This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by H. Y. Margary)
An electric lighting installation consists essentially of a dynamo with some form of engine to drive it, and all necessary wire, lamps, and fittings. Many circumstances govern the choice of engine and dynamo which are quite beyond the province of an architect or builder. The factors with which such are concerned are the prime cost of plant, the cost of upkeep, and the efficiency. The most economical power for driving a dynamo is water, where available. Steam engines and turbines, and coal gas and producer gas engines are used, while high-speed petrol motors, coupled directly to the dynamo, have been recently introduced. The design of the dynamo must of necessity depend upon the nature of the power employed to drive it. In small installations it is usually desirable to have the engine working for part of the day only, so that some means of storing the electricity must be provided. For this purpose secondary batteries or accumulators are employed.
Batteries are of two kinds, primary and secondary. Primary batteries are collections of cells such as that described in Chapter XIV (Simple Magnetic And Electric Phenomena)., and are used for generating currents. They are of little use practically, except for operating bells and telephones, for experimental purposes, or for testing. Secondary batteries are collections of secondary cells, and are used for the purpose of storing electricity. These are of great use practically, as a battery of cells occupying little space can be used for storing sufficient electricity for driving many lamps or motors.
Secondary Cells or "Accumulators" consist essentially of two lead plates immersed in dilute sulphuric acid, and provided with suitable terminals for connecting the ends of an external circuit to the plates.
In practice accumulator cells are made up with a large number of plates, the capacity increasing in direct proportion to the number of plates. The capacity of a cell may also be increased by increasing the size of its plates, but as this tends to weaken the plate it is preferable to increase the number. Plates usually have a superficial area of one square foot.
The plates are supported within a glass box containing dilute sulphuric acid, so that they are entirely immersed. When accumulators are subjected to hard wear, as in motor cars or yachts, the plates are contained within boxes of lead-lined teak. They should be as close together as possible, to reduce the internal resistance of the cell to a minimum, but care should be taken to avoid contact even by small particles becoming detached and lodging between them. The plates are usually separated by strips or rods of ebonite, glass, or glass tubing. Cells are also made in which the plates are separated by layers of thin porous material, and a number of plates are kept together by the pressure of elastic bands, specially prepared to resist the action of the acid. Such cells are made up in sections of several plates and separators, and cells of any capacity may be built up by placing a number of these sections together. The plates may rest upon blocks or strips of wood placed at the bottom of the glass cell, or may be suspended from their edges. Care should be taken not to concentrate the weight of the plates on too small an area of the bottom of the glass cell, otherwise the glass is liable to crack and the acid to be spilt. Sometimes glass ribs are formed upon the bottom of the glass cells upon which the plates are rested. The method of suspending the plates is usually considered more satisfactory, as any small particles of plate which drop off may then accumulate in the space beneath the plates, with very little chance of forming a connection between them.
When cells are received from the makers the plates are usually made up in sections ready for placing in the cells. These should be handled carefully to avoid bending or cracking, and all dust should be removed by the use of a pair of bellows or a soft brush. Any piece of packing material which may have found its way between the plates should be carefully removed by means of a thin slip of wood. The separators should be examined, and any that may have got out of place should be adjusted and any that are broken should be replaced.
Each cell, no matter what its size may be, gives an E.M.F. of 2.2 volts when fully charged, and this pressure continually drops while the cell is being discharged. If the discharge goes on too long the plates are seriously affected, so when the E.M.F. of the cell has dropped to a certain limit the cell should be recharged. This limit is fixed by the makers, and is usually 1.9 volt ; although some makers permit their cells to be discharged until their E.M.F. has dropped to 1.8 volt.
If, therefore, a battery of accumulators is required to give a pressure of 100 volts, the number of cells required will be 100/1.9 = 53. To obtain the combined E.M.F. of the cells they must be connected up in series - that is, the positive pole of each cell must be connected to the negative pole of the next cell, and so on.
