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ELECTRICITY SUPPLY
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See also:ELECTRICITY See also:SUPPLY . I. See also:General Principles.—The improvements made in the See also:dynamo and electric motor between 187o and 188o and also in the details of the arc and incandescent electric See also:lamp towards the See also:close of that See also:decade, induced See also:engineers to turn their See also:attention to the question of the private and public supply of electric current for the purpose of See also:lighting and See also:power. T. A. See also:Edison' and St G. See also:Lane See also:Foxe were among the first to see the possibilities and advantages of public electric supply, and to devise plans for its See also:practical See also:establishment. If a supply of electric current has to be furnished to a See also:building the See also:option exists in many cases of See also:drawing from a public supply or of generating it by a private plant. Private See also:Plants.--In spite of a See also:great amount of ingenuity devoted to the development of the See also:primary See also:battery and the thermopile, no means of See also:generation of large currents can compete in See also:economy with the dynamo. Hence a private electric generating plant involves the erection of a dynamo which may be driven either by a See also:steam, See also:gas or oil See also:engine, or by power obtained by means of a See also:turbine from a See also:low or high fall of See also:water. It may be either directly coupled to the motor, or driven by a See also:belt; and it may be either a continuous-current See also:machine or an alternator, and if the latter, either single-phase or polyphase. The convenience of being able to employ storage batteries in connexion with a private-supply See also:system is so great that unless power has to be transmitted See also:long distances, the invariable See also:rule is to employ a continuous-current dynamo. Where space is valuable this is always coupled See also:direct to the motor; and if a steam-engine is employed, an enclosed engine is most cleanly and compact. Where See also:coal or See also:heating gas is available, a gas-engine is exceedingly convenient, since it requires little attention. Where coal gas is not available, a Dowson gas-producer can be employed. The oil-engine has been so improved that it is extensively used in See also:combination with a direct-coupled or belt-driven dynamo and thus forms a favourite and easily-managed plant for private electric lighting. See also:Lead storage cells, however, as at See also:present made, when charged by a steam-driven dynamo deteriorate less ' See also:British Patent See also:Specification, No. 5306 of 1878, and No. 602 of 1880. 2 Ibid. No. 3988 of 1878.rapidly than when an oil-engine is employed, the See also:reason being that the charging current is more irregular in the latter See also:case, since the single See also:cylinder oil-engine only makes an impulse every other revolution. In connexion with the generator, it is almost the invariable See also:custom to put down a secondary battery of storage cells, to enable the supply to be given after the engine has stopped. This is necessary, not only as a See also:security for the continuity of supply, but because otherwise the See also:costs of labour in See also:running the engine See also:night and See also:day become excessive. The storage battery gives its supply automatically, but the dynamo and engine require incessant skilled attendance. If the building to be lighted is at some distance from the engine-See also:house the battery should be placed in the See also:basement of the building, and under-ground or overhead conductors, to convey the charging current, brought to it from the dynamo. It is usual, in the case of electric lighting installations, to reckon all lamps in their See also:equivalent number of 8 See also:candle power (c.p.) incandescent lamps. In lighting a private house or building, the first thing to he done is to See also:settle the See also:total number of incandescent lamps and their See also:size, whether 32 C.p., 16 c.p. or 8 c.p. Lamps of 5 c.p. can be used with See also:advantage in small bedrooms and passages. Each candle-power in the case of a See also:carbon filament lamp can be taken as equivalent to 3.5 See also:watts, or the 8 c.p. lamp as equal to 30 watts, the 16 c.p. lamp to 6o watts, and so on. In the case of metallic filament lamps about 1•o or 1.25 watts. Hence if the equivalent of too carbon filament 8 c.p. lamps is required in a building the maximum electric power-supply avail-able must be 3000 watts or 3 kilowatts. The next See also:matter to consider is the pressure of supply. If the battery can be in a position near the building to be lighted, it is best to use too-volt incandescent lamps and enclosed arc lamps, which can be worked singly off the too-volt See also:circuit. If, however, the lamps are scattered over a wide See also:area, or in See also:separate buildings somewhat far apart, as in a See also:college or See also:hospital, it may be better to select 200 volts as the supply pressure. Arc lamps can then be worked three in See also:series with added resistance. The third step is to select the size of the dynamo unit and the amount of spare plant. It is desirable that there should be at least three dynamos, two of which are capable of taking the whole of the full load, the third being reserved to replace either of the others when required. The total power to be absorbed by the lamps