Modern electric generators are divided into two distinct classes, those which generate a direct current and those which generate an al ternatirig current. It is customary, with a few exceptions, to term the part of a machine in whose coils the E. M. F. is generated the arma ture, and the parts that produce the flux the field magnets. Dynamo electric machines are further classified as to their mechanical opera tion into machines in which: (a) The armature revolves and the field magnets are stationary; (b) the field magnets revolve and the arma ture is stationary; and (c) the field and armature coils are stationary and iron core re volves. It is obvious from previous mention that a dynamo electric machine may supply either a direct or an alternating current depend ing upon whether it is fitted with a commu tator or with slip rings. Machines termed rotary converters possess both a commutator and slip rings, their function being to convert alternating into direct current or vice-versa. If the ma chine be driven by an external force, either di rect or alternating current may be obtained by making the proper brush connections. The ro tary converter when operated in this manner is termed a double-current generator. As the con struction of the armatures of direct-current machines, rotary converters and a large num ber of alternating current machines is similar with the exception of their commutators or slip rings, the construction of the armature of a direct-current machine will be given in detail.
Direct-current Armatures.—A direct-cur rent rotative armature consists essentially of three parts, an iron core mounted upon a shaft, a number of conductors wound upon the sur face of the core or embedded in slots near the surface, and a commutator. See ELECTIUC DI RECT CURRENT.
The object of the core is to facili tate thepassage of lines of force from one pole of the field magnet to another. If this core were not present fewer lines of force would pass through the armature, and this would tend to decrease the E. M. F. generated by the machine. For equal magnetizing forces a sample of iron may carry 2,500 times the num ber of lines of force that would be carried if air were substituted for it. An iron core in a coil -is 2,500 times as strong a magnet as a coil having no core.
The periphery of the armature core of large machines is usually slotted longitudinally, the purpose of which is to admit the conductors and to decrease the air space, termed the air. gap, between the core and the pole faces, a thing which is very desirable.
The core is composed of iron discs punched from sheets. These discs are punched out whole for small machines, or are made in segments, the junctions of which are staggered in large machines. The discs in machines of large ca pacity are mounted upon a form of proper di mensions, which in turn is fastened to a spider and keyed to the shaft. The punchings are as sembled with their planes perpendicular to the axis of rotation. The reason for using discs in stead of a solid casting is that the iron core is equivalent to a conductor revolving in a field.
Currents, termed Foucault currents, flow through the iron in the direction of the axis if the discs are not properly insulated. This cur rent unduly heats the armature and is there fore undesirable.
Armature Windings.— The wires distrib uted over the periphery of the core of an arma ture constitute the generating part of a direct current machine. Armatures in which the wind ings are only upon the periphery are termed drum armatures. When the core of the arma ture is in the form of a ring and the wires are wound in and out around the ring the armature is called a ring armature. Drum armatures are in use commercially to a greater extent than ring armatures particularly in the case of large ma chines. In the drum armature a greater por tion of the winding is active in producing an electromotive force than in the ring armature. The ring armature has considerable wire upon the inner face of the ring in which very little eletromotive force is induced.
The length of active conductor connected in series on an armature determines the magni tude of the electromotive force generated by a given magnetic field. In machines of large magnitude two volts or more are generated per foot of active conductor.
Many forms• of armature winding are in vogue. The designing engineer is often called upon to design a machine of given capacity which will generate a certain pressure at a defi nite seed. To do this he must place sufficient wire in series to produce the required E. M. F., and this wire must be of sufficient cross-section to carry the current it will be called upon to deliver. To meet these conditions and still have an armature which is not abnormal in size often results in a very complicated series multiple winding. Machines of large capacity usually have more than one pair of poles and have brushes between successive poles around the commutator. These serve to conduct the cur rent to the main circuit.
An economical method of winding drum ar matures consists in the employment of formed coils. These coils are wound upon a collapsible form of dimensions and after being thor oughly insulated and shellacked are removed from the form. They are then inserted in the slots of the armature core. The distance be tween the two halves of the loop is nearly equal to the distance between the centres of two poles of opposite polarity. The terminals of the loop are connected to two adjacent commutator seg ments. The next .loop is placed in slots and similarly connected, care being taken to connect the two coils in series to a common commutator segment. By this method the current tends to flow in. one continuous direction. Many modi fications are made to this form of winding but the principle of connecting the coils so that their E. M. F.'s will be cumulative is common for all machines. Some machines have two or more separate windings upon the same core.