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 ALTERNATORS.]

DYNAMO

winding (Fig. 13 i.) was early employed in the machines of Gramme and Wilde, and later of De Meritens. Owing . ,. to the excellent grip of the wires on the core, ng ' the ring coils are well adapted to withstand centrifugal force, and high peripheral speeds become permissible. The discoidal form of ring winding, with its appropriate form of field-magnet, may be easily derived from Figs. 13 ii. and 25. Drum winding in some of its numerous varieties is, however, more usual at the present time in alternators, as in continuous-current machines, and for the same reasons, namely, that it requires less wire and has less inductance. As in the continuouscurrent dynamo, the coils may be hand-wound, former - wound, or composed of bars united by end-connectors. If the armature core is smooth and the section of wire is small, the drum coil may be former-wound, and with a large number of poles the curvature required to fit it to the circumference of the armature core is so slight that it becomes almost flat; the open space within the inside turn is usually filled with a wood centre, flush with the sides. The complete coils may be placed either on the outside or inside of the armature core (Fig. 13 iii.), according as the poles are external or internal. A single coil per pair of poles may be used, as shown in Fig. 37, but the advantages of its division into two halves, as in Fig. 13 iii., namely, the better utilization of the space at the ends of the core and the gain in effective cooling surface, have already been mentioned. A pair of outside ends of adjacent coils and a pair of inside ends are then alternately connected together, and the maximum difference of potential between any two adjacent wires is equal to twice the voltage of one coil. But if the coils are all connected in series right round the armature, one of the groups, consisting of the adjacent halves of the first and last coils, is subjected to the strain of the total voltage ; hence for high pressures it may be advantageous to insert one dummy coil to separate the first and last coils. Another plan is to divide the armature winding into two halves and to connect these in parallel, as in Fig. 38. Either circuit then carries half the current, but must give the total voltage of the machine, so that twice the number of inductors of half the section are required; there is thus a greater amount of space lost in insulation, and, as with all cases of parallel circuits, it is very necessary to secure close equality of the E.M.F.’s of

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the two branches at every instant, since otherwise the load will not divide equally between them, and a loss of efficiency, with greater heating, will result. As compared with ring winding, the drum coils on a rotating smooth core are not so easily held in place against centrifugal force and the alternating stresses due to the magnetic pull on them ; the wood filling-pieces, being screwed to the core, serve as drivers, 'but with high peripheral speeds the surface, must be covered at frequent intervals by bands of binding wire. It is therefore even more common to employ a toothed armature core, although the embedded wires have more inductance and more reaction on the field ; the alternator then requires more alteration of its excitation if the voltage is to be kept constant under varying loads. When the slots the armature have parallel sides, or the teeth do not close entirely over the opening at the top, the coils may still be wound on a former in the lathe, and afterwards pressed into the slot, which is well lined with micanite, press-spahn, or other highly-insulating material. But if the armature has holes pierced close to the periphery of the discs, hand-winding must be resorted to, and the wires are threaded through the holes in tubes of micanite or paper. The wires themselves must be well insulated, but do not need to be very tight in the holes, as by far the largest part of the mechanical pull is transferred to the iron core. Disc winding (Fig. 13 iv.) has been very successfully used for alternators, as in the large magneto-machines invented by Nbllet, and afterwards constructed and improved by the Alliance Co. (1860), and also in the alternators of Wilde (1866) and Siemens (1878). The connexions of the coils can be followed from Figs. 37 and 38, which are as applicable to disc as to drum winding. It is usual for the armature to revolve, and no iron is employed in the core ; there is therefore no loss from hysteresis, but a somewhat large exciting-current loss is involved by the length of the air-gap between the opposing poles of opposite sign. An undulatory winding which was adopted in some early alternators has since been discarded in favour of coils, as in Fig. 38. In the modern Ferranti alternator, the centres on which the coils are wound are made up of strips of brass soldered together at their inner ends and lightly insulated, to separate the teeth of the comb ; eddy-currents in the core are thus largely prevented, and the advantage of a metal centre is retained. The thin copper strip employed in winding the coils is corrugated along its centre line, to retain it in its place and to prevent it shifting sideways. The very high peripheral speed which is used in this type of machine tends to keep the composite structure of the armature rigid in the plane of rotation. The general idea of the polyphase alternator giving two or more E.M.F.’s of the same frequency, hnt displaced in phase, has been already described. The several phases may be entirely independent, and such was the case with the early pCwarterpolyphase niachines of Gramme, who used four inde- ('ase pendent circuits, and also in the large two-phase alter- aaa frrs " nators designed by Gordon in 1883. If the phases are ° ’ thus entirely separate, each requires two collector rings and two wires to its external circuit, i.e., four in all for two-phase, and six for three-phase, machines. The only advantage of the polyphase machine as thus used is that the whole of the surface of the armature core may be efficiently covered with winding, and the output of the alternator for a given size be thereby increased. It is, however, also possible so to interlink the several circuits of the armature that the necessary number of transmitting lines to the external circuits may be reduced, and also1 the weight of copper in them for a given loss in the transmission. The condition which obviously must be fulfilled, for such interlinking of the phases to be possible, is that in the lines meeting at any common junction the algebraic sum of the instantaneous currents, reckoned as positive if away from such junction and as negative if towards it, must be zero. Thus if the phases be diagrammatically represented by the relative angular position of the coils in Fig. 39, the current in the coils 1 As in the historical transmission of energy from Lauffen to Franks fort (1891).