The installation at the Grosvenor Gallery, London, may be taken to illustrate Class I. or the practical working of distribution by means of transformers.
Fig. 11 represents the arrangement of primary and secondary circuits.
An alternating current is sent through the main L L¹, which is a closed circuit, and a small portion is drawn off wherever there is a secondary generator or transformer T; these instruments are placed in parallel between the conductors in the same manner as a glow lamp; neither main can be called positive or negative, as the current flows backwards and forwards many times in a second. The house wires M M are joined to the secondary circuits, and are quite distinct from the main, which they do not even touch, although sufficiently near to receive an induced current alternating the same as the primary, but of a much lower electro-motive force.
D, alternating current dynamo; E, continuous current dynamo for exciting; L L¹, main primary conductors; M M, secondary conductors; T T, transformers; S¹ S² S³ S⁴ S⁵ S⁶, lamps in parallel.]
The Goulard transformers were used at first, but have been superseded by others designed by Mr. Ferranti; they are of the No. 2 kind, or shell type, and have a core of hoop-iron, on which the two coils are wound; the hoop-iron is then bent over, and the ends joined so as to enclose the coils. The machinery is fixed in a basement excavated under the Grosvenor Gallery; the foundations are of massive concrete, in which stone supports for the engines and dynamos are embedded; the concrete does not touch the walls of the building, but a space of about 1 foot is left, which is filled in with clay; and by this simple plan all vibration of the machinery is isolated from the building. The power is obtained from two horizontal high-pressure engines, each of 600 indicated horse-power, in addition to the original two horizontal high-pressure non-condensing engines, each of 35 nominal horse-power, running at a speed of 55 revolutions per minute, which is maintained constant by means of a governor directly attached to the expansion slide-valve. The four engines drive on to a countershaft, which is cut up into lengths; each section is coupled to a dynamo and exciter by means of a conical friction-disk clutch; this permits of either length being started or stopped without interfering with the other. The speed of each engine is checked by means of a liquid speed-indicator, designed by the author. Two Ferranti alternating current dynamos, each capable of maintaining ten thousand lamps, are driven direct, one dynamo by each length of shafting: they are excited by two continuous current machines, the circuits from which are joined to a regulating apparatus, which by altering resistance keeps the electro-motive force of the large machines proportional to the number of lamps which are to be maintained. At present hand regulation is employed, but it is proposed to use automatic regulation, which will increase the life of the lamps, as they are severely tried by the variation of the current, which is more noticeable than in continuous current installations. The current from the machines is at a potential of 2400 volts, and that from the transformers is 100 volts. The primary wire which carries this high electro-motive force does not enter the houses, as the transformers are, as a rule, fixed in the cellars, and from them the house branch is led in the form of a cable of fine wires, having a total diameter of 7/16 inch; the lamps, which are placed in parallel across this cable, are attached to single No. 18 or No. 20 B. W. G. wires in the usual manner. When first established, the transformers presented an element of danger, in that they, in common with all induction coils, were also condensers, and therefore a dangerous shock might be given to any one touching some unguarded portion of the lighting system. This has been prevented by the simple plan of connecting one of the terminals of every secondary circuit to earth, a method which, however, is not to be recommended, as it throws an additional strain on the insulation of the primary circuit; in fact, by earthing the secondary the insulation is practically reduced to one-half. A safety device should be inserted, which would come into operation on any leakage from primary to secondary, and immediately cut out the transformer.