If the magnet cores are of wrought iron or cast steel, and the yoke is
of cast iron, the last term must be divided into two portions
corresponding to the different materials, i.e. into f(B_m)l_m +
f(B_y)l_y. In the ordinary multipolar machine with as many
magnet-coils as there are poles, each coil must furnish half the above
number of ampere-turns.
[Illustration: FIG. 31.]
Magnetic leakage.
Since no substance is impermeable to the passage of magnetic flux, the
only form of magnetic circuit free from leakage is one uniformly wound
with ampere-turns over its whole length. The reduction of the
_magnetic leakage_ to a minimum in any given type is therefore
primarily a question of distributing the winding as far as possible
uniformly upon the circuit, and as the winding must be more or less
concentrated into coils, it resolves itself into the necessity of
introducing as long air-paths as possible between any surfaces which
are at different magnetic potentials. No iron should be brought near
the machine which does not form part of the magnetic circuit proper,
and especially no iron should be brought near the poles, between which
the difference of magnetic potential practically reaches its maximum
value. In default of a machine of the same size or similar type on
which to experiment, the probable direction of the leakage flux must
be assumed from the drawing, and the air surrounding the machine must
be mapped out into areas, between which the permeances are calculated
as closely as possible by means of such approximate formulae as those
devised by Professor G. Forbes.
Excitation of field-magnet.
In the earliest "magneto-electric" machines permanent steel magnets,
either simple or compound, were employed, and for many years these
were retained in certain alternators, some of which are still in use
for arc lighting in lighthouses. But since the field they furnish is
very weak, a great advance was made when they were replaced by soft
iron electromagnets, which could be made to yield a much more intense
flux. As early as 1831 Faraday[18] experimented with electromagnets,
and after 1850 they gradually superseded the permanent magnet. When
the total ampere-turns required to excite the electromagnet have been
determined, it remains to decide how the excitation shall be obtained;
and, according to the method adopted, continuous-current machines may
be divided into four well-defined classes.
[Illustration: FIG. 32.]
The simplest method, and that which was first used, is _separate
excitation_ from some other source of direct current, which may be
either a primary or a secondary battery or another dynamo (fig. 32).
But since the armature yields a continuous current, it was early
suggested (by J. Brett in 1848 and F. Sinsteden in 1851) that this
current might be utilized to increase the flux; combinations of
permanent and electromagnets were therefore next employed, acting
either on the main armature or on separate armatures, until in 1867 Dr
Werner von Siemens and Sir C. Wheatstone almost simultaneously
discovered that the dynamo could be made _self-exciting_ through the
residual magnetism retained in the soft iron cores of the
electromagnet. The former proposed to take the whole of the current
round the magnet coils which were in series with the armature and
external circuit, while the latter proposed to utilize only a portion
derived by a shunt from the main circuit; we thus arrive at the second
and third classes, namely, _series_ and _shunt_ machines. The starting
of the process of excitation in either case is the same; when the
brushes are touching the commutator and the armature is rotated, the
small amount of flux left in the magnet is cut by the wires, and a
very small current begins to flow round the closed circuit; this
increases the flux, which in turn further increases the E.M.F. and
current, until, finally, the cumulative effect stops through the
increasing saturation of the iron cores. Fig. 33, illustrating the
_series_ machine, shows the winding of the exciting coils to be
composed of a few turns of thick wire. Since the current is undivided
throughout the whole circuit, the resistance of both the armature and
field-magnet winding must be low as compared with that of the external
circuit, if the useful power available at the terminals of the machine
is to form a large percentage of the total electrical power--in other
words, if the efficiency is to be high. Fig. 34 shows the third
method, in which the winding of the field-magnets is a _shunt_ or
fine-wire circuit of many turns applied to the terminals of the
machine; in this ease the resistance of the shunt must be high as
compared with that of the external circuit, in order that only a small
proportion of the total energy may be absorbed in the field.
