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'Power factor?'
1999\02\07@160834 by Mohamed Elegairy

picon face
Hi
I want to use PIC16F84 to measure power factor witch is (as I know ) the
angle between
voltage and current. I know how to detect voltage angle (zero crossing ) but
how to do it for the current should I use current transformer? any other
idea.

Thanks in advance

1999\02\07@162331 by Sean Breheny

face picon face
Hi Mohamed,

How much current will your power factor meter have to handle?

There are standard shunts available (small value power resistors) which
have a very accurate resistance and develop a small but certain amount of
voltage across them in proportion to the current going through them. You
can then amplify this voltage with an op amp and detect its zero crossing,
as well. However, in order to choose the right kind of shunt, you have to
know how much current it needs to handle.

Sean


At 10:55 PM 2/7/99 +0200, you wrote:
>Hi
>I want to use PIC16F84 to measure power factor witch is (as I know ) the
>angle between
>voltage and current. I know how to detect voltage angle (zero crossing ) but
>how to do it for the current should I use current transformer? any other
>idea.
>
>Thanks in advance
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
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1999\02\08@031318 by erik

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face
Mohamed,

I have a handful of 100 Amp shunts.
e-mail me of the list if your interested.

Erik

Sean Breheny wrote:
{Quote hidden}

1999\02\08@055635 by Mohamed Elegairy

picon face
Hi Sean

Current may reach 200 Amps so is it possible to use shunt resistor? if so
from where can I get it and how much it would be?
Thanks

Mohamed Elegairy

E-mail : egairyspamKILLspaminame.com
ICQ     :       14668366



{Original Message removed}

1999\02\08@090623 by Mike Keitz

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On Sun, 7 Feb 1999 22:55:52 +0200 Mohamed Elegairy <.....egairyKILLspamspam.....iname.com>
writes:
>Hi
>I want to use PIC16F84 to measure power factor witch is (as I know )
>the angle between voltage and current.

It's usually shown as the cosine of that angle converted to percentage,
but of course you can compute that in software.

> I know how to detect voltage angle (zero crossing
>) but
>how to do it for the current should I use current transformer? any
>other
>idea.

For small currents (up to tens of amps) it's probably simplest to amplify
the voltage developed across a small resistor in series with the load.
You can even use a section of wire as the resistor but, since the
resistance of wire changes with temperature, that method doesn't have a
lot of absolute precision.  For larger currents use a current transformer
or linear Hall sensor magnetically coupled to a conductor.


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1999\02\08@115846 by dave vanhorn

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face
At 12:44 PM 2/8/99 +0200, Mohamed Elegairy wrote:
>Hi Sean
>
>Current may reach 200 Amps so is it possible to use shunt resistor? if so
>from where can I get it and how much it would be?
>Thanks


At this range, a hall sensor starts getting interesting.
200 gauss around a wire IIRC.

1999\02\15@150525 by John Payson

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face
To really measure the power factor, you should measure power,
RMS voltage, and RMS current.  Measuring the phase difference
between voltage and current will let you determine the power
factor if the current is sinusoidal, but not otherwise.

Because you are interested in measuring the power factor rather
than the absolute amount of current, you probably don't need a
whole lot of precision in your current measurement.  Using a
measured length of wire for the current measurement should thus
be sufficient (even if the resistance changes when the wire heats
up, it won't change much within a 60Hz cycle).

If practical, the 17Cxx parts may be quite useful here since they
have a built-in multiplier; you should be able to, many times per
second, read the instantaneous voltage and current and then total
up:
 sqvoltage += voltage*voltage
 sqcurrent += current*current
 totpower  += voltage*current  [be careful of sign!]

Then after you accumulate readings for awhile, the power factor
should equal totpower / sqrt(sqvoltage*sqcurrent).  Cute, eh?

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