Thank you Eric Lemmon ! Your explanation is the most precise I've read thus far. The last line of your last paragraph said what folks want to hear. I started to respond to this thread earlier, but after a few minutes of organizing my thoughts, my aging pea-brain said, "Forget this and go get another gin & tonic ! What's another few brain cells anyway ?"
de WD7F John in Tucson ----- Original Message ----- From: "Eric Lemmon" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, September 16, 2008 9:59 PM Subject: RE: [Repeater-Builder] Volt-Amp (Re: APC UPS Charging Power) > 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: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of John Barrett > Sent: Tuesday, September 16, 2008 9:13 PM > To: [email protected] > 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: [email protected] > <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: [email protected] > <mailto:Repeater-Builder%40yahoogroups.com> >> <mailto:Repeater-Builder%40yahoogroups.com> >> > [mailto:[email protected] > <mailto:Repeater-Builder%40yahoogroups.com> >> <mailto:Repeater-Builder%40yahoogroups.com>] On Behalf Of >> > Albert >> > Sent: Tuesday, September 16, 2008 2:13 PM >> > To: [email protected] > <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 >> >> > > >