All the plates have lugs of lead cast on to them, and all the positive plates and all the negative plates are connected together by casting stout lead bars on to these lugs. The lugs of one cell are connected to the lugs of the adjacent cells. Clamped connections should be carefully and heavily varnished, to protect them from the acid fumes arising from the cell. The method of burning together is usually employed when the cells are large, but it lacks the convenience of clamping, which enables a cell to be readily removed for repairs.
The maximum charging current is usually fixed by the makers, and this should not be exceeded, otherwise the plates will be damaged and their life considerably reduced. It is usually the same as the maximum discharging current; and it is usual, except under special circumstance, to charge with the maximum current, so as to reduce the time required for charging to a minimum. The E.M.F. required for charging each cell is 2.5 volts, so that a considerable loss of pressure takes place between the charging and discharging current, - so much so that accumulators would not be used at all were it not for the numerous compensating advantages which they possess.
The dynamo for charging the cells should be carefully tested, to see whether the maximum current is in accordance with that specified by the makers of the cells.
The acid should not be placed in the cells until all is absolutely ready for the initial charging. It should be of a particular density specified by the makers of the cell, usually about 1.21, pure distilled water alone being used for diluting the strong acid as received from the manufacturers. When all is ready the charging current should be switched on to the cells, the positive terminal of the dynamo being connected to the positive terminal of the battery, and the negative to the negative.
In charging the positive plates turn the colour of wet chocolate, owing to the formation of peroxide of lead, and the negative plates to the colour of slate, owing to the formation of a spongy surface of lead, the colour becoming deeper as the charge increases. The extent to which they are charged may be roughly gauged by the appearance of the plates. As the charging continues the density of the acid decreases, and when fully charged this should not be less than 1.15. The charging continues until it reaches the value specified by the makers, which with a new set of cells may take twenty-four hours or more. Another indication that the cells are fully charged is the appearance of "boiling" which takes place. This is not due to heat developed within the cell, but is caused by the evolution of gas. On its appearance the charging should be discontinued, when the battery, if the cells are in good condition, will give an E.M.F. which, when divided by the number of cells, will be equal to 2.2 volts.
The boiling of the electrolyte, as the acid is sometimes called, causes a spray to be thrown up from the surface of the acid, which will damage any insulating material or metal in the immediate vicinity. To prevent this spray from becoming wafted about, the cells are covered with sheets of glass, tilted so that the spray that collects thereon may run down and drip back into the cell. If this method is employed, care must be taken that the glass does not touch the plates, otherwise the film moisture upon the glass short-circuits the cell. Another method is to fill the cell with acid to a level half an inch higher than that actually required; then place a peg of wood in one corner of the cell and pour melted paraffin wax or other easily fusible compound upon the surface of the acid. When the wax has cooled the peg in the corner is removed carefully, and the level of the acid is reduced by means of a glass syringe inserted in the hole left by the peg. This method creates rather a nuisance when a cell has to be taken to pieces for repairs. Perhaps the best method is to cover the surface of the acid with crumbled cork or little hollow balls of glass. When cork is used it should be prepared by boiling in paraffin wax so that the acid cannot char it.
As acid fumes pervade all the space in the vicinity of accumulators, these should be set up in a house or shed well isolated from the dynamo, which is easily affected by the corrosion of the metal parts and the consequent deterioration of the insulation. The house should be preferably lined with glazed bricks built in cement mortar to withstand the acid fumes, and should be well lit from a louvered skylight in the roof.
The floor should be of concrete, asphalt, or some impervious material which is not affected by acid, and laid to a fall towards a gully to facilitate washing down.
Strong racks of pitch pine or teak well planed and shellac varnished should be formed to carry the cells. Where space is lacking the cells may be supported in two tiers, but it is better that the accumulator house be long and narrow, so that the cells may be arranged in one tier down the centre, that they may be thoroughly examined from each side, and so that any cell may be easily cut out for repairs. The racks, which should be composed of stout horizontal members braced together at intervals, should be supported at a height of about 30 inches from the floor. The supports should be of glazed brick, as wood becomes saturated with water owing to the constant flushing of the floor.