Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/635

 D Y N AM O if for any reason, such as eccentricity of the armature in the bore of the pole-pieces, the inductive effect of one field is not equal to that of any other, the equality of voltage produced by the two halves of the winding is not affected thereby. In the parallelwound armature inequality of the voltage of the various parallels may be avoided by a combination of the series and parallel methods. For large currents, multiple circuits are possible both with series and parallel winding, and q = ‘lp/ x x, where a; = the number of separate armatures which are virtually wound on the same core. When the core of the armature is constructed of iron or other metal, the passage of it through the lines of the field sets up in its mass E.M.F.’s which are in opposite a Wre core. * directions under poles of opposite sign, and are greatest at the surface where the rate of linecutting is greatest. Hence if the core were a solid mass, a current-sheet would flow along its surface under each pole, and complete its circuit by passing through the inner parts of the core or by returning in a sheet under a pole of opposite sign. The result would be a considerable reduction in the efficiency of the machine, since the power absorbed by these so-called eddy-currents would be entirely dissipated in heating the core. The production of the E.M.F. cannot be prevented, but the paths of the eddycurrents can be broken up, so that they meet with a comparatively high resistance and are reduced in amount. In the first place, the metal core must be divided into laminations whose planes are at right angles to the length of the inductors proper, since these are arranged to secure the maximum E.M.F. The laminae must be slightly insulated from one another; and if the insulation be maintained right up to their edges, the E.M.F.’s which still act across their thickness will not be added up along the length of the core, but will only produce very small currents, which circulate through the interior of the separate laminations. This remedy is practically realized in ring and drum armatures by building up their cores of thin iron plates or discs, strung on the shaft or on a supporting hub, so that their edges are presented to the lines of the field. Each disc is either coated with an insulating varnish or has one of its sides covered with a sheet of very thin paper; or in some cases no special insulating material is employed, and reliance is placed on the thin film of oxide which forms on the disc in the process of manufacture, and which is a poor electrical conductor. The thickness of the discs is usually about one-fortieth of an inch, and if this be not exceeded, the eddy-currents set up in the mass of the core are reduced to an almost negligible amount. The discs are in the ring machine notched with three or more key-ways, and are then passed over a gun-metal hub with three or more arms running parallel to the shaft, which fit into the corresponding slots on the inner periphery of the discs (cp. Fig. 6 and the sections of Fig. 13). In the case of small bipolar drums the discs are often keyed directly to the shaft, but in larger multipolar machines the required radial depth of iron is small, and the discs are then mounted on a hub similar to that of the ring armature, save that it may now be made of cast-iron. If the armature is more than 4 feet in diameter, the discs become too large to be conveniently made in one piece, and are therefore divided into segments, which are built up so that they alternately break joint. In all cases the stout end-plates between which the discs are clamped together must be kept well out of the influence of the field, since otherwise the lines which curve round into the ends of the core from the edges of the poles may cause eddy-currents in their mass. Hence the axial length of the core is often made slightly greater than the corresponding dimension of the poles. In discoidal-ring armatures the laminations must be concentric with the axis of rotation, and a thin ribbon of iron is wound on the sup-

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porting wheel, which may be of cast-iron (cp. Figs. 10 and 13 ii.). When the armature core has been built up and all rough edges smoothed off, it is covered with some material of good insulation, resistance, and durability, such as oiled linen, varnished paper, or micanite sheeting, so that all edges and sur-. * ure faces on which the winding rests are thoroughly protected. In the ring wire-wound armature a length of wire sufficient for one section of the armature is cut off, treated with shellac, and dried ; one end is passed through the interior of the ring, and the required number of loops are wound tightly round the core. The next section is then similarly wound, until finally the whole core is covered, and the beginnings and ends of contiguous sections are united at the commutator to form a continuous helix. If the size of the solid wire becomes too large to be conveniently bent, two or more wires may be wound in parallel, with, however, some necessary loss of space owing to the subdivision. The small drum armature may be similarly hand-wound, the loops of wire overlapping at the ends of the armature core; but the method is accompanied by the disadvantage that if one section has to be repaired, the whole armature may have to be taken to pieces and rewound. Hence various methods have been devised for winding drums with coils formed on shapers previously to fixing them in place. The former-wound coils have the advantage that they are perfectly symmetrical and interchangeable, can be thoroughly insulated before they are placed on the core, and, further, are economical in cost of labour. If, however, the current in each inductor is large, the drum armature must be bar-wound. The inductors are then large and heavy bars of rectangular section, which are laid on the core and joined up in regular sequence by soldering to the end-connectors. These latter are strips of copper from one to two inches wide, which are bent in a double evolute curve so that they span the correct pitch and unite the short end of one bar to the long end of another bar situated at the proper distance apart round the circumference of the core. For multipolar armatures with two or more layers of inductors, “surface” or ‘ ‘ barrel ” winding is now extensively used, in which no separate end-connectors are employed ; at each end the inductors project beyond the core and are bent through an angle corresponding nearly to half the pitch of the poles, the upper layer being bent in the opposite direction to the lower ; the ends of the two layers are then soldered together to form loops lying entirely on the surface of the cylindrical armature. Since any inductor must have a certain width in the direction of rotation, one edge enters or leaves the field sooner than the other edge. Hence when a solid bar is moving through the fringe of lines between the pole-tips, where the density varies considerably, the E.M.F. set up along the one edge is different from that set up along the other, and an eddy-current will flow round the bar, passing down one side and up the other, due to the difference between the E.M.F.’s of the two edges. Thus in a rotating armature an eddy-current will be set up in each and every solid bar as it passes between the pole-tips. To prevent the reduction in efficiency that results therefrom, and the consequent heating of the bar, it is necessary to make use of a stranded bar in which the separate strands, lightly insulated from each other, are twisted so that they pass from one side to the other, at least once midway along the length of the bar. The E.M.F. induced in each strand is then practically the same, since it is the average of the E.M.F.’s of the two sides, and the eddy-current loss is very largely avoided. In every dynamo with rotating armature the driving force has to be transmitted from the shaft or coupling to the hub, thence to the discs, and finally to the inductors. The discs Driv,n . . must therefore be securely keyed to the hub, and S in the smooth-surface armature any slip between the ,nduc j. ors core and the inductors is opposed by the friction be" tween the two. The bands of binding wire, which are placed at intervals along the length of the armature in order to resist the stress of centrifugal force, further increase the friction. In the wire-wound ring the loops themselves grip the core tightly, and are also held in place by passing through the interior of the ring between the arms of the hub. But in the smooth-core drum the firm attachment of the inductors to the core presents greater difficulties, and in order to ensure the positive driving of the bars through the magnetic field against the mechanical drag, it is usual to groove a number of longitudinal slots in the periphery of the core at equal distances apart of some four inches, and to drive tightly into these hard-wood driving strips, which project from the core to a height equal to that of the inductors. Thus the inductors are split up into groups, and the combined drag due to a group is taken up by a corresponding strip. A still more perfect method of driving is found in the toothed or slotted core. This was the original type of armature invented by Pacinotti, but after some considerable use it was largely discarded in favour of the smooth core ; of recent years, however, it has again been reintroduced, with a fuller understanding of the special precautions