and the direction of this force is at right angles to the conductor and
to the field. The rate at which electrical energy is developed, when
this force is overcome by moving the conductor as a dynamo through the
field, is EC = B_gLVC x 10^(-8) watts, whence the equality of the
mechanical power absorbed and the electrical power developed (as
required by the law of the conservation of energy) is easily
established. The whole of this power is not, however, available at the
terminals of the machine; if R_a be the resistance of the armature in
ohms, the passage of the current C_a through the armature conductors
causes a drop of pressure of C_aR_a volts, and a corresponding loss of
energy in the armature at the rate of C_a squaredR_a watts. As the resistance
of the external circuit R_e is lowered, the current C = E_a/(R_e + R_a)
is increased. The increase of the current is, however, accompanied by a
progressive increase in the loss of energy over the armature, and as
this is expended in heating the armature conductors, their temperature
may rise so much as to destroy the insulating materials with which they
are covered. Hence the temperature which the machine may be permitted to
attain in its working is of great importance in determining its output,
the current which forms one factor therein being primarily limited by
the heating which it produces in the armature winding. The lower the
resistance of the armature, the less the rise of its temperature for a
given current flowing through it; and the reason for the almost
universal adoption of copper as the material for the armature conductors
is now seen to lie in its high conductivity.[6]
Since the voltage of the dynamo is the second factor to which its output
is proportional, the conditions which render the induced E.M.F. a
maximum must evidently be reproduced as far as possible in practice, if
the best use is to be made of a given mass of iron and copper. The first
problem, therefore, in the construction of the dynamo is the disposition
of the wires and field in such a manner that the three directions of
field, length of active conductors, and movement are at right angles to
one another, and so that the relative motion is continuous.
Reciprocating motion, such as would be obtained by direct attachment of
the conductors to the piston of a steam-engine, has been successfully
employed only in the special case of an "oscillator,"[7] producing a
small current very rapidly changing in direction. Rotary motion is
therefore universally adopted, and with this two distinct cases arise.
Either (A) the active length of the wire is parallel to the axis of
rotation, or (B) it is at right angles to it.
[Illustration: FIG. 3.]
(A) If a conductor is rotated in the gap between the poles of a
horse-shoe magnet, and these poles have plane parallel faces opposing
one another as in fig. 3, not only is the density of the flux in the
interpolar gap small, but the direction of movement is not always at
right angles to the direction of the lines, which for the most part pass
straight across from one opposing face to the other. When the conductor
is midway between the poles (i.e. either at its highest or lowest
point), it is at this instant sliding along the lines and does not cut
them, so that its E.M.F. is zero. Taking this position as the
starting-point, as the conductor moves round, its rate of line-cutting
increases to a maximum when it has moved through a right angle and is
opposite to the centre of a pole-face (as in fig. 3), from which point
onward the rate decreases to zero when it has moved through 180 deg.. Each
time the conductor crosses a line drawn symmetrically through the gap
between the poles and at right angles to the axis of rotation, the
E.M.F. along its length is reversed in direction, since the motion
relatively to the direction of the field is reversed. If the ends of the
active conductor are electrically connected to two collecting rings
fixed upon, but insulated from, the shaft, two stationary brushes bb can
be pressed on the rings so as to make a sliding contact. An external
circuit can then be connected to the brushes, which will form the
"terminals" of the machine, the periodically reversed or alternating
E.M.F. induced in the active conductor will cause an alternating current
to flow through conductor and external circuit, and the simplest form of
"alternator" is obtained. If the field cut by the straight conductor is
of uniform density, and all the lines pass straight across from one
pole-face to the other (both of which assumptions are approximately
correct), a curve connecting the instantaneous values of the E.M.F. as
ordinates with time or degrees of angular movement as abscissae (as
shown at the foot of fig. 3), will, if the speed of rotation be uniform,
be a sine curve. If, however, the conductor is mounted on an iron
cylinder (fig. 4),[8] a sufficient margin being allowed for mechanical
clearance between it and the poles, not only will the reluctance of the
magnetic circuit be reduced and the total flux and its density in the
air-gap B_g be thereby increased, but the path of the lines will become
nearly radial, except at the "fringe" near the edges of the pole-tips;
hence the relative directions of the movement and of the lines will be
continuously at right angles. The shape of the E.M.F. curve will then be
as shown in fig. 4--flat-topped, with rounded corners rapidly sloping
down to the zero line.
[Illustration: FIG. 4.]
But a single wire cannot thus be made to give more than a few volts, and
while dynamos for voltages from 5 to 10 are required for certain
purposes, the voltages in common use range from 100 to 10,000. It is
therefore necessary to connect a number of such wires in series, so as
to form an "armature winding." If several similar conductors are
arranged along the length of the iron core parallel to the first (fig.
