A }aer fresented at the MMth A anal Conventionof the Am erican Institute of Electrical Engi-neers, BEtffao, A ugust i2ist, o901. PresidentSteinmetz in thc Chair.

METERING OF ELECTRICAL ENERGY.

BY HARRY P. DAVIS.

The practice is now almost universal in the modern centralstation to use the integrating wattmeter for measuring and record.ing and from its reading to comnpute the charges to be made forsupply to the consumers; consequently, it follows that the de-sign of the wattmeter and the conditions under which it isoperated, should be such as to om'it of no question of its accuracyand permanency. These considerations, with the additional oneof small cost of maintenance, are now verv generally recognizedas essential. As the meter accuracy thus bears so intimate a rela-tion to the revenue return to the station, the meter becomes afactor in station engineering of prime importance, and as a re-sult has received a vast amount of study, both at thle hands ofthe user and of the manufacturer.;It is hoped that the following notes from the writer's experi-ence in this field, although developing nothing particularly new.will outline some of the more important features which shouldbe embodied in the ideal meter; and, for comtlparison with a com-nmercial type, data and a brief description will be given of a teston a meter with which the writer has been most closely identified,and which is now largely used in practical operation.

While there are many characteristics wbich a meter mustpossess, only the more important will be pointed out as beingessential to all types of meters. In colnsidering specific featuresof design, however, it is the intentioni to cover only the so-calledmotor types of meters now so generally in use. Experience has

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278 DAVIS OX METERIJVG OF ELECTRIC ENERGY. [Aug. 21

demonstrated that if a meter is, electrically and meehanically welldesigned with reference to the law and conditions u-nder whichit is to operate, and proper care is given to its manuifactuire, itwill be accurate and have the ability to remain in- calibration.These features, namnely, accuracy and permanency, are funda-mental, and the meter which possesses them to a proper degreewill surely possess all of the merits which are usually classed asfundamentals.The meter which is theoretically correct will be inflnenced

only by forces which are directly a product of the quantity whichit is to measure. The revolving elemient will respond in exactproportioin to the torque exerted upon it by this quantity and itschanges in speed will in no way affect this proprotio-nality. Theretardino, force which will act in opposition will be independentof the quantity to be measured, and will increase in proper pro-portion with the speed of the revolving element.A meter properlv designed to emnbody these conditions, will

measure accurately and be uninfluenced by changes in voltageor temperature, and when used in alternating current circuits willalso record true energy under all conditions of inductive or non-inductive load, irrespective of variations in frequency or waveform. While seemingly simple, these conditions can never becompletely attained, and rnany conmpromises muist be made in thedevelopment of the commercial type of imeter; obviously, how-ever, the nearer they are approached the more accurate will bethe meter, and the more extended will be its range of registra-tion.

Commercially the range of accurate registration is most impor-tant for the meter muist account for all the conditions and varia-tions of the load, without being delicate and expensive, qualifica-tions requiring accuracy between wide limits, as it must startand record accurately on very light loads, and also be able tooperate correctly under considerable overload. Further, themeter to be constant and unvarying in its operation, as it mustbe to be a practical and commercial success, the mechanicalmake-up must receive careful consideration, and it is most essen-tial to have:

1st. Simplicity.2d. Proper design of the rotatinig element.3d. Proper design of the enclosing case.The simple meter will have few parts, with every part easily

accessible, and so related to others that the meter may be easily

1901.] DAVIS ON XETERING OF ELECTLRIC ENERGY. 279

taken down and put up without destroying or disturbing in anyway the balance or adjustment of the separate parts. Coinpensa-tiDg devices are necessary to extend the range and to strailghtenoout the calibration curve. The fewer of these devices used,the better, however, and the amount of compensation neededshould be slight, so that their influence may be small.

