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Leading Or Lagging Power Factor


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#1 fancy102

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Posted 11 March 2008 - 05:57 AM


i am carrying out power factor measurement andi need to know the power quality...

my problem is how can i know wheather the current is leading or lagging the voltage??i can only know it through ossiloscope?got any other way to know leading or lagging?thanks

#2 marke

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Posted 11 March 2008 - 07:55 AM

Hello fancy02

I would use a power factor meter in the real world, but if you are asking about lead and lag based on your previous posts and circuit, all you need to do is to look at the polarity of the current at the point of th positive voltage zero crossing.
If the current is positive as the voltage turns positive, then the current is leading, other wise the current is lagging.

Best regards,

#3 fancy102

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Posted 11 March 2008 - 03:08 PM

hi..

you mean i should observe the ossiloscope and look at polarity of current and voltage??no other method to know the leading or lagging besides the ossiloscope?

this is because i want to display the lagging or leading pf as my output so that the user can know whether the current is leading or lagging...thnaks

#4 marke

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Posted 11 March 2008 - 05:36 PM

Hello fancy102

No, not at all.
If you are using your circuit from your previous post, then where you have a square wave indicating the voltage polarity, and a square wave indicating the current polarity, you can add a D flip flop, or a latch and trigger the flip flip on the positive edge of the voltage square wave and feed the current square wave into the data input. If the current is high on the voltage transition from low to high, then the current is positive first , so it leads. If the current waveform is low on the transition, then the current lags. - simple.

Best regards,

#5 fancy102

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Posted 13 March 2008 - 11:54 PM

hi marke,
okok...thanks...

sorry to ask again...for the same circuit that i had post last time, if my load is purely inductive, is it my power factor obtain will almost same as unity power factor?is this because my inductance value is too small and the resistance value of the inductor itself higher than the inductance value?i use 6m Henry inductor connected in series.

best regards

#6 marke

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Posted 14 March 2008 - 08:30 AM

Hello fancy102

If your load is purely inductive, the power factor will be 0 and the current will lag the voltage by 90 degrees.
If your load is purely capacitive, the power factor will be 0 and the current will lead the voltage by 90 degrees.

Best regards,

#7 fancy102

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Posted 16 March 2008 - 03:23 AM

okok...thanks ..

sorry for the last question to ask..the phase angle i measure must be less than 90 degrees right?the phase angle must less than 90 degrees no matter what load i put??

best regards,
fancy

#8 marke

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Posted 16 March 2008 - 06:42 AM

Hello fancy102

Yes that is correct, the angle must be 90 degrees of less, but it can be positive or negative depending on whether the current is leading or lagging.

Best regards,

#9 fancy102

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Posted 16 March 2008 - 12:14 PM

hi marke,

thanks for the guidance given.really appreciate it...thanks very much...good luck in whatever u do!!




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