mark@retix.retix.retix.com (Mark Hoy) (01/06/90)
Is there a <$30 meter that will monitor an appliances power usage? I would like to know how much energy is being used by a couple of fridges, aquariums, etc. I am willing to build the thing if necessary, (but only if it's relatively simple). -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= __/ /. ___ _ _ uunet! | | | . | ___ _| |_ |_| _ _ cepu!retix!mark or mark@retix.com |_ ._| | __|/ ._\|_ _|| |\ \/ / oliveb! /_/ |_| \\___/ |_| |_|/_/\_\ rutgers! Mark Hoy 213 399-1611 800 255-2333 ttidca! -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Dick@cup.portal.com (dick a wotiz) (01/07/90)
> Is there a <$30 meter that will monitor an appliances power usage? I > would like to know how much energy is being used by a couple of > fridges, aquariums, etc. I am willing to build the thing if > necessary, (but only if it's relatively simple). You probably won't want to build a meter that will be accurate enough with an inductive load like a refrigerator. But I've seen surplus real electric meters (like the one on the side of the house) for about $15-$20. You might try Herbach & Rademan (now H & R Corp.) at 401 E. Erie Ave, Philadelphia PA 19134, 215-426-1708. One of their older catalogs has one... they might still have some around. Good luck! Dick Wotiz dick@portal.com {sun|uunet}!portal!dick
jgd@rsiatl.UUCP (John G. De Armond) (01/07/90)
In article <MARK.90Jan5105134@retix.retix.retix.com> mark@retix.retix.retix.com (Mark Hoy) writes: >Is there a <$30 meter that will monitor an appliances power usage? I >would like to know how much energy is being used by a couple of >fridges, aquariums, etc. I am willing to build the thing if >necessary, (but only if it's relatively simple). Regular watt-hour meters such as used in your house entrance don't cost much more than this. Figure on paying between 50 and 70 bux. You could try contacting your utility meter department. They may sell you one. If not, you can buy them from the vendors (Westinghouse, GE, Saganaw, Japan Inc.) directly, though at a bit higher cost. I bought mine directly from my utility. I mounted it in a meter base attached to a wooden frame of appropriate dimentions to keep it level. An outlet box is also supplied. To make the kind of measurement you want, a mechanical watt-hour meter is hard to beat over the long run. A long history of experience, regulatory concern, and good design makes your humble little power meter very accurate. Overall accuracy of 2% at low wattage and 1% at high are typical calibration standards. My work in a utility metrology lab made me view these instruments with a whole new level of respect. BTW, my instrumentation shows that RSIATL, a novell server, and the losses in the UPS cost about $16 a month to operate :-) John -- John De Armond, WD4OQC | The Fano Factor - Radiation Systems, Inc. Atlanta, GA | Where Theory meets Reality. emory!rsiatl!jgd **I am the NRA** |
rich@island.uu.net (Rich Fanning) (01/09/90)
In article <25675@cup.portal.com> Dick@cup.portal.com (dick a wotiz) writes: >You probably won't want to build a meter that will be accurate enough >with an inductive load like a refrigerator. But I've seen surplus >real electric meters (like the one on the side of the house) Do the utility meters on the average house measure true watts or only current? I have always assumed they measured watts, but when I think of it now, why shouldn't the utility charge by the amp? It seems as if the utility charging for "amp-hours", rather than for watt-hours, would be in THEIR best interest. Utilities provide financial incentives for raising the power factor of the load of industrial companies. I have always assumed that the industries get charged for "amp-hours". Do household and industrial customers get metered differently in this regard? -- Rich Fanning | "Thinking is more interesting than {uunet,sun,well}!island!rich | knowing, but not so interesting as | looking" - Goethe
josef@peun11.uucp (Moellers) (01/10/90)
mark@retix.retix.retix.com (Mark Hoy) writes: >Is there a <$30 meter that will monitor an appliances power usage? I >would like to know how much energy is being used by a couple of >fridges, aquariums, etc. I am willing to build the thing if >necessary, (but only if it's relatively simple). >-- You may try and dig into old versions of BYTE (and I mean OLD, i.e. from the time when BYTE was readable and didn't consider "computer" being exactly identical to "IBM compatible") Steve Ciarcia had an article on monitoring power consumption. If You can't find the article(s), give me a "ring", I have a small number of BYTEs and can probably find the article easily (I write this at work and the BYTEs are in the basement of my home) Josef Moellers Josef Moellers | c/o Nixdorf Computer AG USA: uunet!philabs!linus!nixbur!mollers.pad | Abt. DX-PC !USA: mcvax!unido!nixpbe!mollers.pad | Pontanusstr Phone: (+49) 5251 146245 | D-4790 Paderborn