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
Since the whole of the armature current passes round the field-magnet
of the series machine, any alteration in the resistance of the
external circuit will affect the excitation and also the voltage. A
curve connecting together corresponding values of external current and
terminal voltage for a given speed of rotation is known as the
_external-characteristic_ of the machine; in its main features it has
the same appearance as a curve of magnetic flux, but when the current
exceeds a certain amount it begins to bend downwards and the voltage
decreases. The reason for this will be found in the armature reaction
at large loads, which gradually produces a more and more powerful
demagnetizing effect, as the brushes are shifted forwards to avoid
sparking; eventually the back ampere-turns overpower any addition to
the field that would otherwise be due to the increased current flowing
round the magnet. The "external characteristic" for a shunt machine
has an entirely different shape. The field-magnet circuit being
connected in parallel with the external circuit, the exciting current,
if the applied voltage remains the same, is in no way affected by
alterations in the resistance of the latter. As, however, an increase
in the external current causes a greater loss of volts in the armature
and a greater armature reaction, the terminal voltage, which is also
the exciting voltage, is highest at no load and then diminishes. The
fall is at first gradual, but after a certain critical value of the
armature current is reached, the machine is rapidly demagnetized and
loses its voltage entirely.
[Illustration: FIG. 35.]
The last method of excitation, namely, _compound-winding_ (fig. 35),
is a combination of the two preceding, and was first used by S.A.
Varley and by C.F. Brush. If a machine is in the first instance
shunt-wound, and a certain number of series-turns are added, the
latter, since they carry the external current, can be made to
counteract the effect which the increased external current would have
in lowering the voltage of the simple shunt machine. The ampere-turns
of the series winding must be such that they not only balance the
increase of the demagnetizing back ampere-turns on the armature, but
further increase the useful flux, and compensate for the loss of volts
over their own resistance and that of the armature. The machine will
then give for a constant speed a nearly constant voltage at its
terminals, and the curve of the external characteristic becomes a
straight line for all loads within its capacity. Since with most prime
movers an increase of the load is accompanied by a drop in speed, this
effect may also be counteracted; while, lastly, if the series-turns
are still further increased, the voltage may be made to rise with an
increasing load, and the machine is "over-compounded."
Commutation and sparking at the brushes.
At the initial moment when an armature coil is first short-circuited by
the passage of the two sectors forming its ends under the contact
surface of a brush, a certain amount of electromagnetic energy is stored
up in its magnetic field as linked with the ampere-turns of the coil
when carrying its full share of the total armature current. During the
period of short-circuit this quantity of energy has to be dissipated as
the current falls to zero, and has again to be re-stored as the current
is reversed and raised to the same value, but in the opposite direction.
The period of short-circuit as fixed by the widths of the brush and of
the mica insulation between the sectors, and by the peripheral speed of
the commutator is extremely brief, and only lasts on an average from
(1/200)th to (1/1000)th of a second. The problem of sparkless
commutation is therefore primarily a question of our ability to
dissipate and to re-store the required amount of energy with sufficient
rapidity.
An important aid towards the solution of this problem is found in the
effect of the varying contact-resistance between the brush and the
surfaces of the leading and trailing sectors which it covers. As the
commutator moves under the brush, the area of contact which the brush
makes with the leading sector diminishes, and the resistance between the
two rises; conversely, the area of contact between the brush and the
trailing sector increases and the resistance falls. This action tends
automatically to bring the current through each sector into strict
proportionality to the amount of its surface which is covered by the
brush, and so to keep the current-density and the loss of volts over the
contacts uniform and constant. As soon as the current-density in the two
portions of the brush becomes unequal, a greater amount of heat is
developed at the commutator surface, and this in the first place affords
an additional outlet for the dissipation of the stored energy of the
coil, while after reversal of the current it is the accompaniment of a
re-storage of the required energy. This energy, as well as that which is
spent in heating the coil, can in fact, in default of other sources, be
derived through the action of the unequal current-density from the
electrical output of the rest of the armature winding, and so only
indirectly from the prime mover.
In practice, when the normal contact-resistance of the brushes is low
relatively to the resistance of the coil, as is the case with metal
brushes of copper or brass gauze, but little benefit can be obtained
from the action of the varying contact-resistance. It exerts no
appreciable effect until close towards the end of the period of
short-circuit, and then only with such a high-current-density at the
trailing edge of the leaving sector that at the moment of parting the
brush-tip is fused, or its metal volatilized, and sparking has in fact
set in. With such brushes, then, it becomes necessary to call in the aid
of a reversing E.M.F. impressed upon the coil by the magnetic field
through which it is moving. If such a reversing field comes into action
while the current is still unreversed, its E.M.F. is opposed to the
direction of the current, and the coil is therefore driving the armature
forward as in a motor; it thus affords a ready means of rapidly
dissipating part of the initial energy in the form of mechanical work
instead of as heat. After the current has been reversed, the converse
process sets in, and the prime mover directly expends mechanical energy
not only in heating the coil, but also in storing up electromagnetic
energy with a rapidity dependent upon the strength of the reversing
field. The required direction of external field can be obtained in the
dynamo by shifting the brushes forward, so that the short-circuited coil
enters into the fringe of lines issuing from the leading pole-tip, i.e.
