DYNAMO-ELECTRIC MACHINERY. 570 DYNAMO-ELECTRIC MACHINERY. rent generator as well; which will transform direct current into altematinj^, and which will also transform alternating current into direct current. According as tlic machine is used for the first, second, or third of these purposes it is called a double-current generator, an inverted converter, or a rotary converter. As these ma- chines are most commonly employed to trans- form alternating current into direct current, they are commonly termed rotary converters. One of the principal uses of rotary converters is in electric-railway operation, where the distance of transmission is great. For long-distance trans- mission alternating current is desirable, since it may be raised easily to a higher potential, as will be explained below. The current for operat- ing the car-motors is usually, however, direct current, so that when energy is transmitted in the form of alternating currents, rotary- converter substations are placed at intervals along the line to convert the alternating cur- rent to direct current for distribution to the trolley-line. A di/iuiinotor is a transforming device combining both motor and generator ac- tion in one magnetic field by employing two armatures oi' one armiiture having two separate windings. These machines enable one to take direct current from a system of supply at one voltage and deliver it at another voltage to a circuit when it is to be utilized. They are used in electroplating works and for various minor purposes. Motor r/enerators are transforming de- vices consisting of two machines, a motor and generator, mechanically connected together. A booster is a machine inserted in series in a cir- cuit to change its voltage, and may be driven by an electric motor or otherwise. Boosters are used extensively in street-railway systems. AiTKRXATixc - CuRREXT ilACHixES. Dynamo- clectric 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 'three-phase.' which will be frequently >ised 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 winding be added, the phase of which is at right angles to the first, and this winding be also pro- vided with slip rings, it will likewise deliver an alternatin2 current which will be of the same Fio. 15. 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 case of the first winding. The currents in the two windings would, then, be spoken of as quarter-phase or two-phase currents. If the values of the cur- rents are platted as ordinates (verticals) in a diagi-am where the abscissa' (horizontals) repre- sent time, the curves produced will l)e like those shown in Fig. 15, where the distance from i; to b represents the time of one revolution, or two altenialions, 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 circiunference of the armature there were three wires spaced at equal distances, and these wires were connected at a connnoii jimction at one end of the armature and to three separate slip rings at the other end, the currents flowing in the three wires woiild reach their maxinunn at periods one-third of a revolution from each other, and the resulting curve diagram would be Fig. 16. as shown by Fig. 10. To apjily the explanation given above to nuiltipolar madiines. the terms 'coiuplete revolution' and 'circumference of the armature' should be understood to mean time taken by a conductor to pass by two jioles and space upon the armature covered by two poles. Also, while the exi)lanation considers only al- ternating machines which have stationary 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 j)art. Alternating-current motors may be classed as synchronous and as non-synchronous or hnluclion motors. The rotary converter aliove descril)ed represents quite well the first class. Such motors are not in extensive use. on account of the fact that they must he lirought up to full speed be- fore they can be thnjwn 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- synchronous motors an interesting property of ])olyphase or nuiltijihasc current must be de- scribed. In Fig. 7 is shown, diagranimatically, an iron ring provid<'d with two' separate wind- ings, which are sup])lied by alternating currents difFering one-quarter perirtd or revolution in phase. Now if the current be at a maximum in one winding, say the one placed horizontally, and no current 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, s.ay, would become a north pole and the left side a south pole. Again, if we assume equal currents to be flowing in both windings at the same time, the north pole would be in the upper right-hand corner and the south pole in the opposite corner. Now since these two currents vary in magnit>ide and alternate in direction one after the other, the north pole in- duced in the ring will travel around it, making a
