Page:The New International Encyclopædia 1st ed. v. 06.djvu/651

* DYKTAMO-ELECTRIC MACHINERY. 567 DYNAMO-ELECTRIC MACHINERY. sistcii of a copper hollow cylinder closed at one end and hunj; over and inclosing a bar magnet. The evlinder rested upon a vertical pivot at the closed end and the open end dipped into a cup of mercury. hen a current was caused to trav- erse the cylinder lengthwise, the cylinder rotated continuously about the magnet. Kara- day's next machine employed a rotating rec- tangle of wire like that shown in Fig. 3 above, except that no magnet was used such as is shown in the drawing. When the axis of the rectangle •vvas placed east and west and the rectangle was rotated, altenuiting currents could be drawn from slip rings attached to the coil, or, by employing a connnutator. direct current could Ik; obtained. It will be noted that the only magnetic field em- ployed was that of the earth. The iirst machine to give a continuous current was constructed by Sir Charles Wheatstone, an Englishman, in 1841. The armature of his machine was wound with six coils, each of which was connected to the brushes only at the instant when it was at its greatest activity. The use of electromagnets instead of permanent magnets for the field was patented by Wheatstone and Cooke in 1845. A self-excited machine was constructed by Jacob Brett in 1848. The drum-wound armature was introduced ))y Werner Siemens at about the mid- dle of the last century. At about the same time or a little later a type of armature constructed in an entirely different fashion was introduced by Gramme. Instead of winding the wires oh the outside of a cylinder placed between the magnet poles, he wound them upon an iron ring. The Gramme-ring type of armature was at one time extensively used, but it has almost entirely disappeared from use at the present time. De- signs of multipolar machines began to appear about 1880. and it was at about that time also that engineers began to study the principles en- tering into the design of dynamo-electric machines for commercial purposes. Since 1880 the greatest development has been in the direc- tion of improving details of design and of mechanical construction. CoMMERCi.L Types. Turning now to the dif- ferent types of dynamo-electric machines in use, it may be said in general that as they are now built in Europe and in America such machines differ in very few and in non-essential particu- lars. An exception to this statement may, how- ever, be noted in the retention of bipolar de- signs and in the slowness in adapting polyphase machinery in England and also in the use of direct currents at high potentials for power transmission in Italy and France. Another variation from standard forms is presented by some of the machines built by Siemens & Halske, of Berlin. Germany, in which the armature is constructed in the form of a ring rotating out- side of a 'spider' or star-shaped magnet frame. All of these exceptions are gradually disappear- ing from practice and need not be dwelt upon further. Only what may be called the standard types of dynamo-electric machines now in com- mercial use then remain for consideration. As the first example a direct-current series-wound machine such as is used for arc lighting will be described. DiRECT-rfRRrNT MACHINES. In arc lighting the lamp~ are often placed in series (see El.EC- TRir T.ionTiN'r, ). and the current furnished by the dvnamo must at all times be of the same magni- tude, while the pressure or voltage at the ter- minals of the dynamo must vary with (he number of lamps in circuit. Arc-lighting dy- namos are usually series wound. T'he current being always the same, the excitation of the field is also constant. Therefore if the armature rotates with uniform speed the electromotive force is constant. jSow, in order to obtain the variable voltage demanded by the conditions of working, the expedient is resorted to of shifting the col- lecting brushes from the point commonly called the neutral point, at which they so eoiiimutate the armature coils as to place the electromotive forces in scries. When the brushes are shifted in either direction some of the coils act in op- position to other coils, thus counteracting them and reducing the voltage at the brushes. This shifting of the brushes is performed automatic- ally by regulating devices which act immedi- ately when any variation in the current occurs, such as would occur, for example, were some of the lamps cut out of or into the circuit. Various forms of regulating devices are employed, but they consist in almost all cases of an electro- magnet which controls a mechanism that shifts the brushes. For brief descriptions of the regu- lating devices employed in some of the most-used direct-current, constant-current arc-lighting dyn^ mos, consult F. B. Crocker, Electric Light- ing (New York, 1896). As a single illustration the Western electric arc-lighting djmamo will be described. In this machine the brush is con- nected by means of a link and a ball-and-socket joint with a long screw. This screw is held in place by a nut. When the current is normal both the screw and the nut revolve at the same rate, and eonsef|uently there is no end movement of the screw, and the brush remains stationary. An electromagnet energized by a coil, which is in series with the main circuit, attracts an ar- mature whose movement toward the magnet is opposed by the action of a spring which is sus- ceptible of regulation. When the current has too high a value, as it would were a number of the lamps suddenly cut out, the electro-magnet attraets the armature more strongly than or- dinarily. The armature moves toward the mag- net, and by its movement catches a stop in the revolving nut and thereby prevents the revolu- tion of the nut until the resulting longitudinal movement of the screws has shifted the brushes sufliciently to bring the current to its normal value. If the current, on the other hand, be too weak, as it would were a number of lamps added suddenly to the circuit, the spring which is at- tached to the magnet armature overpowers the electro-magnetic attraction. The resulting movement of the armature stops the rotation of the screw and permits the rotation of the nut. This results in a longitudinal movement of the screw and the shifting of the brushes in the op- posite direction. The type of dynamo used for are lighting as just described is a direct-current, constant-cur- rent, series-wound machine. By far the greater number of direct-current generators are con- stant potential machines, and are, if they are not separately excited, either shunt wound or compoimd wound. Generators of this type are used for supplying current for electric .street railways, incandescent lighting, electrolytic work, and other purposes for which a direct cur- rent of comparatively low voltage is desired to