by giving the brushes an "angle of lead." An objection to this process
is that the main flux is thereby weakened owing to the belt of back
ampere-turns which arises (_v. supra_). A still greater objection is
that the amount of the angle of lead must be suited to the value of the
load, the corrective power of copper brushes being very small if the
reversing E.M.F. is not closely adjusted in proportion to the armature
current.
On this account metal brushes have been almost entirely superseded by
carbon moulded into hard blocks. With these, owing to their higher
specific contact-resistance, a very considerable reversing effect can be
obtained through the action of unequal current-density, and indeed in
favourable cases complete sparklessness can be obtained throughout the
entire range of load of the machine with a fixed position of the
brushes. Yet if the work which they are called upon to perform exceeds
certain limits, they tend to become overheated with consequent glowing
or sparking at their tips, so that, wherever possible, it is advisable
to reinforce their action by a certain amount of reversing field, the
brushes being set so that its strength is roughly correct for, say, half
load.
In the case of dynamos driven by steam-turbines, sparkless commutation
is especially difficult to obtain owing to the high speed of rotation
and the very short space of time in which the current has to be
reversed. Special "reversing poles" then become necessary; these are
wound with magnetizing coils in series with the main armature current,
so that the strength of field which they yield is roughly proportional
to the current which has to be reversed. These again may be combined
with a "compensating winding" embedded in the pole-faces and carrying
current in the opposite direction to the armature ampere-turns, so as to
neutralize the cross effect of the latter and prevent distortion of the
resultant field.
Heating effects.
From the moment that a dynamo begins to run with excited field, heat
is continuously generated by the passage of the current through the
windings of the field-magnet coils and the armature, as well as by the
action of hysteresis and eddy currents in the armature and
pole-pieces. Whether the source of the heat be in the field-magnet or
in the armature, the mass in which it originates will continue to rise
in temperature until such a difference of temperature is established
between itself and the surrounding air that the rate at which the heat
is carried off by radiation, convection and conduction is equal to the
rate at which it is being generated. Evidently, then, the temperature
which any part of the machine attains after a prolonged run must
depend on the extent and effectiveness of the cooling surface from
which radiation takes place, upon the presence or absence of any
currents of air set up by the rotation of itself or surrounding parts,
and upon the presence of neighbouring masses of metal to carry away
the heat by conduction. In the field-magnet coils the rate at which
heat is being generated is easily determined, since it is equal to the
square of the current passing through them multiplied by their
resistance. Further, the magnet is usually stationary, and only
indirectly affected by draughts of air due to the rotating armature.
Hence for machines of a given type and of similar proportions, it is
not difficult to decide upon some method of reckoning the cooling
surface of the magnet coils S_c, such that the rise of temperature
above that of the surrounding air may be predicted from an equation of
the form t deg. = kW/S_c, where W = the rate in watts at which heat is
generated in the coils, and k is some constant depending upon the
exact method of reckoning their cooling surface. As a general rule the
cooling surface of a field-coil is reckoned as equal to the exposed
outer surface of its wire, the influence of the end flanges being
neglected, or only taken into account in the case of very short
bobbins wound with a considerable depth of wire. In the case of the
rotating armature a similar formula must be constructed, but with the
addition of a factor to allow for the increase in the effectiveness of
any given cooling surface due to the rotation causing convection
currents in the surrounding air. Only experiment can determine the
exact effect of this, and even with a given type of armature it is
dependent on the number of poles, each of which helps to break up the
air-currents, and so to dissipate the heat. For example, in two-pole
machines with drum bar-armatures, if the cooling surface be reckoned
as equal to the cylindrical exterior plus the area of the two ends,
the heating coefficient for a peripheral speed of 1500 ft. per minute
is less than half of that for the same armature when at rest. A
further difficulty still meets the designer in the correct
predetermination of the total loss of watts in an armature before the
machine has been tested. It is made up of three separate items,
namely, the copper loss in the armature winding, the loss by
hysteresis in the iron, and the loss by eddy currents, which again may
be divided into those in the armature bars and end-connexions, and
those in the core and its end-plates. The two latter items are both
dependent upon the speed of the machine; but whereas the hysteresis
loss is proportional to the speed for a given density of flux in the
armature, the eddy current loss is proportional to the square of the
speed, and owing to this difference, the one loss can be separated
from the other by testing an armature at varying speeds. Thus for a
given rise of temperature, the question of the amount of current which
can be taken out of an armature at different speeds depends upon the
proportion which the hysteresis and eddy watts bear to the copper
loss, and the ratio in which the effectiveness of the cooling surface
is altered by the alteration in speed. Experimental data, again, can
alone decide upon the amount of eddy currents that may be expected in
given armatures, and caution is required in applying the results of
one machine to another in which any of the conditions, such as the
number of poles, density in the teeth, proportions of slot depth to
width, &c., are radically altered.
