frequensy at 50Hz generate by function generator. So, it is consider as low frequensy right? If like this, the value of my capasitor will alter the phase angle??may i know what is the concept of this?i am not so understand,,thanks


besr regards,
fancy102






#4 marke

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Posted 08 March 2008 - 04:12 AM

Hello fancy102

If you are using a function generator, there is a good chance that the capacitor on the output is causing distortion and so the waveform has a harmonic content. This will certainly affect the results that you are measuring.

Best regards,

#5 fancy102

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Posted 08 March 2008 - 05:30 AM

Hi marke,

so u mean the capasitor is causing distortion and thus the waveform is not harmonic?

so what can i do to improve the system so that my measurement can be more accurate?

and is it in practical, the pure capasitive load is not ideal itself. It has series resistance and leakage current?so it is really impossible for me to get phase of 90 degrees as the theory in real life?

thanks a lot!!

best regards,
fancy

#6 fancy102

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Posted 08 March 2008 - 06:23 AM

my result is different based in different capasitive load as below:
1 micro Farad - 80 degrees
2 micro Farad - 64 degrees
3 micro Farad - 54 degrees
4 micro Farad - 46 degrees
5 micro Farad - 40 degrees
6micro Farad - 33 degrees

is it possible the capasitance will effect the phase shift as above?
so what can i do to improve the system so that my measurement can be more accurate?
i am really at a loss at to what to do. thanks...

#7 marke

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Posted 08 March 2008 - 07:13 AM

Hello fancy102

  1. How are you measuring the power factor?
  2. What type of capacitors are you using?


An interesting point that I note is that the problem gets worse with increasing capacitance.
This would happen if the capacitance is overloading the output of the generator.
This would also happen if you are using electrolytic capacitors.

I would suggest that you connect a resistor in series with the output of the generator.

Best regards,

#8 fancy102

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Posted 08 March 2008 - 08:21 AM

hi marke,

i measure the power factor by measuring the current and voltage of the circuit. My power supply come from the function generator 1V,50Hz.

then i measure the voltage and current of the system. First Op-amp is as current to voltage converter and second op-amp is to 180 phase shift of the voltage. Output of the second op-amp is then connect to comparator to become pulse.

then both of my pulse waveform will go into AND gate. Output of AND gate will become input to the microcontroller (PIC16F877A). Then i get the power factor by measuring the phase shifted.

i attach my schematic of the circuit here.

How could i know wheather my capacitance is overloading the output of the generator?Thanks!

best regards,
fancy

Attached Files



#9 marke

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Posted 08 March 2008 - 08:54 AM

Hello fancy102

The circuit should work, but you are introducing some phase shift by the use of a potentiometer in parallel with the capacitor.you need to significantly increase the value of this pot in order to reduce the phase shift from it.

QUOTE
How could i know whether my capacitance is overloading the output of the generator?

The easiest way is to measure the current waveform and the voltage waveform with an oscilloscope.
The two waveforms must be sinusoidal. If they are not, then you have a problem.

QUOTE
What type of capacitors are you using?


Best regards,

#10 fancy102

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Posted 08 March 2008 - 09:23 AM

hi marke,

u mean it by incresing the potentiometer value then my phase shift will be reduced?but from my observation from the ossiloscope, the potentiometer just effect my amplitude of the voltage measured...i am doubt about how to calculate the circuit load with the potentiometer...

ya, i measured it with ossiloscope and i do have sinusoidal waveform. i just use normal capasitor, how can i check whether it is electrolytic or ceramic capasitor?is type of capasitor will effect my power factor?

thanks..

best regards,
fancy



#11 fancy102

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Posted 08 March 2008 - 09:35 AM

if the capasitor is CE-US it is ceramic capasitor??
thanks for fast reply




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