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

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.

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.
>
>
|
| Sean Breheny
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
shb7cornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

Mohamed,

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

Erik

Sean Breheny wrote:
{Quote hidden}

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 : egairyiname.com
ICQ     :       14668366

{Original Message removed}
On Sun, 7 Feb 1999 22:55:52 +0200 Mohamed Elegairy <egairyiname.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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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.

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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