NA TURE 



[November 19, 1891 



at a uniform rate, so that the driving force is intermittent. 

 Both these defects, however, can be much lessened by sub- 

 dividing up the coils wound on the iron ring of the motor, 

 a result that can be attained without increasing the num- 

 ber of main wires beyond three by employing the following 

 device. Imagine one half of each of the three coiis of 



FiG. 26. — Transformation of a three-phase alternate current motor (closed 

 ^vinding), with currents differing by 120° in phase, into a six-phase motor, 

 with currents differing by 60° in phase. 



the motor in Fig. 24 to be wound in the opposite direction ; 

 then an arrangement, symbolically indicated in Fig. 26, 

 w(iuld be obtained, where the six halves of the former 

 three coils, I, II, III, are now called I, I,, IL, II, III, and 

 I Ho, as we go round the triangle, Fig. 26. If now, with- 



but as one-half of the coil is wrapped one way round the 

 iron ring, and the other half the other way, the currents, 

 as far as sending a north pole round the ring is concerned, 

 will have diametrically opposite effects — that is, will differ 

 by 180° in phase. Hence, while the currents in the three 

 coils I, II, 1 1 1, in Fig. 24, differed by 120^ in phase, the cur- 

 rents in the six coils I, II, III, Ij, Il2< III2 (Fig. 27) will 

 differ by 60" in phase, so that, as far as the magnetization 

 of the iron ring is concerned, we have arrived at exactly 

 the arrangement of currents shown in Fig. 18. There is, 

 however, this important difference — that, whereas in 

 Fig. 18 six main wires were required, in Fig. 27 only 

 three are needed. 



The difference in phase between the currents. in the six 

 coils (Fig. 27) and the currents in the mains can be at 

 once obtained from Fig. 25. For it is easy to show that 

 the current 



in I=^+B^ 

 3 



., II = ^^-S 



111 



where A, B, and C represent simply the arithmetical values 

 of the currents in the three main leads. Arithmetically, 

 then, for the same currents in the mains A, B, C, the 

 currents in the three coils 1, II, III of Fig. 27 are the 

 same as the currents in the three coils I, II, III of Fig. 

 24. But while, as far as sending north polarity counter- 

 clockwise round the iron ring is concerned, the current 

 in coil II of Fig. 24 was negative, that in coil II of 



-Six-phase alternate current motor (closed winding) ; currents differing by 60° in phase, and reprei 

 the direction and length of the arrows. 



ted in direction and magnitude by 



out separating any of the connections, the coils L and II 

 be made to change places, as well as lU and III, we 

 obtain the arrangement of winding shown on the motor 

 in Fig. 27. 



Now it is to be observed that since the coils I and Ig 

 (Fig. 27) are in series, being in fatt simply parts of the 

 same coil I of Fig. 26, the current in the one must be, of 

 course, exactly the same as the current in the other ; 



NO. I 151, VOL. 45] 



Fig. 27 is positive. Hence, while it was the projection of 

 O2 (Fig. 25) that gave the current in coil II of Fig. 24, 

 it will be the projection of O2' that will completely 

 represent the current in coil II of Fig. 27, &c. 



Hence, in Fig. 27 the current in the coil I will be O I, the 

 projection of Oi ; the current in the coil II will be OH', the 

 projection of O2' ; that in coil HI will be O HI, the pro- 

 jection of O3 ; that in coil Ig will be O I2', the projection of 