rsd@sei.cmu.edu (Richard S D'Ippolito) (01/11/90)
In article <1262@island.uu.net> rich@island.uu.net (Rich Fanning) asks: >Do the utility meters on the average house measure true watts or only >current? True watt-hours. >I have always assumed they measured watts, but when I think of it now, why >shouldn't the utility charge by the amp? It seems as if the utility >charging for "amp-hours", rather than for watt-hours, would be in THEIR best >interest. Why? A watt is only the rate of energy usage and the utility is in the energy business. Watt-hours are energy. >Utilities provide financial incentives for raising the power factor of the >load of industrial companies. Not really -- they provide financial penalties for not drawing energy at a high power factor. >I have always assumed that the industries get charged for "amp-hours". > >Do household and industrial customers get metered differently in this regard? Industrial and commercial customers large enough or with significant reactive loads get metered on three bases: One meter measures the energy usage, just like it does for a residence. Another meter measures average power factor and the demand (peak usage usually averaged over a 15minute period). Both of these numbers are then used to correct (i.e., increase) the base rate to determine the final bill. Rationale: Drawing power at low power factors means that the feeder losses are higher for the same energy delivery (lower transmission efficiency) and that the both the feeders and transformers have to be larger. Having high peak demands means that the load is variable -- the utility must size its equipment to provide the maximum demand (a capital expense), but the equipment is under-utilized. Consider three customers, one having 10,000w of load connected all of the time, another having 30,000w connected for an eigth-hour shift each day, the third having an electric furnace drawing 300,000w for one 48-minute period each day. All use the same energy -- should they pay the same for service? Residential demand is more steady and predictable, and the power factor is close to unity. More important, all customers are the same, so the rates can be well based on the cost to supply the energy. Rich -- Hitting baseballs and writing software are two professions where you can become a millionare with a 75% performance failure rate. rsd@sei.cmu.edu ------------------------------------------------------------------------
jgd@rsiatl.UUCP (John G. De Armond) (01/11/90)
In article <1262@island.uu.net> rich@island.uu.net (Rich Fanning) writes: >In article <25675@cup.portal.com> Dick@cup.portal.com (dick a wotiz) writes: >>You probably won't want to build a meter that will be accurate enough >>with an inductive load like a refrigerator. But I've seen surplus >>real electric meters (like the one on the side of the house) > >Do the utility meters on the average house measure true watts or only current? >I have always assumed they measured watts, but when I think of it now, why >shouldn't the utility charge by the amp? It seems as if the utility >charging for "amp-hours", rather than for watt-hours, would be in THEIR best >interest. Watt-hour meters measure true watts and are typically certified to meet tolerance specifications down to about 60% PF. Physically, the torque that turns the meter disk is the product of the instantaneous voltage and current components. If both components are in phase, a simple product is representative. At the other extreme, if the power factor is 0, ie, the current and voltage are 90 degrees out of phase as with a purely reactive load, there is no torque generated because the voltage and current components never coincide. The utility is permitted to charge the basic rate for "work units" or watts. They are also allowed to charge a power factor penalty for low power factor. This is normally not done for residentials users, as the assumption is that residential loads are close to resistive. >Utilities provide financial incentives for raising the power factor of the load >of industrial companies. I have always assumed that the industries get >charged for "amp-hours". Not in those terms. For moderate size industrial loads, the utility will take a representative sample of the power factor and calculate a PF penality if the power factor is below a threshold. For large loads, VARs (volts-amps, reactive) are also metered (typically by using a watt meter with a reactive element in the voltage winding to shift the phase 90 degrees.) and charges are calculated directly on the reactive current. Generating stations also meter VARs but for the purpose of adjusting the field excitation on the generator to reduce the VARS as much as is feasible. Varying the field excitation on either a generator or a synchronous motor can make lagging VARs, leading VARs or unity PF. John > >Do household and industrial customers get metered differently in this regard? Yes -- John De Armond, WD4OQC | The Fano Factor - Radiation Systems, Inc. Atlanta, GA | Where Theory meets Reality. emory!rsiatl!jgd **I am the NRA** |
