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through tubes of micanite or specially prepared paper lining the slots; or with singleturn loops, stout bars of copper of Ushape can be driven through the slots and closed by soldered
connexions at the other end.
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 shape of contactmaker charged a condenser with the E.M.F.
B.M.F. produced by the armature at a particular instant during
curse. each period. The condenser was discharged through a
ballistic galvanometer, and from the measured throw the instantaneous E.M.F. could be deduced. The contactmaker 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 smoothcore 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 capacityofthe 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 Al 2 or 1.414 times the effective value, the maximum value of a A 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 thecontinuouscurrent dynamo, but also the more peaked the curve the better must be the insulation.
Since an alternating current cannot be used for exciting the fieldmagnet, recourse must be had to some source of a direct Exalts current. This is usually obtained from a small auxiliary
don.
continuouscurrent 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 fieldmagnet is attached to the bedplate. Although separate excitation is the more usual method, the alternator can also be made selfexciting if a part or the whole of the alternating current is " rectified," and thus converted into a direct current.
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 Quarter already described. The several phases may be entirely
phase independent, and such was the case with the early poly
alter phase machines of Gramme, who used four independent
=tom. circuits, and also in the large twophase 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 twophase and six for threephase 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.' The condition which obviously
' As in the historical transmission of energy from Lauffen to Frankfort (1891).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°; 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
Fin. 39.
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,/ 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 employin 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 sl 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 continuouscurrent 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 fourphase system of distribution. Four collecting rings are necessary if the arma FIG. 40.
ture 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 42. The current in any line is the resultant of the currents in the two phases connected to it, and its effective value is c Alt, where c is the current of one phase.
When we pass to machines giving three phases differing by 120°, 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 11 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// 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
Threephase alternators.
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=cal 3, hence the watts areW=3cE=3CE/ /3 =1.732 EC, as before. Thus any threephase winding may be changed over from the star to the delta connexion, and will then give 1.732 times as much current, but only I/1•732 times the voltage, so that the output remains the same.
The " armature reaction " of the alternator, when the term is
used in its widest sense to cover all the effects of the alternating
Armature current in the armature as linked with a magnetic circuit
or circuits, may be divided into three items which are
reaction
inalfery different in their origin and consequences. In the first
nators. place the armature current produces a selfinduced 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° behind the phase of the current, and this leakage or reactance voltage must be directly counterbalanced electrically by an equal component C in the opposite sense in the voltage from the
main field. The second and third elements are
pendent 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 ampereturns are crossmagnetizing, i.e. produce a distortion of the main flux, displacing its maximum density to one or other edge of the pole. When the coilside 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 fieldmagnet 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 ampereturns 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 coilside and of the different phases of the currents in the polyphase machine, expressions are obtained for the equivalent cross and direct ampereturns 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 continuouscurrent dynamo, for a given voltage under load the excitation between the polepieces X„ must not only supply the net excitation required over the airgaps, armature core and teeth, but must also balance the back ampereturns Xe 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 noninductive, 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 continuouscurrent 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 joint operation of several alternatots so that their outputs may be delivered into the same external circuit is sharply disThe tinguished from the corresponding problem in continuouscoupling current dynamos by the necessary condition that they
of alter must be in synchronism, i.e. not only must they be so
nators. 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.
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 alte Usesrvoz
energy over long distances.' In the early days of nator, electric lighting, the alternatecurrent 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 singlephase and gave the high pressure of Io,000 volts immediately, while more recently the tendency has been to employ stepup transformers and a polyphase system. The advantage of the latter is that the current, after reaching the distant substations, 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 continuouscurrent form.
General: S. P. Thompson, DynamoElectric Machinery—ContinuousCurrent Machines (1904), AlternatingCurrent 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 fur den Dynamobau (Stuttgart, 1902); C. P. Steinmetz, Elements of Electrical Engineering (New York, 1901).
ContinuousCurrent Dynamos: Y. FischerHinnen, ContinuousCurrent Dynamos (London, 1899) ; E. Arnold, Die Gleichstrommaschine (Berlin, 1902) ; F. Niethammer, Berechnung and Konstruktion der Gleichstrommaschinen and 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.)
In the pioneer threephase transmission between Laufen and Frankfort (Electrician, vol. xxvi. p. 637, and xxvii. p. 548), the threephase current was transformed up from about 55 to 8500 volts, the distance being 'To m. A large number of installations driven by water power are now at work, in which energy is transmitted on the alternatingcurrent system over distances of about too m. at pressures ranging from 20,000 to 67 ;000 volts.
782
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