Fig. 70 shows a view of a 250 horse-power Westinghouse electric-railway generator. There are foto• pole-pieces, and over each one is slipped a metal bobbin. which is secured in place by bolts. These bobbins carry the shunt and series coils, which are put On side by side and do not overlap. The pole-pieces are laminated. being built up of thin sheet-iron plates cast into the cylindrical yoke. The bearings and this cylindrical yoke part along a horizontal plane through the artnature-shaft, thus giving ready access to the field-coils and armature. Each brush is held in an independent holder, so that filly one can he raised from the commutator without disturbing the others: and each brush has its own spring. which permits of perfect adjustment. The diametrically op posite brushes are of the same polarity, and are connected to the same terminal of the dynamo. The core of the armature is built up of a large number of thin. soft-iron disks, which are forced together under great press ure, and rigidly fastened to the shaft. This iron core, after having been completely cov ered with special insulating material, forms the foundation on which the wires of the armature are laid. The Westinghouse 500 horse-power railway-gete•ato• is designed On lines similar In the machine just de scribed, but with six poles.
a (1712 JI ipnlar ('out/n Currml Ilyiaime.—In order to construct a machine which should have II large output with a comparatively small weight of mate rial, lilessrs. Ganz &Co., of Bulhi-Pesth, ha ve deigned t he machine shown in the accompanying illustration (Fig. 71). As will be seen, the machine consists of six fixed eleetro-inagnets. which are east in one piece with the journal bearing, while the ring, supported on one side only, rotates over and around the field-mag nets. The armature, which is destined for a 6-pole machine, is wound so that one sixth of the number of coils will generate the necessary difference of potential of 1.500 volts. These six sections are then joined in parallel in order to obtain a current of 35 amperes. Each of these six sections consists of 56 coils, having 12 convolutions each, so that the commutator has 56X 6= 336 sectors or bars. The sixth corresponding commutator-bars are all jointly connected, so that the current is taken off by a single pair of brushes instead of six.
The most recent form of this machine when running at 1,000 revolutions per min. generates a current of 33 amperes at 1,500 volts; and the following data of its construction are of interest : According to the ;Wove data, the total output of the machine is 52.500 watts: this gives 640 watts per kilogramme (about 290 watts per lb.) of copper. the total weight of the machine being 685 kilogrammes. The electrical efficiency of the machine is per cent, according to the above data, which is an unusually high figure, and its commercial efficiency is also high. The machine was specially designed for the transmission of power where small weight and large capacity are desired.
The Wenstrom Dynamo.—This machine, in its improved form, is shown in perspective in Fig. 72, and in longitudinal section in Fig. 73. It is a 4-pole machine, but so constructed that two exciting coils are sufficient, the A-poles being energized direct, and the two consequent S-poles by induction. The brushes are placed at an angle of 90° on the commutator. Usually the space between the magnet-poles and core of the armature most be sufficient not only for the safe revolving of the armature at a high rate of speed. but also for the eon ductors in which the currents are induced. This gap introduces a very high magnetic resistance in the circuit for the magnetic induction. To avoid this, therefore, the conductors are buried in grooves or holes (Fig. 74). The distance between the poles and the core is thus limited to the small gap required for the safe revolving of the armature. The magnetic resistance and the magnetic circuit are by this means reduced in a high degree, and con sequently the magnetizing power necessary for creating the required magnetism in the armature is also decreased.
A test made by Dr. Louis Duncan, Dr. G. A. hiebig. and Mr. W. F. Hasson. of .Johns Hopkins University. on a 400-ampere 110-voltdynaino of this type, gave the following results. The machine was built for 400 revolutions per min., and weighed 7,100 lbs. : in the test it was run at 330 revolutions: The Bradley Dynamo.—The armature of this machine has the conductors wound upon it in one continuous course, so that the distance between each two successive induced portions of the conductors is greater or less than that between the poles of the field magnets, according to the formula: D = d — — ; or, D = — — d.
The induced current flows from one brush to the other through the whole system of con ductors in such manner that, however simple or complex the system of conductors may be, _ the average value of the tance it bet ween every two in duced parts which follow next to one another always con forms to the condition, d = 1 p n where u = average circumfer ence of armature, 0 = num ber of segments of the collec tor, p = any number of field magnets not less than four.
In the machine shown in the engraving (Fig. 75) there are 74 slots cut in the arum ure in. deep, in. wide at t he circumference, and -Apj„- in. wide at the bottom. This leaves the iron the same thickness between Ilse slots at the bottom as at the top, the aim being in I hese machines to have the magnetic circuit tile SUMP its cross-section in all its parts, The sectional area of the yokes, cores of the magnets, and the sum of the eross-seet ;mei of the bodies of iron between the slots which are under the pole-pieces at any one time, are approximately equal, Two hum's are placed in each slut, the lop One being longer than I he bot loin one, so as to project bevond it at each end to join the connectors. The lop bar is in, wide and the bottom in., so each has the same sectional area.