It remains to add, that the rise of temperature which may be permitted
in any part of a dynamo after a prolonged run is very generally placed
at about 70 deg. Fahr. above the surrounding air. Such a limit in ordinary
conditions of working leads to a final temperature of about 170 deg.
Fahr., beyond which the durability of the insulation of the wires is
liable to be injuriously affected. Upon some such basis the output of
a dynamo in continuous working is rated, although for short periods
of, say, two hours the normal full-load current of a large machine may
be exceeded by some 25% without unduly heating the armature.
Uses of continuous current dynamos.
For the electro-deposition of metals or the electrolytic treatment of
ores a continuous current is a necessity; but, apart from such use, the
purposes from which the continuous-current dynamo is well adapted are so
numerous that they cover nearly the whole field of electrical
engineering, with one important exception. To meet these various uses,
the pressures for which the machine is designed are of equally wide
range; for the transmission of power over long distances they may be as
high as 3000 volts, and for electrolytic work as low as five. Each
electrolytic bath, with its leads, requires on an average only some four
or five volts, so that even when several are worked in series the
voltage of the dynamo seldom exceeds 60. On the other hand, the current
is large and may amount to as much as from 1000 to 14,000 amperes,
necessitating the use of two commutators, one at either end of the
armature, in order to collect the current without excessive heating of
the sectors and brushes. The field-magnets are invariably shunt-wound,
in order to avoid reversal of the current through polarization at the
electrodes of the bath. For incandescent lighting by glow lamps, the
requirements of small isolated installations and of central stations for
the distribution of electrical energy over large areas must be
distinguished. For the lighting of a private house or small factory, the
dynamo giving from 5 to 100 kilo-watts of output is commonly wound for a
voltage of 100, and is driven by pulley and belt from a gas, oil or
steam-engine; or, if approaching the higher limit above mentioned, it is
often directly coupled to the crank-shaft of the steam-engine. If used
in conjunction with an accumulator of secondary cells, it is
shunt-wound, and must give the higher voltage necessary to charge the
battery; otherwise it is compound-wound, in order to maintain the
pressure on the lamps constant under all loads within its capacity. The
compound-wound dynamo is likewise the most usual for the lighting of
steamships, and is then directly coupled to its steam-engine; its output
seldom exceeds 100 kilo-watts, at a voltage of 100 or 110. For larger
installations a voltage of 250 is commonly used, while for
central-station work, economy in the distributing mains dictates a
higher voltage, especially in connexion with a three-wire system; the
larger dynamos may then give 500 volts, and be connected directly across
the two outer wires. A pair of smaller machines coupled together, and
each capable of giving 250 volts, are often placed in series across the
system, with their common junction connected to the middle wire; the one
which at any time is on the side carrying the smaller current will act
as a motor and drive the other as a dynamo, so as to balance the system.
The directly-coupled steam dynamo may be said to have practically
displaced the belt- or rope-driven sets which were formerly common in
central stations. The generating units of the central station are
arranged in progressive sizes, rising from, it may be, 250 or 500
horse-power up to 750 or 1000, or in large towns to as much as 5000
horse-power. If for lighting only, they are usually shunt-wound, the
regulation of the voltage, to keep the pressure constant on the
distributing system under the gradual changes of load, being effected by
variable resistances in the shunt circuit of the field-magnets.
