John, You are correct that when the power factor is 1.00, the current is in phase with the voltage, as it will be in a purely resistive circuit. In this unique case, watts equals volt-amperes. However, when the load is reactive, the current either leads or lags the applied voltage. A typical case is a lightly-loaded induction motor, which may exhibit a power factor around 0.65, which is considered to be a "poor" PF. Some of the current drawn by this motor is used to create torque and perform actual work, and this current is nearly in phase with the applied voltage. Some current causes heating of the copper windings and the field core, and this current is also nearly in phase with the applied voltage. Both of these currents consume true power and are measured in watts. However, a significant amount of current does nothing but magnetize the field windings during part of the AC cycle, and that reactive energy is returned to the source as the magnetic field collapses just before the field is built up with the opposite polarity during the other half of the AC cycle. There are additional reactive losses, such as eddy-current and hysteresis losses, but the magnetizing currents are lagging the applied voltage, so they are the cause of "apparent power" which is expressed in volt-amperes reactive, also known as VARs.
As was discussed at great length in this forum several months ago, one cannot measure true power with separate voltage and current meters. That is because the readings of separate meters are not synchronized in time. Even if one uses two "true RMS" DVMs to measure voltage and current, one still winds up with nothing more than apparent power based upon volts times amperes, or volt-amperes. To measure true power, one must use an AC power meter that synchronously measures current and voltage throughout the entire cycle. This can be performed by a device called a "four-quadrant multiplier" which, as its name implies, performs the multiplication of current and voltage through 360 degrees of phase rotation. One such instrument is a VIZ Instruments WD-767 Digital Wattmeter, which sold for about $925. Another device which has become very popular with Hams is the "Kill-A-Watt" meter that costs about $20 from several Internet vendors. Although not as accurate as the WD-767, it does perform quite well. As I stated in an earlier post, the rotating-disk kWH meter on the typical residence measures only true power. It does this magic by combining the magnetic fields of two coils: a voltage coil that is connected in parallel with the load, and a current coil that is in series with the load. The angular displacement of these coil windings is precisely set so that only currents that are exactly in phase with the voltage result in a torque to turn the aluminum disk. A small permanent magnet is placed near the disk so that the speed of the disk is proportional to the amount of true power being consumed. Since speed is movement over time, the disk is geared to a register that accumulates the number of rotations, which represents power over time, which is energy. Thus, the meter measures kilowatthours. You pay only for the true power that you actually use; you do not pay for "apparent power" since volt-amperes is not true power. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of John Barrett Sent: Tuesday, September 16, 2008 9:13 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging Power) This doesn't mesh up with what I've learned about power factor -- the impression that I got was a perfect power factor was 1 (one) (current in phase with voltage, equipment using everything the the power company charges you for to do useful work). Anything other than PF=1 meant that the equipment was using the power less efficiently, and therefore you were paying more in KWH than the work actually performed. That description excludes the possibly that the equipment could use more power than the power company records as being delivered (heck, conservation of energy says that in any case). any load reactance (inductive or capacitive) and the very low PF numbers stated sound more like what I get off my linear power supplies with big capacitance and no power factor correction. In any case, anything other than PF=1 should mean that you are paying for more power than you are actually using. Tell me where I goofed this up ?? Bob M. wrote: > > Thank you Eric. This was the explanation I was looking for. The UPS is > saving me money when drawing 181 Volt-Amps, yet the electric meter is > only recording and charging me for 31 Watts. I wonder if APC did this > on purpose. I don't know how much current is actually being fed to the > batteries; they've been in there for a couple of months and should be > fully charged by now, so it should just be trickling them (eight 12V > 7A SLA cells in series/parallel for 48V). > > Bob M. > ====== > --- On Tue, 9/16/08, Eric Lemmon <[EMAIL PROTECTED] <mailto:wb6fly%40verizon.net> > <mailto:wb6fly%40verizon.net>> wrote: > > > From: Eric Lemmon <[EMAIL PROTECTED] <mailto:wb6fly%40verizon.net> <mailto:wb6fly%40verizon.net>> > > Subject: RE: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging Power) > > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > > Date: Tuesday, September 16, 2008, 9:21 PM > > Albert, > > > > You are forgiven, because you pose an important question! > > > > The spinning aluminum disk in the kilowatthour meter found > > on most > > residential service-entrance panels measures true power in > > kilowatts versus > > time, which equals energy. Thus, your electric utility > > charges you for the > > true power you use, not for volts times amperes- known as > > reactive power. > > Although the utility must provide the capability to supply > > all of the > > amperes you need, some of those amperes are "given > > back" to the utility due > > to a lower than unity power factor. That is why many > > utility companies > > charge a "kVAR Penalty" to certain industrial > > power users whose volt-ampere > > demands far exceed their watt demands, meaning that the > > power factor is low. > > Industrial power users strive to keep their power factors > > at 0.95 or above, > > to avoid some really painful penalties! The power factor, > > or PF, is simply > > watts divided by volts time amperes. > > > > The issue of power factor is why large Diesel generator > > sets have ratings > > such as 1000 kW/1250 kVAR. In simple terms, any AC > > generator requires > > torque (engine horsepower) to meet true power demands, and > > excitation (field > > flux intensity) to meet reactive power demands. When the > > generator load is > > reactive, that is, it has a power factor less than unity, > > the generator must > > not only have the horsepower to supply the energy in watts, > > but it must have > > excess capacity to handle the additional current required > > by motors and > > other low-power-factor loads. In a nutshell, that is why a > > 1000 watt > > generator may be unable to keep running a refrigerator that > > uses only 900 > > watts; the fridge may require 1200 VA to operate because it > > has a low power > > factor, and the small generator has no ability to handle > > such loads. > > Because of its relatively small amount of spinning mass, > > such a small > > generator probably could not even handle the > > refrigerator's starting > > current- which is about 5 to 6 times its running current. > > > > 73, Eric Lemmon WB6FLY > > > > > > > > -----Original Message----- > > From: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > > [mailto:Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com>] On Behalf Of > > Albert > > Sent: Tuesday, September 16, 2008 2:13 PM > > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > > Subject: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging > > Power) > > > > Hopefully, you will forgive me for hijacking the post, but > > this brings up a > > question I have had for a long time. What on earth is a > > "volt-amp"? > > My logic would state that is is the same as a watt, which > > is volts x amps, > > as you probably well know. So what on earth is it? > > > > Confused..... > > > > Albert > >