5), the E.M.F.'s generated in the conductors which at any moment are
under the same pole are similarly directed, and are opposite to the
directions of the E.M.F.'s in the conductors under the other pole (cf
fig. 5 where the dotted and crossed ends of the wires indicate E.M.F.'s
directed respectively towards and away from the observer). Two distinct
methods of winding thence arise, the similarity of the E.M.F.'s under
the same pole being taken advantage of in the first, and the opposite
E.M.F.'s under N and S poles in the second.
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
1. The first, or _ring_-winding, was invented by Dr Antonio Pacinotti of
Florence[9] in 1860, and was subsequently and independently reintroduced
in 1870[10] by the Belgian electrician, Zenobe Theophile Gramme, whence
it is also frequently called the "Gramme" winding. By this method the
farther end of conductor 1 (fig. 5) is joined in series to the near end
of conductor 2; this latter lies next to it on the surface of the core
or immediately above it, so that both are simultaneously under the same
pole-piece. For this series connexion to be possible, the armature core
must be a hollow cylinder, supported from the shaft on an open
non-magnetic spider or hub, between the arms of which there is room for
the internal wire completing the loop (fig. 6). The end of one complete
loop or turn embracing one side of the armature core thus forms the
starting-point for another loop, and the process can be continued if
required to form a coil of two or more turns. In the ring armature the
iron core serves the double purpose of conducting the lines across from
one pole to the other, and also of shielding from the magnetic flux the
hollow interior through which the connecting wires pass. Any lines which
leak across the central space are cut by the internal wires, and the
direction of cutting is such that the E.M.F. caused thereby opposes the
E.M.F. due to the active conductors proper on the external surface. If,
however, the section of iron in the core be correctly proportioned, the
number of lines which cross the interior will bear but a small ratio to
those which pass entirely through the iron, and the counter E.M.F. of
the internal wires will become very small; they may then be regarded
simply as connectors for joining the external active wires in series.
[Illustration: FIG. 7.]
2. The second or _drum_ method was used in the original "shuttle-wound"
armatures invented by Dr Werner von Siemens in 1856, and is sometimes
called the "Siemens" winding. The farther end of conductor 1 (fig. 5) is
joined by a connecting wire to the farther end of another conductor 2'
situated nearly diametrically opposite on the other side of the core and
under the opposite pole-piece. The near end of the complete loop or turn
is then brought across the end of the core, and can be used as the
starting-point for another loop beginning with conductor 2, which is
situated by the side of the first conductor. The iron core may now be
solid from the surface to the shaft, since no connecting wires are
brought through the centre, and each loop embraces the entire armature
core (fig. 7). By the formation of two loops in the ring armature and of
the single loop in the drum armature, two active wires are placed in
series; the curves of instantaneous E.M.F. are therefore similar in
shape to that of the single wire (fig. 4), but with their ordinates
raised throughout to double their former height, as shown at the foot of
fig. 6.
Next, if the free ends of either the ring or drum loops, instead of
being connected to two collecting rings, are attached to the two halves
of a split-ring insulated from the shaft (as shown in fig. 7 in
connexion with a drum armature), and the stationary brushes are so set
relatively to the loops that they pass over from the one half of the
split-ring to the other half at the moment when the loops are passing
the centre of the interpolar gap, and so are giving little or no E.M.F.,
each brush will always remain either positive or negative. The current
in the external circuit attached to the brushes will then have a
constant direction, although the E.M.F. in the active wires still
remains alternating; the curve of E.M.F. obtained at the brushes is thus
(as in fig. 7) entirely above the zero line. The first dynamo of H.
Pixii,[11] which immediately followed Faraday's discovery, gave an
alternating current, but in 1832[12] the alternator was converted into a
machine giving a _unidirected current_ by the substitution of a
rudimentary "commutator" in place of mercury collecting cups.
(B) So far the length of the active wires has been parallel to the axis
of rotation, but they may equally well be arranged perpendicularly
thereto. The poles will then have plane faces and the active wires will
be disposed with their length approximately radial to the axis of the
shaft. In order to add their E.M.F.'s in series, two types of winding
may be employed, which are precisely analogous in principle to the ring
and drum windings under arrangement (A).
3. The _discoidal_ or flat-ring armature is equivalent to a ring of
which the radial depth greatly exceeds the length, with the poles
presented to one side of the ring instead of embracing its cylindrical
surface. A similar set of poles is also presented to the opposite side
of the ring, like poles being opposite to one another, so that in effect
each polar surface is divided into two halves, and the groups of lines
from each side bifurcate and pass circumferentially through the armature
core to issue into the adjacent poles of opposite sign.
4. In the _disk_ machine, no iron core is necessary for the armature,
the two opposite poles of unlike sign being brought close together,
leaving but a short path for the lines in the air-gap through which the
active wires are rotated.