Generators used for supplying current to electric tramways are commonly
wound for 500 volts at no load and are over-compounded, so that the
voltage rises to 550 volts at the maximum load, and thus compensates for
the loss of volts over the transmitting lines. For arc lighting it was
formerly usual to employ a class of dynamo which, from the nature of its
construction, was called an "open-coil" machine, and which gave a
unidirectional but pulsating current. Of such machines the Brush and
Thomson-Houston types were very widely used; their E.M.F. ranged from
2000 to 3000 volts for working a large number of arcs in series, and by
means of special regulators their current was maintained constant over a
wide range of voltage. But as their efficiency was low and they could
not be applied to any other purpose, they have been largely superseded
in central stations by closed-coil dynamos or alternators, which can
also be used for incandescent lighting. In cases where the central
station is situated at some distance from the district to which the
electric energy is to be supplied, voltages from 1000 to 2000 are
employed, and these are transformed down at certain distributing centres
by continuous-current transformers (see TRANSFORMERS and ELECTRICITY
SUPPLY). These latter machines are in reality motor-driven dynamos, and
hence are also called _motor-generators_; the armatures of the motor and
dynamo are often wound on the same core, with a commutator at either
end, the one to receive the high-pressure motor current, and the other
to collect the low-pressure current furnished by the dynamo.
In all large central stations it is necessary that the dynamos should
be capable of being run _in parallel_, so that their outputs may be
combined on the same "omnibus bars" and thence distributed to the
network of feeders. With simple shunt-wound machines this is easily
effected by coupling together terminals of like sign when the voltage
of the two or more machines are closely equal. With compound-wound
dynamos not only must the external terminals of like sign be coupled
together, but the junctions of the brush leads with the series winding
must be connected by an "equalizing" lead of low resistance;
otherwise, should the E.M.F. of one machine for any reason fall below
the voltage of the omnibus bars, there is a danger of its polarity
being reversed by a back current from the others with which it is in
parallel.
Owing to the necessary presence in the continuous-current dynamo of
the commutator, with its attendant liability to sparking at the
brushes, and further, owing to the difficulty of insulating the
rotating armature wires, a pressure of 3000 volts has seldom been
exceeded in any one continuous-current machine, and has been given
above as the limiting voltage of the class. If therefore it is
required to work with higher pressures in order to secure economy in
the transmitting lines, two or more machines must be coupled _in
series_ by connecting together terminals which are of unlike sign.[19]
The stress of the total voltage may still fall on the insulation of
the winding from the body of the machine; hence for high-voltage
transmission of power over very long distances, the continuous-current
dynamo in certain points yields in convenience to the alternator. In
this there is no commutator, the armature coils may be stationary and
can be more thoroughly insulated, while further, if it be thought
undesirable to design the machine for the full transmitting voltage,
it is easy to wind the armature for a low pressure; this can be
subsequently transformed up to a high pressure by means of the
alternating-current transformer, which has stationary windings and so
high an efficiency that but little loss arises from its use. With
these remarks, the transition may be made to the fuller discussion of
the alternator.
_Alternators._
Frequency.
The frequency employed in alternating-current systems for distributing
power and light varies between such wide limits as 25 and 133; yet in
recent times the tendency has been towards standard frequencies of 25,
50 and 100 as a maximum. High frequencies involve more copper in the
magnet coils, owing to the greater number of poles, and a greater loss
of power in their excitation, but the alternator as a whole is somewhat
lighter, and the transformers are cheaper. On the other hand, high
frequency may cause prejudicial effects, due to the inductance and
capacity of the distributing lines; and in asynchronous motors used on
polyphase systems the increased number of poles necessary to obtain
reasonable speeds reduces their efficiency, and is otherwise
disadvantageous, especially for small horse-powers. A frequency lower
than 40 is, however, not permissible where arc lighting is to form any
considerable portion of the work and is to be effected by the
alternating current without rectification, since below this value the
eye can detect the periodic alteration in the light as the carbons
alternately cool and become heated. Thus for combined lighting and power
50 or 60 are the most usual frequencies; but if the system is designed
solely or chiefly for the distribution of power, a still lower frequency
is preferable. On this account 25 was selected by the engineers for the
Niagara Falls power transmission, after careful consideration of the
problem, and this frequency has since been widely adopted in similar
cases.
