590
DYNAMO [alternators. from the measured throw the instantaneous E.M.F. could be In all large central stations it is necessary that the dynamos The contact-maker was then shifted through a small should be capable of being run in parallel, so that their outputs deduced. instantaneous E.M.F. at the new position cormay be combined on the same “omnibus bars, and thence dis- angle, and the to a difierent moment in the period was shape of tributed to the network or feeders. With simple shunt-wound responding
- this process was repeated until the E.M.F. ^ m p°
machines this is easily effected by coupling together terminals ot measured for a complete period could be traced. Various Cu,fe. like sign when the voltage of the two or more machines are curve of the same principle have since been closely equal. With compound-wound dynamos not only must modifications and recently a form of “oscillograph” has been perfected the external terminals of like sign be coupled together, but the used, is well adapted for the purpose of tracing the curves both of iunctions of the brush leads with the series winding must be con- which and of current (see Measurikg Instruments, Electric). nected by an “equalizing ” lead of low resistance; otherwise, should E.M.F. machine on which Joubert carried out his experiments was a the E.M.F. of one machine for any reason fall below the voltage The Siemens disc alternator having no iron in its armature, and -it was of the’omnibus bars, there is a danger of its polarity being reversed found that the curve of E.M.F. was practically identical with a by a back current from the others with which it is in parallel. curve. The same law has also been found to hold true for a Owing to the necessary presence in the continuous-current sine ring or drum armature, but the presence of the iron dynamo’of the commutator, with its attendant liability to spark- smooth-core core enables the armature current to produce greater distorting ing at the brushes, and further, owing to the difficulty of effect, so that the curves under load may vary considerably from insulating the rotating armature wires, a pressure of 3000 volts their shape at no load. In toothed armatures, the broken surface has seldom been exceeded in any one continuous-current machine, of the core, and the still greater reaction from the armature current, and has been given above as the limiting voltage of the class. If, may produce wide from the sine law, the general tendtherefore, it is required to work with higher pressures in order to ency being to givevariations E.M.F. curve a more peaked form. The secure economy in the transmitting lines, two machines must be great convenience of the the assumption that the E.M.F. obeys the coupled in series by connecting together two terminals which are sine law has led to its being very commonly used as the basis of unlike sign. The stress of the total voltage may still fall on for the mathematical analysis of alternator problems; but any the insulation of the winding from the body of the machine; deductions made from this premiss to be applied with caution hence for high-voltage transmission of power over very long dis- if they are likely to be modified byrequire a different shape of the curve. tances, the continuous-current dynamo must yield in convenience Further, the same alternator will give widely different curves even to the’alternator. In this there is no commutator, the armature E.M.F., and still more so of current, according to the nature of coils may be stationary and can be more thoroughly insulated, of the external circuit to which it is connected. As will be explained while further, if it be thought undesirable to design the machine later, the phase of the current relatively to the E.M.F. depends for the full transmitting voltage, it is easy to wind the armature not only on the inductance of the alternator itself, but also upon for a low pressure; this can be subsequently transformed up to a the inductance capacity of the external circuit, so that the hi<di pressure by means of the alternating-cuirent transformer, same current willand different effects according to the amount which has stationary windings and so high an efficiency that but by which it lagsproduce or leads. The question as to the relative little loss arises from its use. With these remarks, the transition advantages of differently shaped E.M.F. curves has led to much may be made to the fuller discussion of the alternator. 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 lightAlternators. ing, motor driving, or incandescent lighting through transformers. The value of the factor K in the electromotive-force The shape of the E.M.F. curve is, however, of great importance in respect, since upon it depends the ratio of the maximum equation of the alternator (I. b) depends on the ratio of one instantaneous E.M.F. to the effective value, and the insulation of the pole-width to the pitch, and upon the distri- the entire circuit, both external and internal, must be capable of Value of K puti0n of the inductors of the coil; in the latter, withstanding the maximum E.M.F. While the maximum value of 'equation'. two cases may be distinguished as “grouped” the sine curve is ^/2 or 1'414 times the effective value, the maxiand “uniform” distribution. By the first, the mum 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 coil is split up into two or more groups, each.of these more heavily insulated than in the continuous-current dynamo, but groups being itself so closely wound that its inductors also the more peaked the curve, the better must be the insulation. may be regarded as in phase, although the several groups The frequency employed in alternating-current systems which are in series differ in phase.1 By the second, the coil is uniformly wound over a certain portion of the peri- for distributing power and light varies between such wide phery of the armature, as in Fig. 13. Thus the distinction limits as 40 and 135; yet in recent times theFrequencyi of the two cases lies in the fact that with grouped distri- tendency has been towards common standards bution a small number of widely distinct phases are of 50 and 60 or 100 as a maximum. High frequencies compounded together, while with uniform distribution a involve more copper in the magnet coils, owing to the large number of phases differing very little from each other greater number of poles, and a greater loss of power in are united. In either case the result must be the same, their excitation, but the alternator as a whole is somewhat namely, a reduction in the effective E.M.F. as compared lighter, and the transformers are cheaper. On the other with the same winding concentrated in a single line, since hand, high frequency may cause prejudicial effects, due to the several components are more or less out of phase. the inductance and capacity of the distributing lines; and Yet the advantages are gained of a smaller inductance and in asynchronous motors used on polyphase systems the a better ventilation of the armature conductors on the increased number of poles necessary to obtain reasonable surface, and still more at the ends of the core. Further, speeds reduces their efficiency, and is otherwise disadvanthe reduction in the E.M.F. only takes place in its full tageous, especially for small horse-powers. A frequency force at no load; when the armature is giving current its lower than 40 is, however, not permissible where arc lightreaction on the field tends to crowd the lines towards one ing is to form any considerable portion of the work and is or other edge of each pole, and so to narrow the effective to be effected by the alternating current without rectificawidth of the pole-face. In actual machines the value of tion, since below this value the eye can detect the periodic alteration in the light as the carbons alternately cool and K usually falls between the limits of 1 and 1'25.2 The first experimental determination of the shape of the E.M.F. become heated. Thus for combined lighting and power curve of an alternator was made by Joubert in 1880. A revolving 50 or 60 are the most usual frequencies; but if the system contact-maker charged a condenser with the E.M.F. produced by is designed solely or chiefly for the distribution of power, a the armature at a particular instant during each period. The still lower frequency is preferable, and on this account 25 condenser was discharged through a ballistic galvanometer, and was selected by the engineers for the Niagara Falls power 3 1 Such is the case of an armature having slots or holes evenly dis- transmission, after careful consideration of the problem. Each ‘of the four methods of forming coils of many turns, tributed round its periphery, each side of a drum coil or each complete namely, the ring, drum, discoidal ring, and disc, have 2 ring coil being divided between two or more slots. ( Kapp, “Alternate-Current Machinery,” Proc. Inst. G.E. vol. xcvii. part iii 1888-89, and Elihu Thomson’s remarks, p. 101; Parshall, already been shown to be applicable to alternators. Rw J 3 Engineering, vol. Ixv. p. 388; Fischer-Hinnen, Elec. Eng. vol. xx. See Forbes, Journ. Inst. Elec. Eng. vol. xxii. p. 493. p. 597; Brousson, Electrical World, 1895, vol. xxvi. p. 236.
