Part 6

One of the first methods of distribution over large areas proposed was by means of motors and dynamos combined. For instance, suppose, in order to keep down the size of the leads, 2,000 volts are used in the mains, a motor capable of working with 2,000 volts is put down where the lights are wanted, and this is made to drive a dynamo giving 100 volts and a large current. Instead of having a separate motor and a dynamo connected by a belt or by coupling the spindles together, it is simpler to make one machine with two armatures, or to have only one armature with two circuits on it. One circuit is wound with fine wire and takes the 2,000 volts and tends to turn the armature round. The other circuit is wound with thick wire giving 100 volts and a large current, and tends to stop the armature, thus absorbing the power supplied by the high-pressure circuit. The direct-current transformer or dynamotor is thus a sort of double dynamo, or dynamo and motor combined. If it gets 2,000 volts and 10 ampères, it would, if there were no waste, give 100 volts and 200 ampères; with a waste of 10 per cent., it will give 100 volts and 180 ampères.

In the United States it is usual to place an alternating current motor in each house to be lighted; but the conditions are quite different there, overhead wires being used extensively. In this country this system is not likely to find favour, and local sub-stations will be used, the high pressure, which is always dangerous to life, will thus be kept out of private houses and offices. There is, then, very little difference in the cost of maintenance of alternating current transformers and dynamotors, and the advantages possessed by alternating current transformers in this respect are more than counter-balanced by the use of motors on direct current circuits.

Dynamotors have not come into general use yet because no stations have been started in this country of the size which demands them. No central station with sub-stations is in operation, but there is every reason to expect several will be soon; and it is very necessary to discuss the various methods, not only in use at this moment but coming into use in the immediate future. The dynamotor itself needs no working out, as any maker of direct-current dynamos can, of course, make them. Messrs. Paris and Scott of Norwich showed some in operation at the Newcastle Exhibition in 1887; the most successful type is that recently invented by Mr. Jas. Swinburne, illustrated by Fig. 29. The backward main round primary or motor magnet is shown on the left, and the forward main round the secondary or dynamo magnet on the right, the outside coil round both magnets is the shunt.

The dynamotor may be made with two circuits on one armature as already explained, or it may have two armatures in separate fields, still making up one machine. The first arrangement has two grave disadvantages. There is difficulty about securing perfect insulation between the two circuits, and this leads to chances of danger in the houses. A dynamotor with two circuits on one armature cannot be compounded, that is to say, it cannot be made to give constant electrical pressure on the mains if the number of lamps is varied. A Swinburne double armature machine can be compounded, not only to give constant pressure with a varying load, but to give constant electrical pressure even if both the load and the pressure on the primary circuit vary. This makes a considerable difference in the copper of the primary leads, as in large and complicated districts it is almost impossible to arrange leads, even when working with high electrical pressure and small currents, so that the electrical pressure remains constant, or even nearly so. A very small variation of the pressure on an incandescent lamp makes an enormous difference in the amount of light it gives, and in its duration. It is, therefore, most important that the E. M. F. on the lamps should be kept absolutely constant.

This difficulty is, of course, insurmountable in the case of alternating current transformers. Alternating current transformers cannot be made to compound, and the loss in leads cannot be corrected by them, so that the lamps burn dull at full load.

If secondary batteries are used at the sub-stations, the reduction of pressure might be effected by them. A number would be charged in series and discharged in parallel. This arrangement needs at least two sets of cells, and cells are expensive; and it is difficult to preserve the insulation of cells with such electrical pressure as 2,000 volts. If cells are used for the purpose of equalising the load or as a safety reserve, it is better to charge them by means of a dynamotor.

INSTALLATION AND WORKING COST OF CENTRAL-STATIONS.

Until the balance-sheet of some large central-station has been published, it is impossible to do more than surmise what relation the earning power of the generating plant bears to the initial cost. Those central-stations which are working successfully in this country at the present time are either too small for a reliable estimate to be formed, or, as in the case of the Grosvenor Gallery, the space is too cramped for the large amount of machinery which it has been found necessary to add in order to meet the increasing demands for light. In order to obtain an approximate idea of the cost of installing a station capable of maintaining 10,000 lights, the following data (Table III.) given in Mr. Crompton’s paper before the Society of Telegraph Engineers are extremely valuable and will be examined with interest.