Alternator construction.
The most usual type of heteropolar alternator has an internal rotating
field-magnet system, and an external stationary armature, as in fig. 10.
The coils of the armature, which must for high voltages be heavily
insulated, are then not subjected to the additional stresses due to
centrifugal force; and further, the collecting rings which must be
attached to the rotating portion need only transmit the exciting current
at a low voltage.
[Illustration: FIG. 36.]
The homopolar machine possesses the advantages that only a single
exciting coil is required, whatever the number of polar projections, and
that both the armature and field-magnet coils may be stationary. From
fig. 8 it will be seen that it is not essential that the exciting coil
should revolve with the internal magnet, but it may be supported from
the external stationary armature while still embracing the central part
of the rotor. The E.M.F. is set up in the armature coils through the
periodic variation of the flux through them as the iron projections
sweep past, and these latter may be likened to a number of "keepers,"
which complete the magnetic circuit. From the action of the rotating
iron masses they may also be considered as the inducing elements or
"inductors," and the homopolar machine is thence also known as the
"inductor alternator." If the end of the rotor marked S in fig. 8 is
split up into a number of S polar projections similar to the N poles, a
second set of armature coils may be arranged opposite to them, and we
obtain an inductor alternator with double armature. Or the polar
projections at the two ends may be staggered, and a single armature
winding be passed straight through the armature, as in fig. 36, which
shows at the side the appearance of the revolving inductor with its
crown of polar projections in one ring opposite to the gaps between the
polar projections of the other ring. But in spite of its advantage of
the single stationary exciting coil, the inductor alternator has such a
high degree of leakage, and the effect of armature reaction is so
detrimental in it, that the type has been gradually abandoned, and a
return has been almost universally made to the heteropolar alternator
with internal poles radiating outwards from a circular yoke-ring. The
construction of a typical machine of this class is illustrated in fig.
37.
[Illustration: FIG. 37.]
Since the field-magnet coils rotate, they must be carefully designed to
withstand centrifugal force, and are best composed of flat copper strip
wound on edge with thin insulation between adjacent layers. The coil is
secured by the edges of the pole-shoes which overhang the pole and
tightly compress the coil against the yoke-ring; the only effect from
centrifugal force is then to compress still further the flat turns of
copper against the pole-shoes without deformation. The poles are either
of cast steel of circular or oblong section, bolted to the rim of the
yoke-ring, or are built up of thin laminations of sheet steel. When the
peripheral speed is very high, the yoke-ring will be of cast steel or
may itself be built up of sheet steel laminations, this material being
reliable and easily tested to ensure its sound mechanical strength. If
the armature slots are open, the pole-pieces will in any case be
laminated to reduce the eddy currents set up by the variation of the
flux-density.
Owing to the great number of poles[20] of the alternator when driven by
a reciprocating steam-engine, the diameter of its rotor is usually
larger and its length less than in the continuous-current dynamo of
corresponding output. The support of the armature core when of large
diameter is therefore a more difficult problem, since, apart from any
magnetic strains to which it may be subjected, its own weight tends to
deform it. The segmental core-disks are usually secured to the internal
circumference of a circular cast iron frame; the latter has a box
section of considerable radial depth to give stiffness to it, and the
disks are tightly clamped between internal flanges, one being a fixed
part of the frame and the other loose, with transverse bolts passing
right through from side to side (fig. 37). In order to lessen the weight
of the structure and its expense in material, the cast iron frame has in
some cases been entirely dispensed with, and braced tie-rods have been
used to render the effective iron of the armature core-disks
self-supporting.
[Illustration: FIG. 38.]
Owing to the high speed of the turbo-alternator, its rotor calls for the
utmost care in its design to withstand the effect of centrifugal force
without any shifting of the exciting coils, and to secure a perfect
balance.
The appearance of the armature of a typical three-phase alternator is
illustrated in fig. 38, which shows a portion of the lower half after
removal of the field-magnet.