If the above elementary dynamos are compared with fig. 1, it will be
found that they all possess a distinctive feature which is not present
in the original disk machine of Faraday. In the four types of machine
above described each active wire in each revolution first cuts the group
of lines forming a field in one direction, and then cuts the same lines
again in the opposite direction relatively to the sense of the lines, so
that along the length of the wire the E.M.F. alternates in direction.
But in the dynamo of fig. 1 the sector of the copper disk which is at
any moment moving through the magnetic field and which forms the single
active element is always cutting the lines in the same manner, so that
the E.M.F. generated along its radial length is continuous and unchanged
in direction. This radical distinction differentiates the two classes of
_heteropolar_ and _homopolar_ dynamos, Faraday's disk machine of fig. 1
being the type of the latter class. In it the active element may be
arranged either parallel or at right angles to the axis of rotation;
but in both cases, in order to increase the E.M.F. by placing two or
more elements in series, it becomes necessary either (1) to employ some
form of sliding contact by which the current may be collected from the
end of one active element and passed round a connecting wire into the
next element without again cutting the field in the reverse direction,
or (2) to form on the armature a loop of which each side is alternately
active and inactive. The first method limits the possibilities of the
homopolar machine so greatly when large currents and high voltages are
required that it is now only used in rare instances, as e.g.
occasionally in dynamos driven by steam-turbines which have a very high
speed of rotation. The second alternative may be carried into effect
with any of the four methods of armature winding, but is practically
confined to the drum and disk types. In its drum form the field is
divided into two or more projecting poles, all of the same sign, with
intervening neutral spaces of equal width, and the span of the loop in
the direction of rotation is at least equal to the width of a polar
projection, as in fig. 8, where two polar projections are shown. Each
side of the loop then plays a dual part; it first cuts the lines of one
polar projection and generates an E.M.F., and next becomes an inactive
connecting wire, while the action is taken up by the opposite side of
the loop which has previously served as a connector but now cuts the
lines of the next polar projection. The E.M.F. is thus always in the
same direction along the side which is at any moment active, but
alternates round the loop as a whole, and the distinctive peculiarity of
the homopolar machine, so soon as any form of "winding" is introduced
into its armature, is lost. It results that the homopolar principle,
which would prima facie appear specially suitable for the generation of
a unidirectional E.M.F. and continuous current, can seldom be used for
this purpose and is practically confined to alternators. It may
therefore be said that in almost all dynamos, whether they supply an
alternating or a continuous current in the external circuit, the E.M.F.
and current in the armature are alternating.
[Illustration: FIG. 8.]
Ring winding was largely employed in early continuous-current dynamos
and also in the alternators of Gramme and H. Wilde, and later of Auguste
de Meritens. Disk winding was also successfully introduced for
alternators, as in the magneto-machines of Nollet (1849) and the
alternators of Wilde (1866) and Siemens (1878), and its use was
continued in the machines of W.M. Mordey and S.Z. Ferranti. But although
the ring, discoidal-ring and disk methods of winding deserve mention
from their historical importance, experience has shown that drum winding
possesses a marked superiority for both electrical and manufacturing
reasons; the three former methods have in fact been practically
discarded in its favour, so that the drum method will hereafter alone be
considered.
The drum coil, composed of several loops wound side by side, may
therefore be regarded as the constituent active element out of which the
armature winding of the modern dynamo is developed. Its application to
the multipolar machine is easily followed from fig. 9, which illustrates
the heteropolar type of dynamo. The span of the loops, which is nearly
180 deg. or across the diameter of the two-pole machine, is reduced
approximately to 90 deg. in the four-pole or to 60 deg. in the six-pole machine
and so on, the curvature of the coil becoming gradually less as the
number of poles is increased. The passage of a coil through two magnetic
fields of opposite direction yields a complete wave of E.M.F., such as
is shown in fig. 6, and the time in seconds taken to pass through such a
complete cycle is the "period" of the alternating E.M.F. The number of
complete periods through which the E.M.F. of the coil passes per second
is called the "periodicity" or "frequency" of the machine. In the
bipolar machine this is equal to the number of revolutions per second,
and in the multipolar machine it is equal to the number of pairs of
fields through which the coil passes in one second; hence in general the
periodicity is pN/60, where N = the number of revolutions per minute and
p = the number of pairs of poles, and this holds true of the E.M.F. and
current round the coil, even though the E.M.F. and current furnished to
the external circuit may be rendered unidirectional or continuous. The
only difference on this point is that in the continuous-current machine
the poles are usually fewer than in the alternator, and the periodicity
is correspondingly lower. Thus in the former case the number of poles
ranges from 2 to 12 and the usual frequencies from 5 to 20; but with
alternators the frequencies in commercial use range from 25 to 120, and
in large machines driven by slow-speed engines the number of poles may
even be as high as 96.
[Illustration: FIG. 9.