Both these defects are practically ob viated in the drum armature. The core of this is cylindrical, being built of thin insulated discs of charcoal-iron keyed to the shaft. Slots are cut at intervals along. the curved face parallel to the axis of the drum, in which are arranged the insulated conductors. This construc tion allows of a very small air-gap be tween armature and pole-pieces, which has the effect of reducing the number of conductors necessary for a given ca pacity. For connecting together the straight segments in the slots, special connecting pieces of thin sheet copper, semi-circular in form, and suitably in sulated, are laid side by side round the shaft, the connecting lugs at their ends forming thus a spiral at each end of the commutator. These are necessary to economize space at the ends of the drum, and also to enable individual segments to be conveniently withdrawn, if neces sary, for replacement. Each coil is ar ranged diametrically to the drum, and the whole number is arranged in one series round the armature.
The commutator for such an arma ture consists of a cylindrical ring, built up of bars of hard-drawn copper in sulated from one another by means of mica spacing pieces, and carried on a cast iron sleeve in such a way as to be mechanically stable and insulated from the shaft. The section and length of the bars is determined by the capacity of the machine, and the maximum cur rent density that is found practicable in leading off the current to the brushes.
The commutator must have as many segments as there are coils in the arma ture, and connections to the segments are made in turn at successive junction points of continuous coils in the series.
Brushes are of laminated copper, cop per gauze embedded in carbon, or car bon alone. They are carried on rocker arms which allow of some adjustment in position round the commutator cir cle, this being necessary to avoid spark ing at the brushes when the load is varied. The holde-:s for the brushes are designed also so that the brushes may be fed radially to the commutator as they wear away, and individual units withdrawn for replacement.
In the Gramme, or ring armature, the lamine are ring-shaped, and supported on a framework of brass or gun-metal, keyed to the shaft. Round the hollow cylinder thus formed, the coils are wound, the return winding being passed through the inside of the cylinder, all coils being in series and connected to the commutator segments as in the drum armature. This form of armature
is suitable only for small size machines, being weak mechanically. It is, more over, harder to wind, and carries a much larger proportion of inactive con ductor than in the drum pattern, since only those segments outside the cylinder contribute to the total E.M.F.
The same general principles apply on the whole as to direct-current machines. The manner of connecting contiguous coils on the armature is different, and of course no commutator is required, its place being taken by collecting rings. The absence of the commutator simpli fies matters considerably, and in par ticular enables high voltages to be devel oped, a matter of considerable economic importance in connection with power transmission over long distances. The necessity of having alternating current of minimum periodicity corresponding to about 60 cycles per second for light ing purposes leads to the adoption of multipolar fields, thus avoiding high rotating speeds. It is quite practicable, and indeed advantageous, to reverse the relative position of armature and field magnets, having the former stationary and the latter revolving, and most mod ern alternating machines are built in this way.
Regarding the magnetization of the field magnets, alternators are dependent for this purpose on direct-current ma chines; in some cases these exciters are built on the shaft of the alternator, but it is more convenient to have them independent. Direct - current machines may also be classified as separately ex cited, in cases where the magnetizing coils of the fields derive their current from a source external to the machine.
Another method is to lead the current in the external circuit round the field magnets. Such an arrangement con stitutes a series dynamo, and in such the building up of the field and the in duced E.M.F. depend on the residual magnetism in the field magnets. Series machines are not practicable except when only small deviations from con stant load occur.
A shunt-round machine is one in which the field magets are wound with coils arranged as a high resistance shunt in parallel with the external cir cuit. Machines of this class are much less affected than the series machines, by variations in the external circuit.
By combining both the series and shunt winding on the one machine there is obtained the compound-wound ma chine, which is practically self-regula ting as regards voltage, with all changes in the external load.