With open slots the coils, after being wound on formers to the required
shape, are thoroughly impregnated with insulating compound, dried, and
after a further wrapping with several layers of insulating material,
finally pressed into the slots together with a sheet of leatheroid or
flexible micanite. The end-connexions of each group of coils of one
phase project straight out from the slots or are bent upwards
alternately with those of the other phases, so that they may clear one
another (fig. 37). A wooden wedge driven into a groove at the top of
each slot is often used to lock the coil in place. With slots nearly
closed at the top, the coils are formed by hand by threading the wire
through tubes of micanite or specially prepared paper lining the slots;
or with single-turn loops, stout bars of copper of [U]-shape can be
driven through the slots and closed by soldered connexions at the other
end.
Shape of E.M.F. curve.
The first experimental determination of the shape of the E.M.F. curve
of an alternator was made by J. Joubert in 1880. A revolving
contact-maker charged a condenser with the E.M.F. produced by the
armature at a particular instant during each period. The condenser was
discharged through a ballistic galvanometer, and from the measured
throw the instantaneous E.M.F. could be deduced. The contact-maker was
then shifted through a small angle, and the instantaneous E.M.F. at
the new position corresponding to a different moment in the period was
measured; this process was repeated until the E.M.F. curve for a
complete period could be traced. Various modifications of the same
principle have since been used, and a form of "oscillograph" (q.v.)
has been perfected which is well adapted for the purpose of tracing
the curves both of E.M.F. and of current. The machine on which Joubert
carried out his experiments was a Siemens disk alternator having no
iron in its armature, and it was found that the curve of E.M.F. was
practically identical with a sine curve. The same law has also been
found to hold true for a smooth-core ring or drum armature, but the
presence of the iron core enables the armature current to produce
greater distorting effect, so that the curves under load may vary
considerably from their shape at no load. In toothed armatures, the
broken surface of the core, and the still greater reaction from the
armature current, may produce wide variations from the sine law, the
general tendency being to give the E.M.F. curve a more peaked form.
The great convenience of the assumption that the E.M.F. obeys the sine
law has led to its being very commonly used as the basis for the
mathematical analysis of alternator problems; but any deductions made
from this premiss require to be applied with caution if they are
likely to be modified by a different shape of the curve. Further, the
same alternator will give widely different curves even of E.M.F., and
still more so of current, according to the nature of the external
circuit to which it is connected. As will be explained later, the
phase of the current relatively to the E.M.F. depends not only on the
inductance of the alternator itself, but also upon the inductance and
capacity of the external circuit, so that the same current will
produce different effects according to the amount by which it lags or
leads. The question as to the relative advantages of differently
shaped E.M.F. curves has led to much discussion, but can only be
answered by reference to the nature of the work that the alternator
has to do--i.e. whether it be arc lighting, motor driving, or
incandescent lighting through transformers. The shape of the E.M.F.
curve is, however, of great importance in one respect, since upon it
depends the ratio of the maximum instantaneous E.M.F. to the effective
value, and the insulation of the entire circuit, both external and
internal, must be capable of withstanding the maximum E.M.F. While the
maximum value of the sine curve is [root]2 or 1.414 times the
effective value, the maximum value of a [Lambda] curve is 1.732 times
the effective value, so that for the same effective E.M.F. the
armature wires must not only be more heavily insulated than in the
continuous-current dynamo, but also the more peaked the curve the
better must be the insulation.
Excitation.
Since an alternating current cannot be used for exciting the
field-magnet, recourse must be had to some source of a direct current.
This is usually obtained from a small auxiliary continuous-current
dynamo, called an _exciter_, which may be an entirely separate
machine, separately driven and used for exciting several alternators,
or may be driven from the alternator itself; in the latter case the
armature of the exciter is often coupled directly to the rotating
shaft of the alternator, while its field-magnet is attached to the
bed-plate. Although separate excitation is the more usual method, the
alternator can also be made self-exciting if a part or the whole of
the alternating current is "rectified," and thus converted into a
direct current.
Quarter-phase alternators.
