Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/634

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starts off again to a third inductor on the same side of the core as the first loop, and lying within its two sides, i.e., the next loop starts with the inductor next but one to the first inductor. The pitch of the front end is therefore less than the pitch of the back end, and differs from it by two. Thus the winding works continually forwards and backwards, until it passes right round the armature and finally closes on itself. The development on a flat surface shows that the completed winding takes the form of a number of partially overlapping loops, whence its name originates. num jer 0 One of these loops is marked in heavy lines in Fig. 19 i., where the obtained, having as many parallel circuits and as many points of firm-line portion gives the development of Fig. 16b, if cut through collection of the current as there are poles. The E.M.I’. of the at the point marked X. The multipolar parallel-wound drum is Z^. T. Zax 10-8 volts, precisely similar, as shown by the dotted additions in Fig. 19 i., armature is then 2p 60 which convert it from a two-pole to a four-pole armature ; the only t whether it be bipolar or multipolar, and the current in any one difference is that the pitch at the back end must in all cases be c odd number less than, but not far different from, r/2p. The inductor is —. All the sets of brushes which are of the same sign an maximum difference of potential in the drum armature exists 2p must be connected together in order to collect the total current C. between neighbouring wires as they successively pass the neutral The potential rises gradually in the armature wires as we pass from points corresponding to the position of the brushes, and is here a negative to a positive brush, or falls as we proceed onwards from equal to the full E.M.F. of the machine. These wires lie side by a positive to a negative brush. The difference of potential between side in the smooth-core armature with one layer, or one on the top adjacent wires is only that due to the added voltage of one in- of the other if there are two layers, as is usually the case in slotted ductor if the winding be in one layer, or to one section if there be several layers ; and as this is but a few volts even in machines giving as much as 3000 volts at the terminals, the insulation of the i 11 i"i wires from one another presents no difficulty. On this score the l ~f 1 ■ 'i 1 -M--:ring winding contrasts favourably with the drum, and it is therei rj-. rr ; i

r fore more especially suited to very high potential machines. If i; i
 * :s ! ) !

i ji i 1! i the increased number of brush-sets in the multipolar parallel- : i j.; 1: wound armature is regarded as a disadvantage, they may again be -iL-.-fii liHA--A-d-fcqUJ. reduced to two by cross-connexion of sectors situated apart; } .I-P but the commutator must then be lengthened p times in order to provide sufficient brushcontact surface to collect the total current C, and, further, the number oi sections must be an even number. It is, however, often required to add together the inductive effect of sections of the winding situated under two or more poles, especially in high voltage machines. This may be done by connecting together in series a set of coils symmetrically situated in fields of like polarity, as diagrammatically shown in Fig. 18 ; the current in the armature is then oidy divided beii. Wave-winding tween two parallel paths, and there are only two sets of brushes, or Fig. 19. Py — 1 and q = 2p — 2. Hence the E. M. F. of the series-wound multi-

the ring and drum armatures of Fig. 16, which are equally true for the discoidal and disc forms. The simple ring winding consisting of a continuous helix is in itself unaffected by the number of poles ; by the mere placing of 2py sets of brushes on the surface ol the commutator at equal distances apart, the winding is at once divided into as many equal and symmetrical paths through the armature, iag Rwinding. . or q ]= 2py.f poles, Hencea ifmultipolar 2py is made equal to 2p ring or the parallel-wound is

t

polar armature is E3 =p. —. r. Za x 10~8 volts. 60 The development of the modern drum winding from the shuttlewound Siemens armature is chiefly due to von Hefner Alteneck of Berlin (1871). From Fig. 16b it will be seen that, to avoid differential action, the width of each winding. in order mus^ exceed the width of a pole-face and may be equal to the pitch of the poles, i.e., the loop may be wound diametrically across the bipolar armature core. If, however, the number of commutator sectors be even and the winding be disposed in one layer, a symmetrical winding cannot be obtained unless the width of the loop falls short of the above maximum width by at least the width of one inductor, i.e., the “pitch” of the loop at the “back” end of the armature farthest from the commutator, or the number of inductors passed through in joining one inductor to another, must not exceed -1. Since - is with an even number of sectors itself also an even number, the pitch at the back end of the armature must then be an uneven number. Even if the armature winding be disposed in two layers, or the commutator sectors be an uneven number, it is not advisable for the loops to be wound diametrically across the armature core ; hence it may be said in general that the pitch of the winding at the back end of the armature must be an uneven number less than t/2, so that the loop spans a chord less than the diameter, and is in effect wound on one side of the core (cp. Fig. 16b). In completing the loop and joining it to the next loop,, two possible cases present themselves. By the first, or lap winding (Fig. 16), the end of the loop is taken to a commutator sector, and thence

armatures ; so that good insulation must be provided between the adjacent sides of the wires in the former case, and between the two layers in the latter case. By the second, or wave, method of drum winding, the end of the first loop in the bipolar armature is taken to a commutator sector on the opposite side of the diametric line to the first Wave loop, and thence the second loop starts with an in- wia(jing. ductor lying outside the first loop and next but one to the original inductor. The method may be extended to multipolar machines, and then gives the series-wound multipolar armature; when the completed winding is developed on a fiat surface (Fig. 19 ii.), it is seen to work continuously forwards m a zigzag wave round the armature, one inductor under each pole being successively joined up until the winding closes on itself. The average of the front and back pitches must be so that, e.g., in a four-pole machine, the possible number of inductors goes up by steps of four., , The equations of the E.M.F. of parallel-wound and series-wound drum armatures are the same as those for the parallel-wound and series-wound rings, the latter in each case being p times the former Thus the chief advantage of the series-wound armature, wfietner ring or drum, is that for a given voltage the number of inductors and the space lost in insulation of the wires is only l/P ol m number of inductors and the space lost in the parallel-woun armature. A further advantage is that the two circuits iron brush to brush consist of inductors influenced by all the poles in the drum, or by all the poles of like sign in the ring, henc