wb8foz@mthvax.cs.miami.edu (David Lesher) (01/11/90)
One question I never asked a utility when I should have... Virtually everything causes inductive power factor problems. You correct for it (typically) with capacitors (some old-timers may call'em condensers, but I think those belong to steam engines and HVAC units :-}). What does the utility think if you offer to correct other people's leading PF by having too many farads? This came up when I had to improve the pf at a pump station. It had one each: 500, 1000, and 1600 hp 4160 volt motors. We went to great expense to switch the correct # of cans in for each motor. But if we had left the worst case setup connected at all times, we would have helped out with all the leading factor from the nearby farms. I never was able to get my boss to understand this, and so we never did get any feedback from the utility. So anyone with direct knowledge of a utility's position on this case? I suspect that the posture would be the same penalty, if only because their pf meter didn't read direction, only scaler amount. -- A host is a host & from coast to coast...wb8foz@mthvax.cs.miami.edu no one will talk to a host that's close..............(305) 255-RTFM Unless the host (that isn't close)......................pob 570-335 is busy, hung or dead....................................33257-0335
roy@phri.nyu.edu (Roy Smith) (01/11/90)
In article <1262@island.uu.net> rich@island.uu.net (Rich Fanning) writes: > Do the utility meters on the average house measure true watts Actually, they measure watt-hours, but you knew that; I'm just being pedantic. Large industrial users are not only "encouraged" to keep a reasonable power factor, but they also get zapped with a hefty penalty if their peak load (I'm not sure if kVA or kW) ever goes past some preset limit. Anybody know what the total average power factor on the power grid is? I would imagine it must be massively lagging, what with motors and transformers being most of the load. I can't think of any typical capacitive loads on the power grid. I know it is possible to make a synchronous motor have a leading power factor with proper external excitation, but have no idea how often that is done. If the above doesn't make sense, try interchanging all instances of leading and lagging; I never could keep them straight. Does current lag voltage or does voltage lead current? Pretty arbitrary, isn't it? I think of capacitive as leading and inductive as lagging. Did I get that right? I know about ELI the ICE man, but that's a pretty bad mnemonic because you can interpret either way! Now, for another question. A while ago, I was wandering around in the basement, looking at the power connections. No, not my basement, where I work basement. The building engineers are wonderfully lax about not locking doors. Anyway, behind the panels with the 5000 amp breakers were a couple of rather ordinary-looking electric meters and boxes with wire seals on them. Some of the boxes said "Con Edison" on them, which seems normal, but some of them said "New York Power Authority" which got me curious. Why would the NYPA have a sealed box in a not-particularly-special building in Manhattan? What's in that box? Could the NYPA actually be interested in the few measly MW our building must draw? Isn't that sort of like the Federal Reserve worrying about whether my personal checkbook gets balanced? -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy "My karma ran over my dogma"
jgd@rsiatl.UUCP (John G. De Armond) (01/11/90)
In article <1452@umigw.MIAMI.EDU> wb8foz@mthvax.cs.miami.edu (David Lesher) writes: > >One question I never asked a utility when I should have... > >Virtually everything causes inductive power factor problems. You >correct for it (typically) with capacitors (some old-timers may call'em >condensers, but I think those belong to steam engines and HVAC units :-}). > >What does the utility think if you offer to correct other people's >leading PF by having too many farads? This came up when I had to >improve the pf at a pump station. It had one each: 500, 1000, and 1600 >hp 4160 volt motors. We went to great expense to switch the correct # >of cans in for each motor. But if we had left the worst case setup >connected at