The general idea of the polyphase alternator giving two or more
E.M.F.'s of the same frequency, but displaced in phase, has been
already described. The several phases may be entirely independent, and
such was the case with the early polyphase machines of Gramme, who
used four independent circuits, and also in the large two-phase
alternators designed by J.E.H. Gordon in 1883. If the phases are thus
entirely separate, each requires two collector rings and two wires to
its external circuit, i.e. four in all for two-phase and six for
three-phase machines. The only advantage of the polyphase machine as
thus used is that the whole of the surface of the armature core may be
efficiently covered with winding, and the output of the alternator for
a given size be thereby increased. It is, however, also possible so to
interlink the several circuits of the armature that the necessary
number of transmitting lines to the external circuits may be reduced,
and also the weight of copper in them for a given loss in the
transmission.[21] The condition which obviously must be fulfilled,
for such interlinking of the phases to be possible, is that in the
lines which are to meet at any common junction the algebraic sum of
the instantaneous currents, reckoned as positive if away from such
junction and as negative if towards it, must be zero. Thus if the
phases be diagrammatically represented by the relative angular
position of the coils in fig. 39, the current in the coils A and B
differs in phase from the current in the coils C and D by a quarter of
a period or 90 deg.; hence if the two wires b and d be replaced by the
single wire bd, this third wire will serve as a common path for the
currents of the two phases either outwards or on their return. At any
instant the value of the current in the third wire must be the vector
sum of the two currents in the other wires, and if the shape of the
curves of instantaneous E.M.F. and current are identical, and are
assumed to be sinusoidal, the effective value of the current in the
third wire will be the vector sum of the effective values of the
currents in the other wires; in other words, if the system is
balanced, the effective current in the third wire is [root]2, or 1.414
times the current in either of the two outer wires. Since the currents
of the two phases do not reach their maximum values at the same time,
the sectional area of the third wire need not be twice that of the
others; in order to secure maximum efficiency by employing the same
current density in all three wires, it need only be 40% greater than
that of either of the outer wires. The effective voltage between the
external leads may in the same way be calculated by a vector diagram,
and with the above _star connexion_ the voltage between the outer pair
of wires a and c is [root]2, or 1.414 times the voltage between either
of the outer wires and the common wire bd. Next, if the four coils are
joined up into a continuous helix, just as in the winding of a
continuous-current machine, four wires may be attached to equidistant
points at the opposite ends of two diameters at right angles to each
other (fig. 40). Such a method is known as the _mesh connexion_, and
gives a perfectly symmetrical four-phase system of distribution. Four
collecting rings are necessary if the armature rotates, and there is
no saving in copper in the transmitting lines; but the importance of
the arrangement lies in its use in connexion with rotary converters,
in which it is necessary that the winding of the armature should form
a closed circuit. If e = the effective voltage of one phase A, the
voltage between any pair of adjacent lines in the diagram is e, and
between m and o or n and p is e [root]2. The current in any line is
the resultant of the currents in the two phases connected to it, and
its effective value is c [root]2, where c is the current of one phase.
[Illustration: FIG. 39.]
[Illustration: FIG. 40.]
[Illustration: FIG. 41.]
Three-phase alternators.
When we pass to machines giving three phases differing by 120 deg., the
same methods of star and mesh connexion find their analogies. If the
current in coil A (fig. 41) is flowing away from the centre, and has
its maximum value, the currents in coils B and C are flowing towards
the centre, and are each of half the magnitude of the current in A;
the algebraic sum of the currents is therefore zero, and this will
also be the case for all other instants. Hence the three coils can be
united together at the centre, and three external wires are alone
required. In this star or "Y" connexion, if e be the effective voltage
of each phase, or the voltage between any one of the three collecting
rings and the common connexion, the volts between any pair of
transmitting lines will be E = e [root]3 (fig. 41); if the load be
balanced, the effective current C in each of the three lines will be
equal, and the total output in watts will be W = 3Ce = 3CE/[root]3 =
1.732 EC, or 1.732 times the product of the effective voltage between
the lines and the current in any single line. Next, if the three coils
are closed upon themselves in a mesh or _delta_ fashion (fig. 42), the
three transmitting wires may be connected to the junctions of the
coils (by means of collecting rings if the armature rotates). The
voltage E between any pair of wires is evidently that generated by
one phase, and the current in a line wire is the resultant of that in
two adjacent phases; or in a balanced system, if c be the current in
each phase, the current in the line wire beyond a collecting ring is C
= c [root]3, hence the watts are W = 3cE = 3CE/[root]3 = 1.732 EC, as
before. Thus any three-phase winding may be changed over from the star
to the delta connexion, and will then give 1.732 times as much
current, but only 1/1.732 times the voltage, so that the output
remains the same.
[Illustration: FIG. 42.]