Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/636

 584

DYNAMO

required in its design, and it has met with great success in the construction of traction dynamos, in which the stresses set up by the varying load are very great. Not only are the bars securely driven by the teeth, but, as previously mentioned, they are largely relieved of the driving stress. A further advantage is due to the fact that the lines snap across the inductors from tooth to tooth with a velocity far in excess ol the speed of rotation. The product of the weaker density in the slots and the increased velocity of the relative movement gives the same E.M.F. as in the equivalent smooth-core armature, but owing to the comparatively uniform density of the lines within any one slot, and the extremely rapid rate at which they cut the inductors, solid bars of much greater width may be used in the toothed armature than on a smooth core without trouble from eddy-currents.1 One disadvantage of the slotted core remains to be mentioned. If the top of the slot be open, and its width of opening be considerably greater than the length of the single air-gap from the iron of the pole-piece to the surface of the core, the lines become unequally distributed not only at the surface of the teeth, but also at the bored face of the pole-pieces ; and this massing of the lines into bands causes the density at the pole-face to be rhythmically varied as the teeth pass under it. No such variation can take place in a solid mass of metal without 2the production of eddy-currents within it; hence if the proportion of the width of slot-opening to the length of airgap is equal to or exceeds 2 : 1, it becomes advisable to laminate the pole-pieces to avoid eddy-currents in them. This precaution is less necessary with half-closed slots, and is entirely unnecessary with tunnel armatures, in which the inductors are threaded through holes pierced close to the surface of the core ; but we are then met with the difficulty that such closed, or nearly closed, slots greatly increase the inductance of the loops, and are therefore disadvantageous to the commutation and sparkless collection of the current. On passing to the second fundamental part of the dynamo, namely, the field-magnet, its functions may be briefly recalled as follows :—It has to supply the magnetic flux; to provide for it an iron path as nearly closed as possible upon the armature, save for the air-gaps which must exist between the pole-system and the armature core, the one stationary and the other rotating; and, lastly, it has to give the lines such direction and intensity within the air-gaps that they may be cut by the armature inductors to the best advantage. Roughly corresponding to the three functions above summarized are the Forms of three portions which are more or less differentiated in the complete structure. These are: (1) the magnet “cores” or “limbs,” carrying the exciting coils whereby the inert iron is converted into an electro-magnet; (2) the yoke, which joins the limbs together and conducts the flux between them; and (3) the pole-pieces, which face the armature and transmit the lines from the limbs through the air-gap to the armature core, or vice versd. Of the countless shapes which the field-magnet may take, it may be said, without much exaggeration, that almost all have been tried ; yet those which have proved economical and successful, and hence have met with general adoption, may be classed under a comparatively small number of types. For bipolar machines the single horse-shoe (Fig. 20), which is the lineal successor of the permanent magnet employed in the first magnetoelectric machines, has been very largely used, and for all outputs up to 150 kilowatts remains one of the simplest, most economical, and most compact types. It takes two principal forms, according as the pole-pieces and armature are above or beneath the magnet limbs and yoke. The ‘ ‘ over-type ” form is best suited to small belt-driven dynamos, while the “under-type” is admirably adapted to be directly driven by the steam-engine, the armature shaft being immediately coupled to the crank-shaft of the engine, and the axis of rotation being thus kept low. In the latter case the magnet must be mounted on non-magnetic supports of gun-metal or zinc, so as to hold it at some distance away from the iron bed-plate which carries both engine and dynamo ; otherwise a large proportion of the flux which passes through the magnet limbs would leak through the bed-plate

across from pole to pole without passing through the armature core, and so would not be cut by the armature inductors. The field-system is thus—to use a somewhat inaccurate expression— ‘ ‘ magnetically insulated ” from the bed-plate, the intervening distance being some eight to ten times the length of the single airgap. Next may be placed the “Manchester'’ field (Fig. 21)—the type of a divided magnetic circuit in which the flux forming one field or pole is divided between two magnets. An exciting coil is placed on each half of the double horseshoe magnet, the pair being so wound that consequent poles are formed above and below the armature. Each magnet thus carries one-half of the total flux, the lines of the two halves uniting to form a Fig. 21. common field where they issue forth into or leave the air-gaps. Or the coils may be divided and placed on the limbs of each horse-shoe, as in Fig. 22, instead of on the yoke, as in Fig. 21. The pole-pieces in both cases may be lighter than in the single horse-shoe tyPe> and the field is much more symmetrical, whence it is well suited to ring armatures of large diameter. Yet these advantages are greatly discounted by the excessive magnetic leakage, and by the increased weight of copper in the exciting coils. Even if the greater percentage which the leakage lines bear to the useful flux is neglected, and the cross sectional area of each magnet core is but half that of the equivalent single horse-shoe, the weight of wire in the double magnet for the same rise of temperature in the coils must be some 40 per cent, more than in the single horse-shoe, and the rate at which energy is expended in heating the coils will exceed that of the single horse-shoe in the same proportion. A somewhat similar form of two-pole field is required hy the discoidal machine, but the poles now face the sides of the flat-ring armature (cp. Fig. 13 ii.) ; opposite poles of similar sign may be joined together by an overarching pole-piece, but owing to the

magnet.

1 2

Elihu Thomson, Elec. Eng. vol. iii. p. 499. Cp. Esson, Elec. Review, vol. xvii., 1885, and Journ. Inst. Elec. Eng. vol. xxvi. p. 596 ; Sayers, Journ. Inst. Elec. Eng. vol. xxii. p. 393.

difficulty of avoiding eddy-currents in the core it is more usual for the magnetic circuits to he entirely divided. Fourthly comes the two-pole ironclad type, so called from the exciting coil being more or less encased by the iron yoke; this latter is divided into two halves, which pass on either side of the armature. Unless the yoke be kept well away from the polar edges and armature, the leakage across the air into the yoke becomes considerable, especially if only one exciting coil is used, as in Fig. 23a ; it is better, therefore, to divide the excitation between two coils, as in Fig. 23b, when the field also becomes symmetrical. From this form is easily derived the multipolar type3 of Fig, 24, which is by far the most usual for any number of poles from four upwards ; its leakage coefficient is but small, and it is economical in weight both of iron and copper. The multipolar discoidal magnet takes the shape shown in the four-pole machine of Fig. 25, with which may be compared Fig. 13 ii. Another four-pole type (Fig. 26) may also be derived from Fig. 23b, if we suppose the 3 For the advantages of multipolar machines, vide Esson, “ Notes on the Design of Multipolar Dynamos,” Journ. Inst. Elec. Eng. vol. xx. p. 265.