November 19, 1891 



NA TURE 



57 



three coils, instead of being wound on ihe ring as indi- 

 cated in Fig. 20, may be wound so as to form a closed 

 circuit, as shown in Fig. 24, With this arrangement of 

 coils it is easy to show that the current 



in I = 

 „ II = 



A + B 



3 ' 

 B -C 



3 ' 



III -C+A 

 " ;;; ^ 



where A, B, and C represent simply the arithmetical 

 values of the currents in the three main leads. And in 

 Fig. 24 the arrows attached to the wires A, B, and C, and 

 to the coils I, II, III, are drawn of such proportional 

 lengths that the above connection between the currents 



Since the three coils both for the open and the closed 

 methods of winding (Figs. 20 and 24) are connected 

 together, and since the current in any one coil varies like 

 the current in the preceding coil, with a lag of 120'', each 

 value of the current may be regarded as travelling round 

 the ring from each coil to the next. This idea has led 

 Mr. Dobrowolski to call such a motor a rotatory current 

 or " drehstrom " motor. 



In joining up a three-phase drehstrom motor, we have 

 to decide whether we shall adopt the arrangement shown 

 in Fig. 20 or that illustrated in Fig. 24. The latter, or 

 closed winding, would be employed when we desired that 

 the maximum potential difference between the terminals 

 of any one of the three coils should be equal to the 

 maximum potential difference between any two of the 

 mains ; while with the open method of winding (Fig. 20) 

 the maximum difference of potential between the terminals 



of any one of the three coils would be only — p, or o'3744 



v3 



Fig. 24. — Three-phase alternate Jcurrent mo<or (closed winding) ; currents differing by 

 120° in phase, and represented in direction and magnitude by the direction and 

 length of the arrows. 



¥ti. 25.— Projections of Oo, 0/3, Oy give direction and 

 relative magnitude of currents in the mains A, B, C 

 of Figs. 20, 24, 27, 29, and 31, and of currents in coils I, 

 If, III of Fig. 20. Projections of Oi, O2, O3 com- 

 pletely repre.sent currents in coils I, II, III of Fig. 24; 

 and projections of Oi, O2', O3 completely rej resent 

 currents in coils I, II, III of Fig. 27. -j 



in the three coils and the currents in the three mains is 

 satisfied. 



Next, to find thedifferencein phase, wedraw three lines.so 

 that their projections on POQ(Fig. 25) represent in direc- 



tionand magnitude 9^^±^\ O^-^S ^^^^^ which 



3 3 3 ' 



is done by taking one-third of the diagonals of each of 

 three parallelograms constructed respectively on Oa and 

 Ofi produced backwa?-ds, on O^ and O7 produced back- 

 ivards, and on Oy and Oa produced backwards. In this 

 way is obtained the three-legged figure, Oi, O2, O3, with 

 three equal sides making angles of 30" with Oa, 0/3, Oy, 

 respectively ; then O I, O 1 1, and 1 1 1, the projections of O i, 

 O2, and O3 on POO, give us the direction and magnitude 

 ofthe three currentsin the coils I, II, III(Fig.24). Hence 

 we see that the current in coil I lags 30° behind the current 

 in A, the current in coil 1 1 30° behind the current in B, and 

 similarly the current in coil 111 30' behind the current 

 inC. 



times the maximum potential difference between any two 

 of the mains. The open method has the further advant- 

 age that the middle point where the single current branches 

 into two (Fig. 20) can be permanently connected with the 

 earth ; so that, while the maximum potential difference 

 between each pair of mains may be, say, 20,000 volts, the 

 potential of no point of the whole system can ever differ 

 from that of the earth by more than 10,000 volts, a result 

 which of course enables the insulation of separate aerial 

 conductors to be more easily carried out. 



The open method of winding has therefore been adopted 

 for the transformers at Lauffen and at Frankfort, as well 

 as for the motor at Frankfort ; but, for the reasons which 

 follow, the actual winding employed is more complex 

 than that indicated in Fig. 20. 



In addition to the defect possessed by the two-phase 

 alternate current motor arising from the variation in the 

 strength of the rotating magnetic field, there is another 

 defect caused by the rotation of the field not proceeding 



NO. 1 151. VOL. 45] 



