54 



NA TURE 



[November 19, 1891 



SOME NOTES ON THE FRANKFORT INTER- 

 NATIONAL ELECTRICAL EXHIBITION} 



V. 



The Evolutio7t of the Mtiltifhase Alternate Ctirreiit 

 Motor. 



THE two-phase alternate current motor described in 

 Part IV. has the disadvantage that, not merely does 

 the magnetic field rotate, but it also varies in strength : 

 this causes the driving force to fluctuate, and diminishes 

 the power that the motor would otherwise give out. That 

 there is this variation in strength of the magnetic field 



[F1G.J12 (repeated). — Rotating magnetic field produced by two alternating 

 currents, differing by 90° m phase. 



may be seen from Fig. 17, where the continuous curves 

 I and II show at any moment the strengths of the 

 currents in the coils I and II, Fig. 12 (repeated for 

 reference from the last article) ; while the dotted curves 

 lo, II2, in Fig. 17, give the values at any moment of the 

 currents in the coils L,, IL, in Fig. 12. For example, 

 when the time equals a (Fig. 17), the currents flowing in 



all the four coils will always help one another to mag- 

 netize the iron ring, hence the magnetizing force at any 

 moment will be appro.ximately proportional to the number 

 of convolutions in one of the coils multiplied by the arith- 

 metical iwm of the ordinates of all the four coils I, II, 

 lo, and II2, that is, multiplied by twice the ordinate of the 

 upper or summation curve. 



If the maximum ordinate of either of the curves I or 

 I2 be called H, the ordinate of the upper or summation 

 curve is equal to H when the time is a, c, or ^, correspond- 

 ing with the illustrations marked a^ c, and e in Fig. 12 ; 



whereas the ordinate of this summation curve is - H, 



\'2 



or I'4I4H, when the time is b, d, or f, corresponding with 

 the illustrations marked b, d, and /in Fig. 12. 



Hence, if K be the number of convolutions in one of 

 the coils, the sum of the products of the current into 

 the number of convolutions, or the number of ampere- 

 tterns, as it is called, will vary between two values pro- 

 portional to HK and )*4I4 HK — that is, will vary by 41 "4 

 per cent. The variation in the magnetism produced by 

 such a change in the number of ampere-turns will be 

 less than 41-4 per cent., and much less if the magnetic 

 induction be considerable ; still, the fluctuation in the 

 strength of the rotating field may be greater than is 

 desirable. 



If in place of the two pairs of coils (Fig. 12) there be 

 three, I, Ij, H, II2, HI, lHa, as in Fig. 18, and if the 

 alternating currents passing through these three circuits 

 be of the same maximum altitude and periodic time, but 

 differ by 60° in phase, the variation in the number of 

 ampere-turns will be much less than if only two alter- 

 nating currents be employed. For on examining the sum 

 of the ordinates of the three continuous curves, I, II, 

 III (Fig. 19), which are the curves of three such 

 currents, we see that the sum, or the ordinate or the top 

 curve, varies between 2H sin 60", when the time equals /, 

 and H -f 2H sin 30°, when the time equals t . Hence this 

 sum varies between V3H and 2H, corresponding with a 

 change of only 14 per cent, in the magnetizing force, and 

 with less than 14 per cent, in the magnetism produced. 



Such a system, however, would require six wires, 

 whereas the same reduced maximum variation of the 

 number of ampere-turns can be practically attained by 

 employing only three wires conveying three alternate 



Fig. 17. — Two harmonic alternating currents of the same period and maximum amplitude, but difteriiig by 90° in phase. 



the coils I and II have respectively their maximum 

 value and nought ; whereas when the time equals b, the 

 currents flowing in each of these coils is the same, being 

 equal to 2 x sin 45*^ into the maximum value. 



Now if each of the four coils occupies only a small 

 portion of the ring, as shown in Fig. 12, the currents in 



' Continued from vol. xliv. p. 619. 



NO. II5T, VOL. 45] 



currents differing by 120" in phase, and by joining up the 

 motor as shown in Fig. 20. 



That it is possible to use only three wires, so that either 

 wire always acts as the return wire for the currents in 

 the other 'two, arises fiom the fact that the algebraical 

 sum of three harmonic alternate curi-ents of the same 

 period and maximum amplitude but differing by 120° in 



