The coils for the field-magnets are wound on spools which are slipped over the castings and fastened firmly in position. These being well protected. the liability of mechanical in jury is reduced to a minimum. In case it is necessary to replace a coil, or to remove the armature, the upper half of the field-casting can be readily removed, leaving the pins easily accessible. For the purpose of energizing the field-magnets the dynamos are furnished with small exciting dynamos of Ilse reet-ell rrent type. It has been found desirable in some special eases to make the smaller sizes of alternating-current dynamos self-exciting. and to this end the armatures are wound with an extra or special coil for furnishing current to ener gize the fields. The exciler is usually placed as shown in Fig. 8$. behind the alternating dynamo, driven by a belt from a small pulley attached to the annature-shaft. One exciter is usually employed with each alternating-current dynamo, but when several dynamos are oper ated in the same station it is often found more convenient to employ exciters, any one of nyhieh is of sullivieut capacity for all the machines. By this arrangement an accident to one exciter need not atfeet the general service.
The accompanying diagram (Fig. 110) and table give the various dimensions, weights, ca pacity, etc., of these machines: — - Ganz do Co.'s type of alternating-current dynamo very largely employed in Europe is that built by Nessrs. Ganz & Co., of Buda-Pestb. Hungary. In its largely form the Ganz alternator had a star-shaped field-magnet of non-laminated iron revolving within a cylindrical armature, the core of which was composed of thin Hug-shaped iron plates held in a frame. The armature-coils were flat bobbins laid upon the Inner surface of the armature-core side by side, with insulated filling-in pieces interposed. The magnetic resistance of the iliterpolar spaces was in this arrangement necessarily high, and in the later machines this difficulty has been overcome by employing an ar mature-core with a series of in ternal racinotti projections. These project ions form the cores of the armature-bobbins, and to avoid the heating of the pole pieces. the field - magnets are now built up of U-shaped iron plates F, as shown in Fig. 111. These plates arc laid upon each other, and arranged round the spindle so as to form a star, alternate layers being arranged 10 break joint, as shown by the dotted lines in the illustration. The plates are fastened together by insulated bolts B, and the existing coils are wound upon separate formers, slipped over the magnet-cores, and held in position by bobliiii-holders and screws strong enough to resist the action of centrifugal force. The armature-core, which for merly was continuous, is in the new machines subdivided into miniber of T-shaped sections, Ilic central stem of the T form ing the Pacinotii projection Icing very short, and of equal width and length with the magnet. These sections are so ar ranged that each with its armature-bobbin (.:10 be removed without disturbing the rest of the machine. The illustration also shows the must rnetion of the rmaturc-sections, and the 1111111 nor of supporting them. The frame of the machine consists of Iwo ring-shaped casting: held
strong holls, and,. ill ioldit inn, there are iron triversers to which the segments are II"' figure.. the section at 1 is taken dose to nue of the east-iron rings, showing the flange In whirl' the Ira arc bolted. The section at is taken at some he I nversers and the plates of the armature-core: and the series at III Is taken at :mother internwiliale point, :Mowing the method by which the armature-core is bolted to the traversers. The armature-plates are held together by ribbed bronze plates, which also serve to hold the armature-coil in its place. In larger armatures bronze plates are also inserted at intermediate points, and they serve for the attachment of the armature-section to the traversers by means of insulated bolts and nuts, as shown at Ill.
Fig. 92 shows a complete machine intended for an output of 80 kilowatts. In this ma chine there are 14 poles and 14 armature-sections, which can be coupled to give a pressure of either 2,000 or 4.000 volts with a current of respect ively 20 or 40 amperes. The speed of the machine is 360 revolutions per min., giving 5,040 reversals, or a frequency of 42. The to tal weight of the iron core, both in field-magnets and armatures, is 1 ton 7 cwt., and the total weight of copper is 030 lbs. The re sistance of the armature is 1.038 ohms for a 2.000-volt machine, giving a loss of 2.08 per cent by ohmic re sistance in the armature circuit. The resistance of the field-magnet circuit is 3.24 ohms, and a current of 28.7 amperes is required at full output, entailing a loss of 333 per cent for excitation. Some experi ments were made with this machine to ascertain the various losses. When driv en in a non-excited field by a belt at the normal speed, 4'07 horse-power was consumed in journal friction and windage.
The field-magnets were then excited so as to produce a terminal pressure of 2,000 volts, but no current was allowed to flow through the armature. Under these conditions the power ab sorbed was 9.81 horse-power, showing that hysteresis and Foucault currents absorbed 0.74 horse-power. The total commercial efficiency. ineluding the power required to excite the machine, is at full output 87 per cent ; and this figure is somewhat increased when the ma chine is direct driven by a steam-engine. In order to facilitate the cleaning of the arma ture-coils, Messrs. Ganz & Co. have from the very first arranged the frame. of the arma ture in such a way that it could be shifted longitudinally beyond the space occupied by the Aeld-magnets, so as to expose the whole of the in ternal surface, and make it easily ac cessible, both for cleaning and for the renewal of a coil should it have been damaged. In the new type of ma chine, the armature itself is, how ever, kept fixed, and the magnet wheel is arranged to slide longitu dinally, for which purpose one of the standards is fitted similarly to the slide-rest of a lathe. In order to slide back the magnets, the pulley must be removed, and the standard on the opposite side can then be drawn back by means of a ratchet bar, screw, and nut to its outermost position.