Although the figures given are necessarily empirical and open to criticism, the cost with both systems of distribution is approximately the same, and may be taken roughly at £5,860 per 1,000 lights, which amount, according to Professor Forbes, would be reduced to £3,914 per 1,000 lights if the installation was put down according to American practice, and at the initial cost of the Westinghouse alternating current system.

Mr. Crompton also compares the working cost of the two systems (Table IV.).

TABLE III.

_Cost of 10,000-Light, or 600-Kilowatt_,[6] _Plant._

A.T.—ALTERNATING TRANSFORMER DISTRIBUTION.

Generating station, buildings, chimney shaft, £ water tanks, and general fittings 11,000 Dynamos and exciters—865 kilowatts, including spare sets, divided as convenient 5,540 Motive power, _i.e._, engines, boilers, steam and feed connections, belts, &c., at £8 12_s._ per I.H.P. 12,470 500 transformers, _i.e._, one to every pair of houses, at £15 each 7,500 2,000 yards primary or charging main, exterior to area of supply, at £308 per 100 yards 6,160 20,000 yards distributing main, 50 mm. sectional area, at £91 7_s._ (_see_ Table I.) 14,270 Regulating gear 500 ——————— £57,440 =======

B.T.—ACCUMULATOR TRANSFORMER DISTRIBUTION.

Generating station, buildings, chimney stack, £ water tanks, and general fittings 8,000 Dynamos—600 kilowatts, in six sets of 100 kilowatts each 4,800 Motive power, _i.e._, engines, boilers, steam and feed connections, &c., at £8 12_s._ per I.H.P. 8,600 4 groups of accumulators, in all 240 cells, in series, at £40 per cell, including stands 9,600 2,000 yards charging main, at £306 17_s._ 6_d._ per 100 yards (_see_ Table II.) 6,137 20,000 yards distributing main, 161·25 mm. sectional area, at £100 12_s._ 6_d._ (_see_ Table II.) 20,125 Regulating gear 2,500 ——————— £59,762 =======

[6] Kilowatt equals 1,000 watts.

TABLE IV.