all times, we would have helped out with all the leading >factor from the nearby farms. I never was able to get my boss to >understand this, and so we never did get any feedback from the >utility. > > >So anyone with direct knowledge of a utility's position on this >case? I suspect that the posture would be the same penalty, if >only because their pf meter didn't read direction, only scaler >amount. I can't state utility policy but I can discuss the engineering issues involved. In short, you'd do as much harm with your capacative reactance as does the neighbors with inductive reactance. Here's why. Your excess capacity causes leading VARs to flow through your substation transformer, through the distribution wiring, and through the transformers feeding the inductive load. Wattless amps caused by leading PF cause the exact same IR heating as lagging PF. In other words, you've not done anything helpful but have hurt the overall situation by causing leading VAR heating your power feed. But it gets worse. The power companys have pretty accurate models for calculating what a PF will be given a typical distribution of loads. Using this model, they can correct the PF close to the source. You will commonly see this done via capacitor banks mounted on distribution poles. Here in Marietta, Cobb EMC tends to correct in a residential setting on a neighboorhood boundary and in light industrial areas, at the industrial park feed. This isolates the wattless current and IR losses to the pole pigs and the low voltage (7200 v or 14.4 kv) distribution wiring. This is a good compromize between the cumulative losses in the pole pigs versus the cost of capacitor banks. In order to size this correction capacity, some underlying assumptions have to be made. For instance, that a typical house will have a heat pump of between 3 and 5 horsepower as the primary reactive load. Or that an office building will have primarily inductive loads in the form of air conditioning and fluroscent lamps. They cannot anticipate that some well intentioned customer is feeding leading VARs back into the line, especially of the magnitude you mentioned. In other words, they want you to be typical. The best thing you can do is to correct the PF to unity. This will make the utility happy and will reduce your energy costs. Realize that while wattless current itself represents no energy consumption and therefore is not metered, the heat loss in the conductors and transformers IS energy and you DO have to pay for it. Conceptualize the equivelant circuit of an inductor and a resistor in series. The larger proportion of impedance that is represented by the resistance, the larger proportion of the amperage is representative of work energy. In other words, the phase vector rotates toward 0 as the reactive component becomes less significant. So because you have to pay for the real watts dissipated as IR losses in wiring and transformers, you should correct the PF as close to the source as possible. I like to see capacitor banks mounted right at the site of the motor downstream of the starter. This has a side benefit of reducing arcing in the starter and prolonging contact life. John -- John De Armond, WD4OQC | The Fano Factor - Radiation Systems, Inc. Atlanta, GA | Where Theory meets Reality. emory!rsiatl!jgd **I am the NRA** |
jgd@rsiatl.UUCP (John G. De Armond) (01/11/90)
In article <1990Jan11.015852.7031@phri.nyu.edu> roy@phri.nyu.edu (Roy Smith) writes: > > Anybody know what the total average power factor on the power grid >is? I would imagine it must be massively lagging, what with motors and >transformers being most of the load. I can't think of any typical capacitive >loads on the power grid. I know it is possible to make a synchronous motor >have a leading power factor with proper external excitation, but have no idea >how often that is done. Actually, by the time you get to a 500 kv backbone or to a generating station, the PF is pretty close to unity. I'd have to get some of my books out to refresh actual numberes. At the hub where I trained, we handled something on the order of 12,000 megawatts. Seems to me like the megaVAR recorder only went to 20 MVAR or so. Generators are typically run a bit over-excited in order to feed some leading VARS down the line and cancel inductive VARs in the distribution system. As I mentioned in an earlier post, PF correction is generally taken care of on a local basis. The reason is simple. Because of the sheer size and the resultant limitation on engineering margins of, say, a 5000 MVA 500 KV transformer, it is important that it handle high PF power. One can easily engineer 2 or 3:1 safety margins into pole pigs. Not so with these behelmoths. > Now, for another question. A while ago, I was