In the revolving element, we come to the consideration of whatis by far the Imost important feature, in meehanical parts of themeter as a whole. The shaft must be short and stiff, and theentire revolving element must be simple and light in weight;this last consideration is most essential, affecting as it does in avital manner the portability, life and ability of the meter toremain in calibration. When made sufficiently light all possi-bility is eliminated of injury to the jewel support, and the meterwill respond instantly to sudden changes or fluctuation of load.Artifieial, complicated and delicate devices for obviating injuryto jewels or to act as substitutes for them, should be unnecessaryif careful attention is given to these cornsiderations in the de-sign and proportions of the revolving element, for unless verywell designed such devices are liable in the long run to betroublesome.No practicable form of bearing for a commercial meter in the

writer's opinion, will have less friction or be more durable thana jewelled step bearing, provided the weight of the revolvingelement is not greater than a few gramines, with a speed ofrotation as slow as is consistent with accurate registration onlight loads. It is important also that meters used in alterniatingcurrent 'eircuits should be so proportioned that there will be novibration effects' as they cause wear on the revolving elornent-and its bearings, and in commercial use will attract the cus-tomer's attention and cause him. dissatisfaction, owing to hisbelief that the noise evidences operation of the meter at times ofno load.

Aniother and most potent factor having influence on the abilityof a meter to remain in calibration is one entirely independent ofthe working parts of the'meter proper, and yet requires as care-ful designing and proportioning in every way; and that is theenclosing case. This is a part of the meter, bowever, which,until recently, has been sadly neglected, and has resulted inbeing the cause of more trouble, in practical operation, tlhan allother causes put together. A meter with a thin sheet metal

280 DAVIS ON JVIETERING OF ELBCTR[C ENERGY. [Aug. 21

cover cannot be made tight, and soon becomes filled with dustand dirt, spiders, cockroaches, flies, etc. A meter is a pre-cisioni instrument, and those with experience know the effect ofa sm-all amount of dirt on accuracy. The well designed case^should be of light but substantial cast material, with the coverdesigned to make a tight joint with the base portion, and soarraniged that it can be hermnetically sealed, thereby effectuallyenclosing the interior of the meter. This requires the meehanismof the meter to be in a separate compartment from the terrni-nals, but results to its advantage, as it enables connections to theeircuit to be made, without any possibility of injury to them-eter. A meter case made in this way will be dust, insect andmoisture proof, and at the same time one wlhich cannot be tam-pered with. These details, while purely meehanical, cannot betoo highly emphasized, as they have so important a bearing onthe permanency of the calibration cuirve. These are also points-of design not always apparent and recognized for their fullvaltue on first examination and a test of a mneter, but whose.worth can only be established by the time test of actual opera-tion.The fact has been briefly noted of the qualifications and the

wide limits which' the calibration curve of the meter must possessto enable it to be elastic enough to meet the exacting require--inents of the modern central station. As these requirementshave been the keynote in meter developmlent in the last five orsix years, and as a proper appreciation of their value is most iun-portant in central station enigineering, the matter is again referredto, and will be explained more in detail. The gradual increasein residential lighting to its present large proportion of' everystation's load presented new problems, many being of the great--est commercial importance. Residential lighting, especially inthe districts far removed from the station, involves a high in-stallation cost, with a corresponding low return per lanmp in-stalled. In these installations the mnaximum dernan-d rarelyexceeds one-tlhird of the lamps connected, while one or two lampsmay be burned for a considerable period. In central stations,supplying such loads it is not an exaggeration to state that tento fifteen per cent. of the 24 hiour output is used in supplyingindividual installations, burning only one or two lamps for anygreat length of tirme, yet each having a total of 20 or 30 lampsinstalled. Tl'ese are loads which, when measured by a miieter-

