The "armature reaction" of the alternator, when the term is used in
its widest sense to cover all the effects of the alternating current
in the armature as linked with a magnetic circuit or circuits, may be
divided into three items which are different in their origin and
consequences. In the first place the armature current produces a
self-induced flux in local circuits independent of the main magnetic
circuit, as e.g. linked with the ends of the coils as they project
outwards from the armature core; such lines may be called "secondary
leakage," of which the characteristic feature is that its amount is
independent of the position of the coils relatively to the poles. The
alternations of this flux give rise to an inductive voltage lagging
90 deg. behind the phase of the current, and this leakage or reactance
voltage must be directly counterbalanced electrically by an equal
component in the opposite sense in the voltage from the main field.
The second and third elements are more immediately magnetic and are
entirely dependent upon the position of the coils in relation to the
poles and in relation to the phase of the current which they then
carry. When the side of a drum coil is immediately under the centre of
a pole, its ampere-turns are cross-magnetizing, i.e. produce a
distortion of the main flux, displacing its maximum density to one or
other edge of the pole. When the coil-side is midway between the poles
and the axes of coil and pole coincide, the coil stands exactly
opposite to the pole and embraces the same magnetic circuit as the
field-magnet coils; its turns are therefore directly magnetizing,
either weakening or strengthening the main flux according to the
direction of the current. In intermediate positions the ampere-turns
of the coil gradually pass from cross to direct and vice versa. When
the instantaneous values of either the cross or direct magnetizing
effect are integrated over a period and averaged, due account being
taken of the number of slots per coil-side and of the different phases
of the currents in the polyphase machine, expressions are obtained for
the equivalent cross and direct ampere-turns of the armature as acting
upon a pair of poles. For a given winding and current, the determining
factor in either the one or the other is found to be the relative
phase angle between the axis of a coil in its position when carrying
the maximum current and the centre of a pole, the transverse reaction
being proportional to the cosine of this angle, and the direct
reaction to its sine. If the external circuit is inductive, the
maximum value of the current lags behind the E.M.F. and so behind the
centre of the pole; such a negative angle of lag causes the direct
magnetizing turns to become back turns, directly weakening the main
field and lowering the terminal voltage. Thus, just as in the
continuous-current dynamo, for a given voltage under load the
excitation between the pole-pieces X_p must not only supply the net
excitation required over the air-gaps, armature core and teeth, but
must also balance the back ampere-turns X_b of the armature.
Evidently therefore the characteristic curve connecting armature
current and terminal volts will with a constant exciting current
depend on the nature of the load, whether inductive or non-inductive,
and upon the amount of inductance already possessed by the armature
itself. With an inductive load it will fall more rapidly from its
initial maximum value, or, conversely, if the initial voltage is to be
maintained under an increasing load, the exciting current will have to
be increased more than if the load were non-inductive. In practical
working many disadvantages result from a rapid drop of the terminal
E.M.F. under increasing load, so that between no load and full load
the variation in terminal voltage with constant excitation should not
exceed 15%. Thus the output of an alternator is limited either by its
heating or by its armature reaction, just as is the output of a
continuous-current dynamo; in the case of the alternator, however, the
limit set by armature reaction is not due to any sparking at the
brushes, but to the drop in terminal voltage as the current is
increased, and the consequent difficulty in maintaining a constant
potential on the external circuit.
The coupling of alternators.
The joint operation of several alternators so that their outputs may
be delivered into the same external circuit is sharply distinguished
from the corresponding problem in continuous-current dynamos by the
necessary condition that they must be in synchronism, i.e. not only
must they be so driven that their frequency is the same, but their
E.M.F.'s must be in phase or, as it is also expressed, the machines
must be in step. Although in practice it is impossible to run two
alternators in series unless they are rigidly coupled together--which
virtually reduces them to one machine--two or more machines can be run
in parallel, as was first described by H. Wilde in 1868 and
subsequently redemonstrated by J. Hopkinson and W.G. Adams in 1884.