_Working Expenses and Maintenance of 10,000-Light, or 600-Kilowatt, Plant._

+-----------------------------------+ | Direct Alternating | | Transformer System. | -------------------------------+-----------------+-----------------+ _Materials_— | £ _s._ _d._ | £ _s._ _d._ | Coals: 4,380 tons at 17_s._ | 3,723 0 0 | | ” 2,550 ” 17_s._ | · · | | Oil, water, and petty stores: | | | 1,500 hours at 7_s._ 6_d._ | | | 7,250 hours at 1_s._ | 925 0 0 | | 1,400 hours at 5_s._ | · · | | Total cost of material +-----------------+ 4,648 0 0 | | | | _Labour_— | | | 2 foreman drivers at 45_s._; | | | 6 drivers at 30_s._; | | | 9 firemen at 24_s._; | | | sundry labour | 1,388 8 0 | | | | | 1 foreman driver at 45_s._; | | | 2 drivers at 30_s._; | | | 3 firemen at 24_s._; | | | sundry labour | | | | | | _Salaries_— | | | 1 chief at £500; | | | 2 assistants at £200 each; | | | 4 clerks at £80 each | 1,220 0 0 | | | | | 1 chief at £500; | | | 1 assistant at £200; | | | 4 clerks at £80 each | · · | | +-----------------+ 2,608 8 0 | _Maintenance of Plant_— | | | Motive power and dynamos: | | | 10 per cent. on £18,010 | 1,801 0 0 | | 10 per cent. on £13,400 | · · | | Buildings and fittings: | | | 5 per cent. on £11,000 | 550 0 0 | | 5 per cent. on £8,000 | · · | | Transformers: | | | 10 per cent. on £7,500 | 750 0 0 | | Accumulators: | | | 15 per cent. on £9,600 | · · | | Mains: | | | 7½ per cent. on £20,430 | 1,532 5 0 | | 2½ per cent. on £26,262 | · · | | Regulating gear: | | | 10 per cent. on £500 | 50 0 0 | | 10 per cent. on £2,500 | · · | | +-----------------+ 4,683 5 0 | | +-----------------+ | |11,939 13 0 | | +-----------------+ 2,100 units × 365 days | | | = 766,500 units. | | | Cost per unit | · · | 3·75_d._ | ==================================================================== | Continuous Battery | | Transformer System. | -------------------------------+-----------------+-----------------+ _Materials_— | £ _s._ _d._ | £ _s._ _d._ | Coals: 4,380 tons at 17_s._ | · · | | ” 2,550 ” 17_s._ | 2,167 0 0 | | Oil, water, and petty stores: | | | 1,500 hours at 7_s._ 6_d._ | | | 7,250 hours at 1_s._ | · · | | 1,400 hours at 5_s._ | 350 0 0 | | Total cost of material +-----------------+ 2,517 0 0 | | | | _Labour_— | | | 2 foreman drivers at 45_s._; | | | 6 drivers at 30_s._; | | | 9 firemen at 24_s._; | | | sundry labour | | | | | | 1 foreman driver at 45_s._; | | | 2 drivers at 30_s._; | | | 3 firemen at 24_s._; | | | sundry labour | 975 0 0 | | | | | _Salaries_— | | | 1 chief at £500; | | | 2 assistants at £200 each; | | | 4 clerks at £80 each | · · | | | | | 1 chief at £500; | | | 1 assistant at £200; | | | 4 clerks at £80 each | 1,020 0 0 | | +-----------------+ 1,995 0 0 | _Maintenance of Plant_— | | | Motive power and dynamos: | | | 10 per cent. on £18,010 | · · | | 10 per cent. on £13,400 | 1,340 0 0 | | Buildings and fittings: | | | 5 per cent. on £11,000 | · · | | 5 per cent. on £8,000 | 400 0 0 | | Transformers: | | | 10 per cent. on £7,500 | · · | | Accumulators: | | | 15 per cent. on £9,600 | 1,440 0 0 | | Mains: | | | 7½ per cent. on £20,430 | · · | | 2½ per cent. on £26,262 | 656 10 0 | | Regulating gear: | | | 10 per cent. on £500 | · · | | 10 per cent. on £2,500 | 250 0 0 | | +-----------------+ 4,086 10 0 | + +-----------------+ | | 8,598 10 0 | + +-----------------+ 2,100 units × 365 days | | | = 766,500 units. | | | Cost per unit | · · | 2·7_d._ | -------------------------------+-----------------+-----------------+

With the exception of the amount allowed for depreciation of the accumulators, which time alone can show to be correct, the expenses may be said to be over rather than under-estimated; the 15 per cent. depreciation given in Table IV. is under what has hitherto been found necessary to allow for the renewal of the plates of a secondary battery.

If the mean of the two results in Table IV. are taken, the working cost per Board of Trade Unit will be 3·22_d._, which shows that with both systems, after making due allowance for interest on capital, directors’ fees, bad debts, and other sundries omitted by Mr. Crompton, there is a probability of a very fair return on the capital expenditure, and the prospect of a handsome dividend for an electric lighting company who can sell electricity at the average price of 7_d._ per Unit.

* * * * *

The cost of maintaining and working electric lighting plant at private installations is usually much in excess of a supply from a central-station; but where the installation is over 500 lights, the difference is not very great.

The working cost at the Athenæum Club of 387 lamps for the past year is given as follows:—

£ _s. d._ Gas for gas-engine 446 7 10 Oil ” ” 71 3 8 Water ” ” 35 0 0 Wages 175 2 1 Sundries 30 3 0 Maintenance of lamps, etc. 98 4 1 Repairs 103 1 1 ————— £959 1 9 —————

Average cost of lighting by gas and oil for previous years, for two-thirds number of lights £840.

At the Naval and Military Club, 420 lights cost £821 18_s._ for the same period, a steam-engine being used instead of a gas-engine.

The annual report of the cost of the electric light at the South Kensington Museum shows that in a larger installation, consisting both of arc and incandescent lamps, the annual cost of the latter is much less than in either of the clubs mentioned. At the Museum there are 860 16 candle-power lamps, working 655½ hours per annum, or 562,387 lamp hours; the total cost for working last year was £386, which includes £66 for repairs of engines, boilers, dynamos, and maintenance of lamps; but rent, interest on capital, depreciation of plant, and management is not included. The light is used only three evenings a week, so that the wages of the attendants are proportionately in excess of what they would be in a central-station.