1901.] DA VIS ON METERING -OF ELECTRIC ENERG Y. 281

unsuited to their character, can be supplied indefinitely, with butslight return to the station, owing to the inability of the meterto register the small load with any degree of accuracy. Thiscondition was becoming apparenit as early as 1896Q and muchpressure was brought to bear on the manufacturers to produce ameter suitable to meet these requirements, natmely, a meter withan unusually long range. Most of the difficulty was inability tostart, and inaeurate registration on light loads, and the metershaving no overload capacity it was necessary to install one havinga normal capacity muieh greater than the average load to be meas-ured. A meter was needed which would recoird accurately atleast withini two per cent. whether one lamp or the entire numberinstalled, were bturning, or, as previouasly stated, long range of re-gistration in the meter was necessary. The fact was also Apparentfrom the nature of the loads that if a meter could be designed whlichcould stand a considerable overload for short periods, and registerthe samie within fair accuracy, it would allow of a ineter beinginstalled of mucli smnaller rated capacity than that of the installa-tion, making it practicable to use a meter whose capacity vouldcorrespond to the average, instead of the maxitnum load of theinstallation. The use of a meter of small capacity would alsohave the advantage of giving a better registration for light loads,and proper selection of the size of meter to be installed wouldensure accurate registration of the smallest load whieh would beplaced on the circuit. Such were the conrditions to be met, andmeter design in the last few years has shown surprising develop-ment in this direction. In the central station, if careful atten-tion is given to the ftting of the3 mueter to the load to be mneas-ured with -special refereniee to installations of this nature, wheethe necessity is so great for a meter which will start and recordaccurately at light load, and at the same tinme hlave ability to takecare of an occasional overload, it is safe to say that a-very con-siderable increase of revenue will result. This amount will besufficient, in nany instances where improper nieters are in use,to pay in a very short time for the entire cost of the new metersand their inistallation, and to inerease the revenue on the entiresystem by a very respectable percentage.

In a large central station, whose cuistomers inelude mnany largeapartment houses, and where particular attention has beeni paidto proper mletering of this class of enstomers, it is customary,

282 DAVIS ON METERING OF ELECTRIC KNERGY. [Aug. 21

when a wbole apartment house is to measure on one meter (theentire installation will average 300 lamps, with about 12 lampsper apartment) to install a mieter having a capacity of about25 per cent. of the total number of lamps wired, and if eachapartment is to be metered separately, a meter is used having50 per cent. of the lights wired.

It is readily seen, that the use of meters of such small ratedcapacity, to that required for the total numuber of lamps installed,results in very accurate return of the light loads, which are usualfor this class of installations. To perfornm such service, it re-quires great overload capability for the mneter to be able to carrvcurrent now and then so greatly in excess of the normal, withouta large drop and heating effect in the series coil. It is, there-fore, important that this coil be of very low resistance, thus re-quiring it to be sr iall, with few convolutions of wire. This willresult in a small mxagnetizing force, and it is necessary in orderto obtain the requisite turning moment on the revolving part,that the magnetic organization of the meter be nearly a closediron circuit. It should be also noted in this connection that aseries coil so designed will have a loss at full load, so slight as tobe negligible, which is a valuable feature, as a large loss in theseries coil of a meter causes a drop in voltage which will affectthe regulation of the circuit, a loss, too, which occurs when theload on the systemn is the greatest. The use of a closed magneticcircuit has another important and useful bearing on the accuracyof the meter; as it makes it proof fromn the effects of externalstray fields, concentratihg as it does its own magnetizing field.To show to what extent these conditions have been realized in

commercial formr, a single phase integrating wattmeter of West-inghouse manufacture was selected at random from stoek fortest. This wattrneter is of the induction type, in which themagnetic field produced by the windings induce secondary cur-renlts in an aluminium disk, causing it to rotate, in the samiemanner as the armature of an induction m-notor. The design ofthis meter has been carefully worked out, to give the best allaround result in actual use. The coils are wound on iron cores,with very nearly closed magrnetic circuits, thus greatly reducingthe magnetizing forces necessary. This is particularly noticeablein the series coil which in an 80-ampere meter consists of buitone turn of wire having a length of about three inches. Meters

1901,] DA lY ON METERING OF ELECT'RIC ENERGY 283

of other capacities have a proportionate number of turns. Asso small a magnetizing force is required, it is evident that theinduction drop will be small, and as there are so few turns ofwire, it can be made with a very low resistance, conditionswhich are very necessary to enable the meter to stand a heavyoverload. The aluminium disk with its shaft is very light,weighing but 15 grammes.

