The fundamental equation of the electromotive force of the dynamo in
its fully developed forms (1 a) (and 1 b) may be compared with its
previous simple statement (1.). The three variable terms still find
their equivalents, but are differently expressed, the density B_g
being replaced by the total flux of one field Z_a, the length L of the
single active wire by the total number of such wires [tau], and the
velocity of movement V by the number of revolutions per second. Even
when the speed is fixed, an endless number of changes may be rung by
altering the relative values of the remaining two factors; and in
successful practice these may be varied between fairly wide limits
without detriment to the working or economy of the machine. While it
may be said that the equation of the E.M.F. was implicitly known from
Faraday's time onwards, the difficulty under which designers laboured
in early days was the problem of choosing the correct relation of Z_a
or [tau] for the required output; this, again, was due chiefly to the
difficulty of predetermining the total flux before the machine was
constructed. The general error lay in employing too weak a field and
too many turns on the armature, and credit must here be given to the
American inventors, E. Weston and T.A. Edison, for their early
appreciation of the superiority in practical working of the drum
armature, with comparatively few active wires rotating in a strong
field.
The armature core.
_Continuous-current Dynamos._--On passing to the separate consideration
of alternators and continuous-current dynamos, the chief constructive
features of the latter will first be taken in greater detail. As already
stated in the continuous-current dynamo the armature is usually the
rotating portion, and the necessity of laminating its core has been
generally described. The thin iron stampings employed to build up the
core take the form of circular washers or "disks," which in small
machines are strung directly on the shaft; in larger multipolar
machines, in which the required radial depth of iron is small relatively
to the diameter, a central cast iron hub supports the disks. Since the
driving force is transmitted through the shaft to the disks, they must
in the former case be securely fixed by keys sunk into the shaft; when a
central hub is employed (fig. 19) it is keyed to the shaft, and its
projecting arms engage in notches stamped on the inner circumference of
the disks, or the latter have dovetailed projections fitting into the
arms. The disks are then tightly compressed and clamped between stout
end-plates so as to form a nearly solid iron cylinder of axial length
slightly exceeding the corresponding dimension of the poles. If the
armature is more than 4 ft. in diameter, the disks become too large to
be conveniently handled in one piece, and are therefore made in
segments, which are built up so as to break joint alternately. Prior to
assemblage, the external circumference of each disk is notched in a
stamping machine with the required number of slots to receive the
armature coils, and the longitudinal grooves thereby formed in the
finished core only require to have their sharp edges smoothed off so
that there may be no risk of injury to the insulation of the coils.
[Illustration: FIG. 19.]
Armature winding.
With open slots either the armature coils may be encased with wrappings
of oiled linen, varnished paper and thin flexible micanite sheeting in
order to insulate them electrically from the iron slots in which they
are afterwards embedded; or the slots may be themselves lined with
moulded troughs of micanite, &c., for the reception of the armature
coils, the latter method being necessary with half-closed slots.
According to the nature of the coils armatures may be divided into the
two classes of coil-wound and bar-wound. In the former class, round
copper wire, double-cotton covered, is employed, and the coils are
either wound by hand directly on to the armature core, or are shaped on
formers prior to being inserted in the armature slots. Hand-winding is
now only employed in very small bipolar machines, the process being
expensive and accompanied by the disadvantage that if one section
requires to be repaired, the whole armature usually has to be dismantled
and re-wound. Former-wound coils are, on the other hand, economical in
labour, perfectly symmetrical and interchangeable, and can be thoroughly
insulated before they are placed in the slots. The shapers employed in
the forming process are very various, but are usually arranged to give
to the finished coil a lozenge shape, the two straight active sides
which fit into the straight slots being joined by V-shaped ends; at each
apex of the coil the wire is given a twist, so that the two sides fall
into different levels, an upper and a lower, corresponding to the two
layers which the coil-sides form on the finished armature. Rectangular
wire of comparatively small section may be similarly treated, and if
only one loop is required per section, wide and thin strip can be bent
into a complete loop, so that the only soldered joints are those at the
commutator end where the loops are interconnected. But finally with
massive rectangular conductors, the transition must be made to
bar-winding, in which each bar is a half-loop, insulated by being taped
after it has been bent to the required shape; the separate bars are
arranged on the armature in two layers, and their ends are soldered
together subsequently to form loops. As a general rule, whether bars or
former-wound coils are employed, the armature is barrel-wound, i.e. the
end-connexions project outwards from the slots with but little change of
level, so that they form a cylindrical mass supported on projections
from the end-plates of the core (fig. 19); but, in certain cases, the
end-connexions are bent downwards at right angles to the shaft, and they
may then consist of separate strips of copper bent to a so-called
butterfly or evolute shape.
After the coils or loops have been assembled in the slots on the
armature core, and the commutator has been fixed in place on the shaft,
the soldering of the ends of the coils proceeds, by which at once the
union of the end of one coil with the beginning of the next, and also
their connexion to the commutator sectors, is effected, and in this lies
the essential part of armature winding.
[Illustration: FIG. 20.