5. Separate-Circuit, Dynamo.—Another type of self-exciting machine is one so arranged that a set of coils, either wound on the same core as the main armature or con stituting a separate armature, but rotating in the same field, feed the exciting field-magnets. This method has been applied for exciting alternate-current machines, and more recently by Thomson and Lahnieyer for motor-dynamo distribution. It has also been proposed by Edi son for low-tension electric railways, with the rails as conductors.
6. Combination Methods of Pield Excitati-on.—Besides the above simple methods of field excitation, a variety of plans have been invented for securing absolute regulation without ex ternal means. These methods consist in various combinations of the series—shunt, separate, and magneto methods. Among these is the series and shunt, or -com pound" dynamo. In this method. patented by Brush in this country, and shown in Fig. 21, the field-magnets have both a series and a shunt winding. The acticn produced by this combination is to keep the field magnets of constant strength at all external loads. In the plain shunt machine the current in the shunt diminishes as the current in the ex ternal circuit increases, or, to put it in another way, as the resistance of the external circuit decreases. By adding the series winding, the cur rent passing around the liehl-magnets is increased to the same amount as that in the shunt decreases; hence, the magnetism remains constant. There arc two ways of connecting the shunt to the series-eirenit, the one just described and the series and long shunt. Thzt latter has, how ever, not been put into practice. The compound-machine is in extensive use for ineandesct-Lt-lighting, especially on shipboard. and is specially adapted for maintaming constant potential. Various combinations have also been designed to obtain constant current automatically, among them the shunt and separate, invented by 1)eprez; the shunt and mag neto, invented by Perry; and the shunt and series, by S. I'. Thompson. Although theoretically possible. the methods of compounding for con stant current are not as a rule carried out in practice, the methods of regulation employed being applied by external regulators, the principal ones being described below.
7. Other Methods of Regulation.—The method of regulation most generally employed in series (are-light, constant-current) machines consists in shifting the brushes so as to reduce or increase the potential in proportion to the resistance of the internal circuit. At the posi tion of maximum load the brushes make contact at or very close to the "neutral point," but as the load decreases the brushes are shifted away so that they approach nearer and nearer a posi tion of right angles to the first position. This method of regulation is carried out in the Thom son-flonston, Wood, llochhausen, Maxim, and Western Electric Co.'s, and a number of other are-machines. (For details of operation, see the description of these machines given below.)
The automatic regulator of Brush employs a variable shunt resistance connected to the terminals of the field-magnet (Fig. 22). the resistance of the shunt being controlled by an eleetro-magnet placed in the main circuit. The dynamo at D pours its current into the cir cuit, leaving the commutator by the upper brush, whence it flows through the field-magnets 1' and round the circuit of lamps L L, back to the negative terminals. Suppose, now. some of the lamps to be extinguished by switches which short-circuit them ; the resistance of the circuit being thus diminished, there will be at once a tendency for the current to increase above its normal value, unless the electromotive force of the dynamo is at once correspond ingly reduced. This is done by the solenoid B in the circuit. When traversed by the normal current, it attracts its armature A with a certain force just sufficient to keep it in its neutral position. If the current increases, the armature is drawn upward, and causes a lever to com press a column of retort carbon-plates C, which is connected as a shunt to the field-magnet.
These plates when pressed together miniuer well, but when the pressure is diminished their imperfect contact partially interrupts the shunt circuit and increases its resistance. When A rises and compresses C, the current is diverted to a greater or less extent from the field-mag nets, which arc thus under control.
For regulating the potential of constant potential (incandescent-light) shtint-machines, Edison first employed a variable resistance placed in series with the field-magnet coils. The arrangement is shown in Fig. 23. As the potential increases, resistances are thrown by mov ing- the handle of the rheostat le, which diminishes the current in the field-magnet coils, and hence their magnetic power, and thus reduces the potential of the machine to its normal value. On a decrease of potential, due to increased load, the rheostat resistance is reduced, which reverses the action just stated. The operation of the rheostat has also been carried out automatically in various ways. Besides the methods just enumerated, others have been em ployed. In Lane-Pox's regulator, a high-resistance relay is connected as a shunt to the mains, and actuates the rheostat as described above.. Regulation can also be effected by winding the field-ma.g,mets in sections, and cutting these sections in, or out, in proportion to the load. This method has been employed by Deprez, Brush, Ilochhausen, Van Depoele, and others. Still another method consists in Id icing a magnetic shunt across the field-magnets, awd thus diverting the lines of force from the armature as the load decreases. This has been carried out by Goolden and Trotter in their constant-current machine (see below).