wandering around in >the basement, looking at the power connections. No, not my basement, where I >work basement. The building engineers are wonderfully lax about not locking >doors. Anyway, behind the panels with the 5000 amp breakers were a couple of >rather ordinary-looking electric meters and boxes with wire seals on them. >Some of the boxes said "Con Edison" on them, which seems normal, but some of >them said "New York Power Authority" which got me curious. Why would the >NYPA have a sealed box in a not-particularly-special building in Manhattan? >What's in that box? Could the NYPA actually be interested in the few measly >MW our building must draw? Isn't that sort of like the Federal Reserve >worrying about whether my personal checkbook gets balanced? Judging by the breaker sizes you quote and looking at your address, I'm assuming your building represents a significant load. Most likely, the sealed box contains load shedding equipment that allows NYPA to shed non-essential loads like A/C during power emergencies. Did you notice phone wiring going to any of these? Other common instrumentation is real-time telemetry so that the utility can observe usage patterns. It is very common to contract for a certain pattern and to charge more for or even shed loads that significantly deviate from the contract. One other note. I've done at least a little bit of almost everything a line crew for a utility ever has to do, including working 181 KV lines hot and climbing around energized 500 KV yard structures. Yet the thing that scares me the most is to be in a customer equipment vault as you've described. I've seen the aftermath of more than one breaker panel explosion. I had a close friend critically burned when he operated the handle of a 5000 amp, 480 volt breaker and it exploded. I'm as nervous as a whore in church in these places. I'd advise you make your "yard and dock" tours rapidly :-) John -- John De Armond, WD4OQC | The Fano Factor - Radiation Systems, Inc. Atlanta, GA | Where Theory meets Reality. emory!rsiatl!jgd **I am the NRA** |
rsd@sei.cmu.edu (Richard S D'Ippolito) (01/12/90)
In article <1990Jan11.015852.7031@phri.nyu.edu> Roy Smith writes: > Anybody know what the total average power factor on the power grid >is? I would imagine it must be massively lagging, what with motors and >transformers being most of the load. I can't think of any typical capacitive >loads on the power grid. Small utilities have banks of powerfactor correction capacitors which they manually switch in and out -- larger ones have gone to reactor sets switched automatically by thyristors. Large industries also do the same. Rotating machines were more common in the past. The average depends on the limit settings in the utility's reporting equipment (mostly software) and the alertness of the operator. Bank switching too often lowers the life of the equipment and causes line transients, which is why the thyristor controlled units are preferred. Of course, they cost a lot more -- it's an economic decision, as all utility decisions are. Typically, they keep it to within 10% of unity either way. >I know about ELI the ICE man, but that's a pretty bad mnemonic because you >can interpret either way! You may know the phrase, but you certainly don't understand its interpretation. The order of the letters is the key. Voltage (E) across an inductor (L) leads (comes before) the current (I). Rich -- Hitting baseballs and writing software are two professions where you can become a millionare with a 75% performance failure rate. rsd@sei.cmu.edu ------------------------------------------------------------------------
wb8foz@mthvax.cs.miami.edu (David Lesher) (01/12/90)
Most everything John said is true in the general case. Won't argue a
word. But this case was such that I thought it might fly. First the
(co-op) utility didn't seem to give a gnat's _ss what the power factor
was, except that THEIR supplier soaked them if it was too poor. This
station was a long way from anything except a bunch of dairy farms,
including the sub. Their guy apologized for the pf charge (!) and
explained that our {town,leg,branch?} was already 'a problem' because
of the milking machines.{ :*}) While we were a big customer,
seems to me $10K/month, we didn't vote @ the meetings......
I would have dearly loved to mount those cans inside the starters, but
some spoilsport pointed out it was against the law to have two objects
occupying the same space at the same time..... (space was TIGHT!) We
looked at placing them right outside the Electro-center wall, but ran
into some non-electrotechnical problem, maybe other equipment in the
way or such. In any case, we put them over by the utility feed, with
remote contactors to switch them in and out.