and See also:motors (if any) being given, together with an See also:allowance for extensions, the size of the dynamos can be settled, and the power of the engines required to drive them determined. A See also:good rule to follow is that the indicated See also:horse-power (I.H.P.) of the engine should be See also:double the dynamo full-load output in kilowatts; that is to say, for a 10-kilowatt dynamo an engine should be capable of giving 20 indicated (not nominal) H.P. From the I.H.P. of the engine, if a steam engine, the size of the See also:boiler required for steam See also:production becomes known. For small plants it is safe to reckon that, including water See also:waste, boiler capacity should be provided equal to evaporating 40 lb of water per See also:hour for every I.H.P. of the engine. The See also:locomotive boiler is a convenient See also:form; but where large amounts of steam are required, some modification of the See also:Lancashire boiler or the water-See also:tube boiler is generally adopted. In settling the electromotive force of the dynamo to be employed, attention must be paid to the question of charging secondary cells, ii these are used. If a secondary battery is employed in connexion with too-volt lamps, it is usual to put in 53 or 54 cells. The electromotive force of these cells varies between 2.2 and 1.8 volts as they See also:discharge; hence the above number of cells is sufficient for maintaining the necessary electromotive force. For charging, however, it is necessary to provide 2.5 volts per See also:cell, and the dynamo must therefore have an electromotive force of 135 volts, plus any voltage required to overcome the fall of potential in the See also:cable connecting the dynamo with the secondary battery. Supposing this to be ro volts, it is safe to install dynamos having an electromotive force of 15o volts, since by means of resistance in the See also: The size of the secondary cell will be determined by the nature of the supply to be given after the dynamos have been stopped. It is usual to provide sufficient storage capacity to run all the lamps for three or four See also:hours without assistance from the dynamo. As an example taken from actual practice, the following figures give the capacity of the plant put down to supply 500 8 c.p. lamps in a hospital. The dynamos were 15-unit See also:machines, having a full-load capacity of 10o amperes at 150 volts, each coupled direct to an engine of 25 H.P. ; and a double plant of this description was supplied from two See also:steel locomotive boilers, each capable of evaporating 800 lb of water per hour. One dynamo during the day was used for charging the storage battery of 54 cells; and at night the discharge from the cells, together with the current from one of the dynamos, supplied the lamps until the heaviest See also:part of the load had been taken; after. that the current was See also:drawn from the batteries alone. In working such a plant it is necessary to have the means of varying the electromotive force of the dynamo as the charging of the cells proceeds. When they are nearly exhausted, their electromotive force is less than 2 volts; but as the charging proceeds, a See also:counter-electromotive force is gradually built up, and the engineer-in-See also:charge has to raise the voltage of the dynamo in See also:order to maintain a See also:constant charging current. This is effected by having the dynamos designed to give normally the highest E.M.F. required, and then inserting resistance in their field circuits to reduce it as may be necessary. The space and attendance required for an oil-engine plant are much less than for a steam-engine. Public Sup ply.—The methods at present in successful operation for public electric supply fall into two broad divisions:—(r) continuous-current systems and (2) alternating-current systems. Continuous-current systems are either low- or high-pressure. In the former the current is generated by dynamos at some pressure less than 500 volts, generally about 46o volts, and is supplied to users at See also:half this pressure by means of a three-See also:wire system (see below) of See also:distribution, with or without the addition of storage batteries. The general arrangements of a low-pressure continuous-current See also:town supply station are as follows:—If steam is the See also:motive Low- power selected, it is generated under all the best pressure conditions of economy by a battery of boilers, and See also:con supplied to engines which are now almost invariably sinuous supply. coupled direct, each to its own dynamo, on one See also:common bedplate; a multipolar dynamo is most usually employed, coupled direct to an enclosed engine. See also:Parsons or See also:Curtis steam turbines (see STEAM-ENGINE) are frequently selected, since experience has shown that the costs of oil and attendance are far less for this type than•for the reciprocating engine, whilst the See also:floor space and, therefore, the building cost are greatly reduced. In choosing the size of unit to be adopted, the engineer has need of considerable experience and discretion, and also a full knowledge of the nature of the public demand for electric current. The rule is to choose as large See also:units as possible, consistent with security, because they are proportionately more economical than small ones. The over-all efficiency of a steam dynamo—that is, the ratio between the See also:electrical power output, reckoned say in kilowatts, and the I.H.P. of the engine, reckoned in the same units—is a number which falls rapidly