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measurement of power factor


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#1 Lewis

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Posted 03 October 2002 - 01:16 PM

Hello.

Could anybody tell me how to measure the power factor without the power factor meter?I mean I want to make a circuit that can measure the power factor.

Thanks ;q

#2 marke

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Posted 03 October 2002 - 07:57 PM

Hello Lewis

Assuming that you are measuring the powerfactor of a circuit that is drawing a sinusoidal current, you can measure the power factor by measuring the angle between the voltage and the current. This is easily done using two comparators and an XOR gate. Use one comparator to monitor the voltage, giving you an output indicating the polarity of the voltage, and a second comparator indicating the polarity of the current. (the current can be monitored by the use of a CT) take these two square wave outputs and feed them into a XOR gate. The output of the gate will be a pulse stream with the mark space ratio indicating the phase angle, so you can filter this and measure the average level.
If the current is not sinusoidal, than you need to measure the rms current and the rms power. Power factor = KVA / KW

Best regards,

#3 Lewis

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Posted 04 October 2002 - 02:09 PM

Hello Marke

The waveforms acquired from CT is analogue signal, how to convert it to digital signal (square wave)?
Could you explain more about the comparators?
Besides, have any other methods to measure the p.f. ? Do you know the "C/k value"?

Thank you

#4 marke

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Posted 04 October 2002 - 09:06 PM

Hello Lewis

Yes the output of a CT is analogue.
A comparator is an electronic component, similar to an opamp that has an inverting input and a non inverting input and a digital output. If the inverting input is lower than the non inverting, the ouput is high. If the inverting input is higher than the non inverting input, the output is low. Have a look at the data for an LM339 and you will see more.

Best regards,

#5 mbitzer

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Posted 22 October 2002 - 02:02 PM

In another message I had seen......
Power factor=kVA/kW but it should be
Power factor=kW/kVA.

#6 marke

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Posted 22 October 2002 - 05:48 PM

Hello mbitzer,
You are correct, power factor = kw/kva
Best regards

#7 Guest__*

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Posted 10 September 2004 - 10:54 AM

is there any single ic chip to measure power factor

#8 Guest__*

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Posted 10 December 2004 - 07:35 AM

;f;
so, setting up a clock and a counter, counting when both U & I is positive, I can measure the p.f.? This means it is quikly to measure p.f. using soc, e.g. 8051. Why there is so many algorithm based on DFT to measure p.f.?

#9 marke

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Posted 22 January 2005 - 07:41 PM

Power factor comprises two components, displacement power factor and distortion power factor.
In the case of a motor load, the power factor is essentially displacement power factor.
Displacement power factor could be measured by using an 8051 to measure the percnetage of each cycle that the U & I are both positive. Distortion power factor would ned to have an analysis of the harmonic currents flowing.

Best regards,

#10 will

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Posted 23 January 2005 - 07:08 AM

Try this circuit it is made for singal phase the components are not hard to find,the more current you draw the bigger the wire.15a=14G wire.

this is the site

http://www.designnot...ITS/pwrfact.htm

Will

#11 Guest__*

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Posted 23 January 2005 - 06:02 PM

can i use 89c51 for power factor measurement...i am developing a circuit for power factor correction..can u suggest techniques for pf measurement using 89c51?

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Posted 27 January 2005 - 04:37 PM

Anybody knows a circuit to measure power factor on 3phase motor?
The circuit in the link http://www.designnot...ITS/pwrfact.htm
is just for one phase!
Instead of current transformer can I use hcpl-788j?

#13 marke

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Posted 27 January 2005 - 06:16 PM

Use the same circuit for each phase, (three circuits) and combine the outputs.

You need a Current transformer to mearsure the current. An opto isolator will not carry the current.

Best regards,

#14 Guest__*

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Posted 27 January 2005 - 06:40 PM

What current transformer should I choose to support a current of 15.9A at 400VAC (3phase - I leave in Portugal and F-N itīs 230VAC). The current transformer with ratio 1:1000 thatīs oK? You said that I had to do the same circuit in each phase (I supposed that) but I only need one current transformer, thatīs right?!
In each comparator I have to supply them with current and voltage... The current we use current transformer and the voltage we supply directly,passing by resistances, the Amp-Ops?
Just one more question... and if we havenīt Neutro (just 3phase) how we connect the circuit?
Thankīs!
Regards

#15 Guest__*

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Posted 22 February 2005 - 10:32 AM

yea hi just been thinking abut it and i dont axually know why i can never get my power factor to 1. it sits at the mo @ 0.96 but i can get it no closer.

out of intrest any one know why that is or wat i can do to get it higher


cheers willie

#16 marke

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Posted 22 February 2005 - 05:47 PM

Hello willie

If the power factor is purely displacement power factor, then you can in theory correct it to 1 provided that you have small enough steps in your capacitor bank.

Three possible reasons for not getting to 1.0 are:
1. Poor power factor less than 1 due to presence of distortion (or harmonic) power factor.
2. Meter not accurate around 1.0
3. Capacitor steps not small enough. If you add too much cpacitance, you will go from a lagging power factor less than 1, to a leading power factor less than 1.

Best regards,

#17 Guest__*

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Posted 23 February 2005 - 08:10 AM

yea i think that the meatre is slightly off...

however was chating with a mate in the electricty board he came to a conclusion that for some strange reason (in real life senairos) that it is imposable to get 1...

any thoughts
cheers

#18 chives

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Posted 23 February 2005 - 10:04 AM

i am studying PFC as part of an engineering services surveying degree.
can anyone spare me 10 minutes to review a question that i am completely stuck on.

my email address is lionel.hives at cyrilsweett.com

cheers

#19 marke

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Posted 24 February 2005 - 09:15 AM

Hello chives

Post your question in the student section and we will look at it. - we do not do your homework for you, but will offer some guidence as to how to solve the problem.
I have edited your email address. I recommend that you do not post complete email addresses with the '@' because these addresses will be collected by automatic spam spiders crawling the web and you will get loads of spam.

Best regards,

#20 Guest__*

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Posted 26 February 2005 - 06:07 AM

hi,
I am also studying/making energy power saving device for single phase.is it possible to use capacitor to 200W heater coil that is use in my incubator.I have 9 doors of incubator having 400W heaters each doors.or can i used thyristor unit to breakdown my electric bills?pls help, any suggestion pls.....




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