DYNAMO-ELECTKIC MACHINERY. 566 DYNAMO-ELECTRIC MACHINERY. Fig. 9. current draamo, by the machine itself. There are two ways in which this self-excitation may be accomplished. If current is taken from the positive brush and led direct- ly through the mag- net-winding and back to the negativebru.sh, as shown by Fig. 8. the magnet is in parallel or shunt w ith the external cir- cuit, and the ma- chine is called shunt- wound. In a shunt- wound machine only part of the current generated passes through the magnet- winding. If, how- ever, all of the current coming from the armature passes through the field-magnet and then goes to the e.xternal circuit, as shown by Fig. 9, the ma- chine is called se- ries wound. If both shimt and se- ries windings are employed, the ma- chine is defined as compoiiml wound. Compound wind- ings are of two classes, defined as long shunt and " short shunt. In the former the cur- rent used in the shunt windings is alsopassed through the series winding along with the main current, as ^^°- ^"^ shown by Fig. 10. In the latter the current from the sliunt coils passes directly back to the arma- ture, avoiding the series turns, as shown by Fig. 11. The field-magnets of alternators are excited by a current from a source exterior to the machine, usually from an auxiliary direct-current dyna- mo, called the ex- citer. The relative arrangement of the field - magnet wind - ing and the exterior circuit in separately excited generators is shown by Fig. 7. The machines shown in these various dia- grams, and in fact all the machines so far mentioned, have had bnt two m.ag- FiQ. 11. netie poles. It is evident, however, that there may be as many pairs of poles as is desired, so long as the armature connections are arranged to correspond. Machines having only one pair of poles are called bipolar machines; machines hav- inK two or more pairs of poles arc called multi- polar. Most machines now built in the United Fig. 12. States, except those of very small size, are multi- polar machines, a typical form being shown in Fig. 12. There are usually as many brushes bearing on the commutator as there are field- poles, but this is not .absolutely nec- essary, since the armature conduct- ors may be so con- nected as to re- quire only two brushes. The lat- ter practice is fol lowed in street- railway motors. Mateiiials. The materials entering into the construc- tion of the various parts of dynamo- [ electric machines are as follows : The magnetic circuit in the field is usually com- posed of cast iron or steel. The pole pieces may be of the same material as the field, or they may be composed of soft iron lamiiur placed edge on to the shaft of the machine. In small and medium- sized machines the base and pillow blocks of the armature shaft arc nearly always east in one piece with the field. The body of the armature upon which the conductors are wound is always built up of iron laminie placed edge on to the shaft. The object of using lainina> here and in the pole pieces is to prevent electric currents from flowing in the iron of the armature itself. Such currents would absorb power and needlessly heat the ariiiature. The conductors themselves in both the field and the armature windings are copper wires or bars carefully insulated from the iron parts and from one another by cotton thread, or cloth, jiapcr, cardboard, mica, etc. The commutator of direct-current m.achines is built up of copper segments separated by strips of mica. The brushes that bear upon the com- mutator in most modern machines take the form of blocks of carbon. In alternating-current machines there is no commut.itor, it being re- placed by cop2)er or brass rings. HiSTomcAi. DEVEi.opjfENT. In the preceding paragraphs the several parts of dynamo electric machines and the functions which Ihcv exercise have been described, and the attempt has been made to elucidate briefly and non-technically the principles at the basis of their operation. An account will now be given of their development, and afterwards the several types of machine in commercial use will be described. The principle of electromagnetic induction, upon whidi the operation of the dynamo-electric ma<hine is based, was discovered by Michael Faraday in 1831. In his first experiments Fara- day produced a current in a coil of wire by starting or stopping a current in a neighboring coil. He then generated currents in a coil by moving it before the poles of a magnet. His first electric motor was a homo-polar machine. A disk of copper was so mounted that it could rotate between the poles of a permanent magnet transversely to the direction of the lines of force. When a current was applied at the axis of the disk and led ofT at the periphery the disk rotated continuously. Faraday constructed a number of machines upon this principle, one of which con-
