DTRECT-CURRENT MACTITICES. In are lighting the lamps are often placed in series (see Tmc LteirrtNe.), and the current furnished by the dynamo must at all times be of the same magni lode, while the pressure or voltage at the ter minals of the dynamo must vary with the number of lamps in circuit. Are-lighting dy namos are usually series wound. The 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. Now, 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 commutate the armature coils us to place the electromotive forces in series. XViien the brushes are shifted in either direction sonic 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 whieh 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 sonic of the most-used direct-current, consta nt-eurrent are-lighting dymt MOS, consult F. B. Crocker, Electric Ligh t iny (New York, I896). As a single illustration the Western electric arc-lighting dynamo 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 consequently there is no end movement of the screw, and the brush remains stationary. An eleetro-magnet 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 electromagnet attracts 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 sufficiently 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 arc lighting as just described is a direct-cur•ent, constant-eur i-ent. series-wound machine. By far the greater number of direct-current generators are eon slant potential machines, and ore, if they are not separately excited. either shunt wound or compound 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 (air rent of comparatively low voltage is desired to he supplied at a constant potential irrespective of the load. In separately excited or shunt wound machines the regulation of the current to constant potential is usually performed by in tiling a rheostat or variable resistance in cir cuit with the field winding. fly increasing or decreasing this resistance. usually by hand, the field current and. therefore, the electro unitive force. are decreased or increased. In compound ound machines the regulation is aecomplished as follows: When there is no load upon the gen orator the excitation of the magnets is produced entirely by the shunt winding, but as the load is increased the current flowing through the series coils aids that in the shunt coils, thus producing stronger fields and increasing the electro motive force at the generator terminals. Small direct current generators. \Outlier operated at con stant eurrent or constant potential. are usually driven by belting from the engine or other prime motor. but large machines. in modern practice. are connected directly to the shaft of the prime motor. Typical direct-current generators are shown on the adjoining pages.