* * * * *

The cost of arc lighting for street purposes may be estimated from the following tenders. At Taunton the local electric light company offered to extend the lighting of streets from 29 to 60 arc lamps of 1,200 candle-power nominal on the Thomson-Houston system, at the following rate:—

Per annum. £ _s. d._ Burning on average of 6 hours per night each lamp 17 7 6 7 ” ” 18 12 6 8 ” ” 19 17 6

The posts and supports to be provided and fixed by the company, or, if the town council found the same, the company would allow a deduction at the rate of 5 per cent. per annum upon the outlay made by the council. The lamps are usually about 400 feet apart.

The actual cost of operating arc lights on this system is given in the following detailed expenses of a six hours’ run of a 50-light plant for the street lighting of an American city:—

2,600 lb. Ind. nut and slack coal, at $1·30 per ton $1·69 Engineer, one night, at $50·00 per month 1·67 Superintendent or electrician, one night, at $50·00 per month 1·67 Trimmer, one day, at $40·00 per month 1·33 48 pairs of carbons, at $18·50 per month 89 Waste, &c., at $20·00 per year 05 Water rent, at $40·00 per year 11 Half-pint cylinder oil, at 60c. per gallon 04 One pint engine and dynamo oil, at 50c. per gallon 06 One day repairs on machine and lamps, including globes, at $120·00 per year 33 One day taxes on 50-light plant, assessed at $5,000, at 2¼ per cent. 31 One day interest on 50-light plant ($10,000), at 6 per cent. 1·67 ————— Making a total of $9·82 10½_d._ or 20·45 cents. per lamp. £15 10_s._ per annum.

If, in addition to the 50 street lights, 33 other arc lights are maintained, the total cost is reduced for a six hours’ run to $13·25, or £2 15_s._ 2_d._ for the 83 lights, 8_d._ or 15·96 cents per lamp, £12 3_s._ 4_d._ per annum.

Table V. has been calculated by M. Decker, of Nuremburg, and gives the comparative cost of working 150 lamps by electricity and by gas. The gas price (1) is that paid in Paris, namely, 6_s._ 9_d._ per 1000 cubic feet; column (2) is the price usually taken commercially, which includes the fixed charges. The price of electricity is given: 1st, when a steam-engine is available; 2nd, when it is necessary to lay down a special engine; 3rd, when a gas-engine is used the gas is charged at a trifle over the price in column (1).

TABLE V.

_Total Cost per Hour and per Lamp._

Legend for Table V. A = Number of hours’ work per year. B = Number of hours’ work per day. C = Steam-engine (existing). D = Hydraulic Motor. E = Steam-engine to be erected. F = Gas engine G = (1) Gas at 6_s._ 9_d._ per 1,000 ft. H = (2) Gas at 8_s._ 9_d._ per 1,000 ft.

+------+--------+--------+---------+---------+-------+------ | | | | | | | A | B | C | D | E | F | G | H | | | | | | | --------+------+--------+--------+---------+---------+-------+------ | | Pence. | Pence. | Pence. | Pence. | Pence.| Pence. 500 | 1·38 | 0·485 | | 1·055 | 1·216 | 0·418 | 0·552 800 | 2·19 | 0·371 | | 0·780 | 1·007 | 0·399 | 0·513 1,200 | 3·29 | 0·314 | | 0·608 | 0·865 | 0·380 | 0·352 3,600 | 9·87 | 0·219 | 0·152 | 0·352 | 0·485 | 0·361 | 0·465

Arc Lamps. 500 | 1·38 | 5·235 | 4·246 | 10·459 | 11·485 | | 800 | 2·19 | 3·971 | 3·089 | 7·552 | 9·272 | | 1,200 | 3·29 | 3·087 | 2·441 | 6·004 | 7·581 | | 3,600 | 9·87 | 2·185 | 1·510 | 3·591 | 5·586 | | --------+------+--------+--------+---------+---------+-------+------

UNDERGROUND OR OVERHEAD WIRES.