as the load decreases, but at full load may reach some such value as 8o or 85%. It is common to specify the efficiency, as above defined, which must be attained by the plant at full-load, and also the efficiencies at See also:quarter- and half-load which must be reached or exceeded. Hence in the selection of the size of the units the engineer is guided by the See also:consideration that whatever units are in use shall be as nearly as possible fully loaded. If the demand on the station is chiefly for electric lighting, it varies during the hours of the day and night with tolerable regularity. If the output of the station, either in amperes or watts, is represented by the ordinates of a See also:curve, the abscissae of which represent the hours of the day, this load See also:diagram for a supply station with lighting load only, is a curve such as is shown in fig. 1, having a high See also:peak somewhere between 6 and 8 P.M. The area enclosed by this load-diagram compared with the area of the circumscribing rectangle is called the load-See also:factor of the station. This varies from day to day during the See also:year, but on the See also:average for a See also:simple lighting load is not generally above ro or 12%, and may be See also:lower. Thus the total output from the station is only some ro% on an average of that which it would be if the supply were at all times equal to the maximumdemand. Roughly speaking, therefore, the total output of an electric supply station, furnishing current chiefly for electric lighting, is at best equal to about two hours' supply during the day at full load. Hence during the greater part of the twenty-four hours a large part of the plant is lying idle. It is usual to provide certain small sets of steam dynamos, called the daylight 320 280 zoo ,,200 x/60 120 eo eo B '0 /2 2 See also:NOON machines, for supplying the demand during the day and later part of the evening, the See also:remainder of the machines being called into requisition only for a See also:short time. See also:Provision must be made for sufficient reserve of plant, so that the breakdown of one or more sets will not cripple the output of the station. Assuming current to be supplied at about 46o volts by different and separate steam dynamos, Dy1, Dye (fig. 2), the machines are connected through proper amperemeters and volt-meters with See also:omnibus bars, 01, 02, 03, on a See also:main switch-See also:board, so that any dynamo can be put in connexion or removed. The switchboard is generally divided into three parts—one See also:panel for the connexions of the See also:positive feeders, F1, with the positive terminals of the generators; one for the negative feeders, F3, and negative generator terminals; while from the third (or See also:middle-wire panel) proceed an equal number of middle-wire feeders, See also:F2. These sets of conductors are led out into the See also:district to be supplied with current, and are there connected into a distributing system, consisting of three separate insulated conductors, D1, D2, D3, respectively called the positive, middle and negative distributing mains. The lamps in the houses, H3, H2, &c., are connected between the middle and negative, and the middle and positive, mains by smaller supply and service wires. As far as possible the See also:numbers of lamps installed on the two sides of the system are kept equal; but since it is not possible to See also:control the See also:consumption of current, it becomes necessary to provide at the station two small dynamos called the balancing machines, BI, B2, connected respectively between the middle and positive and the middle and negative omnibus bars. These machines may have their shafts connected together, or they may be driven by separate steam dynamos; their See also:function is to supply the difference in the total current circulating through the whole of the lamps respectively on the two opposite sides of the middle wire. If storage batteries are employed in the station, it is usual to install two See also:complete batteries, Si, S2, a 6 AM /Z 2 8 /0 /2 4 e PM Three-wire system. which are placed in a separate battery See also:room and connected between the middle omnibus See also:bar and the two See also:outer omnibus bars. The extra electromotive force required to charge these batteries is supplied by two small dynamos bI, b2, called boosters. It is not unusual to join together the two balancing dynamos and the two boosters on one common bedplate, the shafts being coupled and in See also:line, and to employ the balancing machines as electromotors to drive the boosters as required. By the use of reversible boosters, such as those made by the Lancashire Dynamo & Motor See also:Company under the See also:patents of Turnbull & McLeod, having four field windings on the booster magnets (see The Electrician, 1904, p. 303), it is possible to adjust the relative See also:duty of the dynamos and battery so. that the load on the supply dynamos is always constant. Under these conditions the main engines can be worked all the time at their maximum steam economy and a smaller engine plant employed. If the load in the station rises above the fixed amount, the batteries discharge in parallel with the station dynamos; if it falls below, the batteries are charged and the station dynamos take the See also:external load. The general arrangements of a low-pressure supply station are shown in See also:figs. 3 and 4. Additional information and CommentsThere are no comments yet for this article.
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