One chuckle. This was in northern WI. Forget heat pumps, and
a/c. On the other hand, a 125kv mosquito killer might have been
nice ;-) What's its pf????
--
A host is a host & from coast to coast...wb8foz@mthvax.cs.miami.edu
no one will talk to a host that's close..............(305) 255-RTFM
Unless the host (that isn't close)......................pob 570-335
is busy, hung or dead....................................33257-0335jgk@osc.COM (Joe Keane) (01/13/90)
My two cents... Yes, a typical electronic power supply will often behave like a slightly capacitive load. A full-wave rectifier conducts twice a cycle, with peak current a bit before the peak voltage. Of course, you have to consider the magnetizing inductance of the power transformer, plus those nasty harmonics. A _good_ switching power supply looks like a resistive load, the resistance slowly changing in response to changes in output current and input voltage. Power factor generally isn't a big problem in residential areas. The utilities often have capacitor banks (switched or unswitched) on feeders, but these are more for voltage regulation than balancing reactive power. The voltage on a line with a capacitor bank will be higher than one without, and will have less voltage fluctuation. For industrial loads it's a different story. The utility charges for power factor, so it's often in the customer's best interest to buy a capacitor bank. Some industrial loads are notoriously badly behaved. Think about a rock crusher or an electric arc furnace. The classic `bad' load is a strip-mining shovel. If the operator screws up and stalls the motor, it draws a ridiculous amount of lagging KVARs and very little real power. Plus it's usually in the middle of nowhere, with no other loads around. What to do? In article <1990Jan11.015852.7031@phri.nyu.edu> Roy Smith writes: > Anybody know what the total average power factor on the power grid >is? I would imagine it must be massively lagging, what with motors and >transformers being most of the load. I can't think of any typical capacitive >loads on the power grid. How about charging current? A long transmission line is a sizable capacitor, and you're charging it up to a couple hundred kilovolts 120 times per second. This won't supply all the leading KVARs you need, but it definitely needs to be taken into account.
roy@phri.nyu.edu (Roy Smith) (01/13/90)
I claimed ELI the ICE man was ambigious. In <5606@ae.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) replied: > You may know the phrase, but you certainly don't understand its > interpretation. The order of the letters is the key. Voltage (E) across > an inductor (L) leads (comes before) the current (I). But it is equally true that Current (I) through an inductor (L) lags (comes after) the Voltage (E). Is 3 > 2 or is 2 < 3? -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy "My karma ran over my dogma"
myers@hpfcdj.HP.COM (Bob Myers) (01/16/90)
>I claimed ELI the ICE man was ambigious. >In <5606@ae.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) replied: >> You may know the phrase, but you certainly don't understand its >> interpretation. The order of the letters is the key. Voltage (E) across >> an inductor (L) leads (comes before) the current (I). > But it is equally true that Current (I) through an inductor (L) >lags (comes after) the Voltage (E). Is 3 > 2 or is 2 < 3? Right. I "comes after" E in an L. E-L-I, ELI...get it? Nothing ambigous about that! Or are we having problems with the phrases "leads", "lags", "comes before" and "comes after"? Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
roy@phri.nyu.edu (Roy Smith) (01/16/90)
Richard S D'Ippolito says: > The order of the letters is the key. Voltage (E) across an inductor (L) > leads (comes before) the current (I). And Bob Myers says: > I "comes after" E in an L. E-L-I, ELI...get it? Nothing ambigous > about that! This is getting silly. I think we're arguing orthagonal arguments, i.e. both Richard and Bob have answered a different question than the one that was asked. Given both (correct) interpretations of ELI the ICE man above, tell me, does an inductive load have a leading or lagging power factor? At this point, I'm not so much interested in the right answer, but in somebody showing me how you can unambigiously derive the sign of the phase of the power factor from the mnemonic. The problem is not in understanding the physics, but in remembering the arbitrary naming convention. -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy "My karma ran over my dogma"
tomb@hplsla.HP.COM (Tom Bruhns) (01/17/90)
jgk@osc.COM (Joe Keane) writes: > >A _good_ switching power supply looks like a resistive load, the resistance >slowly changing in response to changes in output current and input voltage. > >---------- Just to continue the basenote drift and give y'all some more food for thought (munch, munch :-) -- two questions (with additional sub- questions :-) : What sort of resistance do you suppose a switching supply looks like? What happens when you hook huge numbers of these up to the power grid? Assume that the switching supply draws a pulse of current synchronized to the line voltage, centered around the peak of the voltage. Thus the current isn't leading or lagging the voltage. What is the power factor? Why?