Their E.M.F.'s should be as nearly as possible in synchronism, but,
as contrasted with series connexion, parallel coupling gives them a
certain power of recovery if they fall out of step, or are not in
exact synchronism when thrown into parallel. In such circumstances a
synchronizing current passes between the two machines, due to the
difference in their instantaneous pressures; and as this current
agrees in phase more nearly with the leading than with the lagging
machine, the former machine does work as a generator on the latter as
a motor. Hence the lagging machine is accelerated and the leading
machine is retarded, until their frequencies and phase are again the
same.
Uses of alternators.
The chief use of the alternator has already been alluded to. Since it
can be employed to produce very high pressures either directly or
through the medium of transformers, it is specially adapted to the
electrical transmission of energy over long distances.[22] In the early
days of electric lighting, the alternate-current system was adopted for
a great number of central stations; the machines, designed to give a
pressure of 2000 volts, supplied transformers which were situated at
considerable distances and spread over large areas, without an undue
amount of copper in the transmitting lines. While there was later a
tendency to return to the continuous current for central stations, owing
to the introduction of better means for economizing the weight of copper
in the mains, the alternating current again came into favour, as
rendering it possible to place the central station in some convenient
site far away from the district which it was to serve. The pioneer
central station in this direction was the Deptford station of the London
Electric Supply Corporation, which furnished current to the heart of
London from a distance of 7 m. In this case, however, the alternators
were single-phase and gave the high pressure of 10,000 volts
immediately, while more recently the tendency has been to employ step-up
transformers and a polyphase system. The advantage of the latter is that
the current, after reaching the distant sub-stations, can be dealt with
by rotary converters, through which it is transformed into a continuous
current. The alternator is also used for welding, smelting in electric
furnaces, and other metallurgical processes where heating effects are
alone required; the large currents needed therein can be produced
without the disadvantage of the commutator, and, if necessary,
transformers can be interposed to lower the voltage and still further
increase the current. The alternating system can thus meet very various
needs, and its great recommendation may be said to lie in the
flexibility with which it can supply electrical energy through
transformers at any potential, or through rotary converters in
continuous-current form.
AUTHORITIES.--For the further study of the dynamo, the following may
be consulted, in addition to the references already given:--
_General_: S.P. Thompson, _Dynamo-Electric
Machinery--Continuous-Current Machines_ (1904), _Alternating-Current
Machinery_ (1905, London); G. Kapp, _Dynamos, Alternators and
Transformers_ (London, 1893); _Id., Electric Transmission of Energy_
(London, 1894); Id., _Dynamo Construction; Electrical and Mechanical_
(London, 1899); H.F. Parshall and H.M. Hobart, _Electric Generators_
(London, 1900); C.C. Hawkins and F. Wallis, _The Dynamo_ (London,
1903); E. Arnold, _Konstruktionstafeln fuer den Dynamobau_ (Stuttgart,
1902); C.P. Steinmetz, _Elements of Electrical Engineering_ (New York,
1901).
_Continuous-Current Dynamos_: J. Fischer-Hinnen, _Continuous-Current
Dynamos_ (London, 1899); E. Arnold, _Die Gleichstrommaschine_ (Berlin,
1902); F. Niethammer, _Berechnung und Konstruktion der
Gleichstrommaschinen und Gleichstrommotoren_ (Stuttgart, 1904).
_Alternators_: D.C. Jackson and J.P. Jackson, _Alternating Currents
and Alternating Current Machinery_ (New York, 1903); J.A. Fleming,
_The Alternate Current Transformer_ (London, 1899); C.P. Steinmetz,
_Alternating Current Phenomena_ (New York, 1900); E. Arnold, _Die
Wechselstromtechnik_ (Berlin, 1904); S.P. Thompson, _Polyphase
Electric Currents_ (London, 1900); A. Stewart, _Modern Polyphase
Machinery_ (London, 1906); M. Oudin, _Standard Polyphase Apparatus and
Systems_ (New York, 1904). (C. C. H.)