2141smh@rduxb.UUCP (henning) (03/26/85)
> Actually, power is the vector product of voltage and current. > > This brings up a question. Do the meters used by the power companies > measure the total power used (volts X amps X power factor X time) ... Please. Total power is meaningless. Please use energy, which is the integral of power with time and is usually specified as Joules (Watt-Seconds) or Watt-Hours or Killowatt-Hours.
fish@ihlpg.UUCP (Bob Fishell) (03/26/85)
*** AC T YOUR AGE *** MUST we pollute this heretofore relatively sane newsgroup with discussions of who's too ignorant to know whatever? SHEESH! All the guy wanted to know was whether there was something he could buy, ready made, that would plug in between the wall and an appliance to tell how much juice it was using. Instead, somebody has to give hime grief and start one of these silly-ass discussions. I don't know of anything you can go out and buy that would do the job, although you could probably build one. All you need to know is the current and phase. Perhaps our friend is aware of that, but would rather buy one off the shelf than spend a couple of weekends burning his fingers and sniffing rosin. As far as I remember, the integrating wattmeters the power company uses (the gumball machine on the side of your house) neglect phase angle and therefore do *not* correct for power factor. Therefore, if you have a lot of inductive load in your house, idling motors and such, you're losing money. The little gadgets they sell for your refrigerator are supposed to correct for power factor and make the ol' ice box look more like a purely resistive load. The power company itself uses capacitors to correct for power factor at the feed to some factories (that use a lot of induction motors), but they neglect to extend the courtesy to us paean consumers. /_\_ Bob Fishell ihnp4!ihlpg!fish
notes@harpo.UUCP (03/27/85)
Leave blank The watthour meter on the side of your house goes to great pains to make sure it properly includes the effects of phase angle. A modern meter can measure the real part, as opposed to magnitude that neglects phase angle, to an accuracy of 2% at power factors as low as 0.3. In addition, the voltage calibration is even better from below 90 volts to over 250 volts. They have a slight sensitivity to frequency variations, although this is a negligable effect because the frequency is well controlled. The only thing they're poor at is accounting for the effects of harmonics. Therefore, if you have a resistive load, during a brownout the meter will drop as the SQUARE of the voltage. Also, if you connect a large capacitor or inductor it will not register. This is for residential use. For commercial use, they may have additional meters to measure reactive power as well as peak demand (load).
brian@sdcc3.UUCP (Brian Kantor) (03/27/85)
To measure the cost of running an appliance, you need the integral of energy used over a period of time. Since the power company probably bills you for kilowatt-hours, its reasonable to calculate it that way. The gumball machine on the side of your house does that, but its probably set up for 230V service, which is likely what you have. (You know, three wires, usually a black and a red or blue hot wire, and a white neutral. You may also see a green or bare ground wire.) Anyway, the outlets in your house are (except for the electric dryer, oven, stove, or maybe a big air conditioner) single phase - i.e., they have only got two wires (plus possibly a round pin - the safety ground). Construction sheds, outhouses, street lights, and cable tv amplifier boxes frequently have small single-phase power meters on them. So here's the deal: you can probably buy an old two-wire single-phase meter from the power company. Mount it in a nice box (varnished maple is nice), mount a receptacle on the back, add a power cord, and voila! you have a meter you can use to figure out how much it costs per appliance. You can't reset it to zero, but just note down the start and end readings for each appliance over a period of several days (a week is good for getting a real average). That's the number of kilowatt hours used. Just multiply it by your power company's usurious rates and you'll know how broke you are going to be. Several friends and I got together to build one of these; the surplus meter cost about $35 as I recall. I now know that heating my waterbed costs about $15 a month, my refrigerator about $20 a month, my computer about $10, my furnace $4 (blowers, you know), and the TV set about $8. I can't measure the stove, oven, nor electric dryer because the meter won't handle three-wire 230V circuits. I haven't measured the electric-belly-button-lint-remover either (yet). (Welcome to San Diego, home of the second highest electric rates in North America.) Brian Kantor UC San Diego decvax\ brian@ucsd.arpa akgua >--- sdcsvax --- brian ucbvax/ Kantor@Nosc ``You unlock this door with the key of inebriation...''
thomson@uthub.UUCP (Brian Thomson) (03/28/85)
There seems to be a great deal of misinformation here.
1) The wattmeter on the side of your house does indeed measure
(and integrate) power, and will not overcharge you if you have
low power-factor loads.
2) Large industrial power users are also charged for true power, but
with a contractually-determined power factor penalty. That is, the
user promises to maintain a power factor of .95 lagging or better, and
the power utility charges them an appropriately small penalty. If the
user only promised .8 or better, the penalty would be greater.
Of course, the utility may conduct power factor spot-checks to be
sure you're holding up your end of the bargain.
They care about power factor because distribution losses are proportional
to current, not to power.
This is how it was explained to me one summer when I worked in a
substation in a nickel smelter in Manitoba. One of my functions was
to monitor power factor and tell the electric furnace operators when
to add or remove capacitors. We were a rather heavy power user - I
seem to recall that the wattmeters wouldn't register at anything below
about 5 megawatts.
--
Brian Thomson, CSRI Univ. of Toronto
{linus,ihnp4,uw-beaver,floyd,utzoo}!utcsrgv!uthub!thomsonkarsh@geowhiz.UUCP (Bruce Karsh) (03/30/85)
> To measure the cost of running an appliance, you need the integral of > energy used over a period of time. Since the power company probably > bills you for kilowatt-hours, its reasonable to calculate it that way. To measure the cost of running an appliance, you need the integral of power used over a period of time. d Energy / dt = Power. -- Bruce Karsh | U. Wisc. Dept. Geology and Geophysics | 1215 W Dayton, Madison, WI 53706 | This space for rent. (608) 262-1697 | {ihnp4,seismo}!uwvax!geowhiz!karsh |