Fig. 29.
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.