myers@hpfcdj.HP.COM (Bob Myers) (01/18/90)
> This is getting silly. I think we're arguing orthagonal arguments, >i.e. both Richard and Bob have answered a different question than the one >that was asked. Given both (correct) interpretations of ELI the ICE man >above, tell me, does an inductive load have a leading or lagging power >factor? At this point, I'm not so much interested in the right answer, but Lagging. But you're right - the extra piece of information you need to carry around with you when considering power factors is that they are referred to as "leading" or "lagging" depending on whether the CURRENT is leading or lagging the VOLTAGE, rather than the other way around. Don't give up on Eli so fast, though, because that mnemonic has 'em in the right place; the current (I) is first in the C word (ICE), and last in the L word (ELI). If you are faced with a power factor that someone has called "inductive", you know from ELI that it must has the current lagging the voltage. Similarly, you can correctly match up "L" or "C" with the sign of the phase of the current. (NOT the sign of the power factor; PFs are always a positive number, distinguished ONLY by "leading" or "lagging".) If you like, just keep in mind that the reference for phase is always the voltage. Shouldn't be hard, as most folks are biased toward thinking in voltage terms rather than currents anyway. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers%hpfcla@hplabs.hp.com | sentient life-form on this planet.
jgk@osc.COM (Joe Keane) (01/20/90)
I write: >A _good_ switching power supply looks like a resistive load, the resistance >slowly changing in response to changes in output current and input voltage. In article <5170068@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes: >What sort of resistance do you suppose a switching supply looks like? The term `switching power supply' often means just a normal power supply (transformer -> rectifier -> big capacitor) with a switching voltage regulator. Mr. Bruhns is right about these: the rectifier only draws current on a small part of the cycle, putting nasty harmonic currents on the line. Anyway, it's good for a power supply to look like a resistor from the line side, and for a large one it becomes an important issue. There are a number of techniques for achieving this; more phases and resonant shunts are classic, but you can also use a switcher. It's interesting, but you could write a whole book on it...
michael@xanadu.com (Michael McClary) (02/02/90)
In article <1452@umigw.MIAMI.EDU> wb8foz@mthvax.cs.miami.edu (David Lesher) writes: > >One question I never asked a utility when I should have... > >Virtually everything causes inductive power factor problems. You >correct for it (typically) with capacitors (some old-timers may call'em >condensers, but I think those belong to steam engines and HVAC units :-}). > >What does the utility think if you offer to correct other people's >leading PF by having too many farads? They don't like that, either. Their resistive losses go with I-squared-times-R, and generators and transformers are limited by heat, and thus handle a maximum current, not a maximum power. So utilities want as much of the current to be in-phase as can be managed. (By the way, idling synchronous motors can also "generate" VARs {be capacitive}. Factories would just switch to all synchronous motors if the utility paid for VAR generation.)
michael@xanadu.com (Michael McClary) (02/02/90)
By the way: Though utilities meter real power to big users with both a 15-min recorder (for finding the demand peak and the primetime higher-rate) and an integrating watt-hour meter (for backup and to generate pulses for the 15-min recorder), they generally meter VARs with an integrating meter that's read monthly, and assume the power factor is constant. Thus, fancy switching is wasted, if you're interested in your bill rather than doing the power company a favor.