THE DYNAMO. or spindle S, which revolves in the two bearings F F. Upon one end of the shaft is fixed the driving pulley P. The iron core is overwound with a large number of insulated copper conductors or coils, the ends of which are connected to the circular commutator C. Two stationary metallic brushes, B B, press upon the latter, and convey the current generated in the coils of the armature through the flexible conductors or "leads," L L, to the terminals T T, from whence it is conveyed to the external or working circuit. When the machine is in action, the armature and conductors or coils are caused to rotate within the magnetic field, produced by the electro magnet M, by means of a belt passing over the pulley P. In consequence, E. M. F.s are induced in the conductors, which, on being commutated at the commutator, are transmitted through the medium of the brushes and flexible leads to the two terminals T T, giving rise to a difference of potential between the latter. are In the classification of dynamos the two principal divisions I. Direct current dynamos, and 2. Alternating dynamos, and in each class there are many forms of construction. The direct current dynamos are again divided into three classes, thus, I. Series wound. 2. Shunt wound. 3. Compound wound. THE DYNAMO. The manner in which the wiring of the fixed magnets are connected to the armatures gives rise to the foregoing classification: Series Dynamos.-The manner in which the connections of the series wound dynamo are arranged is shown in Fig. 60. FIG. 60. SERIES WOUND DYNAMO. The coils of the field magnet are wound with a few turns of thick insulated wire, and being joined in series with the armature, the whole of the current generated in the latter passes direct THE DYNAMO. through them, and thence to the external circuit. The current in passing through the coils of the field magnet energizes the latter, and creates a magnetic field between the two poles N S, in which the armature revolves. FIG. 61 SHUNT WOUND DYNAMO. Shunt Wound Dynamos.-The shunt wound dynamo differs from the series wound machine, in that an independent circuit is used for exciting its field magnet. This circuit is composed of a large number of turns of fine insulated copper wire, THE DYNAMO. which are wound round the field magnet and connected to the brushes, so as to form a shunt or "by pass" to the brushes and external circuit. Fig. 61 shows the connections of the shunt wound dynamo, from which it will be seen that two paths are presented to the current as it leaves the armature, between which it divides in the inverse ratio of the resistance; whilst one part of the current flows through the magnetizing coils, the other portion flows through the external circuit. In all well designed shunt dynamos, the resistance of the shunt circuit is always very great, as compared with the resistance of the armature and external circuit, and the strength of the current flowing in the shunt coils rarely exceeds 12 amperes in even the largest machines. Compound Wound Dynamos.-These are in fact a combination of the series wound and the shunt wound machines. See Fig. 62. The field magnet of this dynamo is wound with two sets of coils, one set being connected in series, and the other set in parallel, with the armature and external circuit. For the purpose of automatically maintaining a constant pressure in incandescent lighting the compound wound dynamo is now generally employed. Separately Excited Dynamos.-This type of machine is not so extensively used as the self-exciting type, owing principally to the fact that an independent dynamo or battery is necessary for exciting its field magnet. It finds its chiefest, application in the electric transmission of power, and in charging accum THE DYNAMO. ulators, and in all cases where a high E. M. F. is required with a varying current. The connections of this dynamo are shown in Fig. 63, where N S are the poles of an electro magnet which is excited by the current generated by the FIG. 62. +. COMPOUND WOUND DYNAMO. independent dynamo or battery D. The armature A revolves in the space between the two pole pieces, and the two brushes B1 B2 press upon the commutator C, and convey the current generated in the armature to the external circuit E. The |