Post by thread:Power factor?

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

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 Personal page: http://www.people.cornell.edu/pages/shb7 spam_OUTshb7TakeThisOuTcornell.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 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 > Personal page: http://www.people.cornell.edu/pages/shb7 > .....shb7KILLspam@spam@cornell.edu Phone(USA): (607) 253-0315 ICQ #: 3329174

On Sun, 7 Feb 1999 22:55:52 +0200 Mohamed Elegairy <.....egairyKILLspam.....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. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

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?