LTERNATI - CURRENT SI ACHINES. Dynamo electric machines for generating alternating current have been mentioned several times in the preceding pages. Before proceeding to dis cuss this type of dynamo, it will be desirable to define the terms 'single phase.' 'two-phase.' and 'llirce•phase,' NVI1 iell will be frequently used in this discussion. All the alternators so far mentioned have been single-phase machines. If in Fig. 3 slip rings had been substituted for the commu tator, the armature winding there shown would deliver a current which would reverse twice dur ing, each revolution of the armature. If another be added. the phase of which is at right angles to the first, and this winding lie also pro vided with slip rings, it will likewise deliver an alternating current which will be of the same periodicity as that delivered by the first. It is evident, however. that the current in the second winding will come to its maximum value one quarter period later than in the ease of the first winding. The currents in the two windings would. then. be spoken of as quarter-phase or two-phase currents. If the vallies of the cur rents arc platted as ordinates (vertieals) in a diagram Where the abscissle (horizontals) repre sent time. the curves produced will be like those shown in Fig. 15, where the distance front a to b represents the time of one revolution, or two alternations, and the distance from a to c repre sents one-quarter of a revolution. or the time elapsing between the instant when the current in the first winding, is zero to the instant when the current in the second winding is zero, If about the circumference of the armature there were three wires spaced at equal distanees, and these Wires were connected at a common junction at one end of the armature and to three separate slip rings at the other end. the currents flowing in the three wires would reach their maximum alt periods one-third of a revolution from each other, and the resulting curve diagram would be as shown by Fig. le,. To apply the explanation given above to multipolar machines. the terms 'complete revolution' and 'circumference of the armature' should be understood to mean time taken by a conductor to pass by two poles and space upon the armature covered by two poles. Also, while the explanation considers only al ternating machines which have stathula•y fields and rotating armatures. it is evident that the ro tative mobility of the two may be reversed. In fact, in most large machines directly connected to the prime motor. the field is the rotating part.
Alternating-current motors may be classed as synchronous and as non-synchronous or induction motors. The rotary converter above described represents quite well the first class. Snell motors are not in extensive use. on amount of the fact that they must he brought up to full speed be fore they can be thrown into circuit.. They. of course. run at the same speed as, or keep in step with, the generator which is furnishing the cur rent. In order to explain induction or non synch•onous motors an interesting property of polyphase or multiphase eurrent must be de scribed. In Fig. 7 is shown, diagrammatically, an iron ring provided Om' separate wind ings, which are supplied by alternating eurrents differing one-quarter period or revolution in phase. Now if the current he at a maximum in one winding. say the one placed horizontally, and no eurrent exists in the other winding, the upper side of the ring will become a north pole, say. and the lower side a south pole. Likewise, if a current were flowing only in the vertical winding, the right side, say, would become a north pole and the left side a smith pole. Again, if we assume equal currents to be flowing in both windings at the ranee liner, the north pole would be in the upper right-hand turner and the south pole in the opposite corner. Now since these two
enrr•nts vary in magnitude and alternate in direction one after the other, the north pole in dueed in the ring will travel around it, making a complete revolution during one period. If then an armature were mounted within the ring and this armature were provided With a winding which was short-cir•uited upon itself, it is evi dent that the moving magnetic field would in duce currents in this winding. These currents would in turn react upon the field of force and tend to move nem-. it. that is, the armature would be carried around with the magnetic field and the device would become an induction motor. It will be obvious that three-phase current, could be used as well as two-phase currents. Single phase induction motors are also made in small sizes. but the principle according to wield' they operate- is somewhat more intricate than that governing polyphase motors. Polyphase-indue tion motors are capable of being used for prac tically all the purposes to which series or shunt wound direct-•urrent motors are applied. They are simple in construction. have no moving con tacts. and do not easily get out of order. They have been extensively employed for power dis tribution in mills and factories, and in Europe they are used to propel electric-railway ears. The characteristics of the induction motor vary greatly according to the way in which it is •on structed. It may be built to have great starting torque. and in other ways possess the valuable qualities of the direet-current series-wound mo tor, or it may he so constructed that it will run at nearly constant speed over a wide range of load, thus resembling a shunt-wound dire•t•cur rent motor.
Tim conclusion, it may not be amiss. since al ternating-current apparatus is so extensively in use. to discuss briefly some of its general char neteristics and deviees for consideration. For long-distance power t ransmission a It erna t lug cur rents are used almost exclusively. This is due to the Met that a large alternating current of low potential y he converted into a small cur rent of high potential by a simple device known as a transformer (see TaaNsroammts), and by tluis means a great saving in the copper required in the transmission may be accomplished. At the distant end of the line another trans formation may he used to `step down' nr re convert the current to one of low potential, For ion;.7-distance t ran-mission work three-phase currents are peeediarly adapted. on account of the fact that a three-abase currant of a given potential may he transmitted with only three fourths of the copper required for a single-please, two-phase, or direct current working at the same voltage. See POWER, 'FRAN smissioN or.
Among the many books available to the stu dent of dynamo-eleetric maehinery, the follow ing will be found helpful: For the general reader, Thompson, By»amo-Elect•ie Machinery New York. tti9O) : Sheldon. Dyne! me- Electric Machinery: veil. i.. barrel Curre»1 Machines (New York. 1901); vol. ii.. .1i/craft/big Curre»t Machines (New York. 1902) ; Crocker, Electric Lighting New York, 1896). For the engineer: 1Viener, Prartical Calculation of Dynamo-Elec t•ic Machinrs (New York, 149st ; Jackson, rice( ro-.I/ ay» et ism and the construction of Dynamo and .1./tcrnot ing Current Machinery (New York, 1493) ; Foster, Electrical E»gi»cers Pocket Book (New York, 1901; Steinmetz. Ele ments of Theoretical Electrical Engineering (New York, 1900). For current developments in dyna mo-electric machinery. the engineering papers and the printed matter of the electrical manufactu• ing companies should be consulted.