Figure 1 is the calibration curve of the meter mentionedabove. This shows that from, 1/50 of full load, to 11 times fullload this meter is correct withini 11 per cent. and at twice fullload it is still within 3 per cent. The small loss in series coil ofthis meter can best be appreciated when it is stated that duringthis test it carried withotut injurv to itself for a considerableperiod a load of six times the rated full load capacity of themeter, representing a heating effect in the series circuit of 36times that occurring at its normal rating.

CALIBRATION CURVE10 AMP. 100 VOLT. 7200 ALTS. WESTINGHOUSE SINGLE PHASE INTEGRATING WATTMETER.

o REGISTERED

10 20 30 40 55 60170 80 90 105 110 120 130 110 1 0 loS 1,0 180 190 2007cLOAD

Fig. 1.

The writer would not consider this paper comnplete without abrief reference to the metering of polyphase currents, as thissubject has now becomne a question of importance, owing to therapid increase in the use of polyphase apparatus. The type ofmeter to use, the- way to read the counters, the manner of con-necting, and the limitations of the various connections, are notvery generally unlderstood, causing a g,reat many difficulties andquestionls to be coonstantly arising. The energy delivered to apolyphase circuit can be measured by the use of one single-phasewattmzeter, two single-phase wattmeters, or one polyphasewat,tineter. A two-phase circuit mnay be treated as two simrilar

284 DAViS ON M1TERING OF ELECTRIC ENERGY. [Aug. 21

single-phase circuits, and with a single-phase Alneter connectedin one of them, its reading will 'indicate one-half the totalenergy supplied in the two circuits. It is obvious that to havethe readings of any value for both circuits, it is absolutelynecessary that the loads on the two phases be balanced; that is,of the same amount.The use of a single-phase meter on a three-phase circuit is

more complicated, as it is necessary to make special connectionsto obtain practicable readings, the c6onectioius being such as tomake the meter show one half of the total energy in the wholecircuit. In order to have the reading, bowever, of total energycorrect, the loads on the phases must be balanced.

With two single-phase meters, a two-phase circuit ean bemetered correctly whether balanced or uinbalanced, the sum ofthe readings of the two meters in all cases giving the total energyin the circuit.

In a three-phase circuit the same is true, except that the meterswill not give similar readings when the power factor of the cir-cuit is less than unity, anid with the power factor lower thaln 0.5one of the meters will reverse and run backwards. It is neces-sary, therefore, in order to arrive at the total energy, to use thealgebraic sum of the readings of the two meters. This diffieultycan be overcomne anid the meter nmade to read equal amounts,with varying power factors and balanced loads, by the use ofspecially connected meters, which will give an apparent trans-formation of the three-phase circuit to a two-phase circiuit beforeit is measured. The use of two single-phase meters is undesira-ble, owing to the necessity of reading two meters and adding thereading, to arrive at the total energy, and it is further aggravatedby the negative values shown on one nmeter on three-plhase cir-cuits of low power faetor, anid if conn<ected to obviate thetrouble, the connectionis are clifficult and complicated to be made.

'hlie use of a polyphas'e meter overcomes a1ll of these difcintil-ties, as it can be connected either to a two or three-phase circuit,the connections being straightforward, and what is most impor-tant, its reading will be on a single dial, and will show the totalenergy irrespective of unbalaneing of the loads or chlanges inpower factor. It can be seen, therefore, that the whole questionof using single-phase meters on polyphase circuits depends eTl-

1901.] DAVISD ON ETERING OF ELECTRIC ENERGY. 285

tirely on the balancing of the loads on the several phases, andthe power factor of the circuit. In view of the uncertainty ofthe polyphase circuits in this respect, the exclusive use of poly-phase meters is to be recommended, ensuring, as they do, accu-rate registration without regard to these irregularities.