December 3, 1891] 



NATURE 



105 



SOME NOTES ON THE FRANKFORT INTER- 

 NA TIONAL ELECTRICAL EXHIBITION> 



VI. 



The Frankfort Motor and the Lauffen Dynamo. 



IN all the motors described in Part V. the magnetism 

 of the stationary iron ring is being rapidly reversed, 

 while that of the moving interior varies at a rate which 

 is equal to only the difference between the speeds of the 

 rotatory magnetic field and the rotating central portion 

 of the motor. This difference is always comparatively 

 small in a well-designed multiphase motor, even when 

 loaded, and becomes practically nought for light loads. 

 Hence we may regard a multiphase motor as roughly one 

 in which the magnetism of the rotatmg iron interior re- 

 mains unchanged relatively to the iron itself, while that of 

 the stationary iron ring varies rapidly. 



certain that the smaller density of the lines of force in the 

 stationary ring do not more than compensate for the in- 

 creased length of their path, a point to which we venture 

 to think Mr. Dobrowolski has not given sufficient atten- 

 tion when coming to this decision to turn the multiphase 

 motor inside out. 



However, be that as it may. Fig. 32 shows the multi- 

 phase motor of 100 horse-power nominal, which the 

 considerations described in this and the previous article 

 led Mr. Dobrowolski to construct for being worked at 

 Frankfort by a portion of the power generated at Lauffen 

 109 miles away. 



To lead the currents coming along the mains to|the 

 rotating interior of the motor, three rubbing contacts must 

 be employed ; indeed, six contacts become necessary if 

 we desire to be able to couple up the coils on the motor 

 in open or closed circuits (Figs. 20 and 24, pp. 56 and 57) 

 — an arrangement provided for in the Frankfort motor 



Fig. 32.— The Frankfort loo j 



-power rotatory current motor. 



Now whenever the magnetism of iron is being rapidly 

 reversed, there is loss of power ; the magnitude of the 

 loss, for a given rate of reversal and quality of iron, de- 

 pending on the intensity of the magnetization and the 

 mass of the iron acted on. And, as the length of the 

 lines of force in the outer stationary iron ring of the 

 multiphase motor is necessarily longer than in the interior 

 rotating portion, Mr. Dobrowolski concluded that less 

 power would be wasted if he inverted the functions of 

 the stationary and rotating parts, sending the multiphase 

 alternate currents round the interior rotating laminated 

 iron drum, and attaching the short-circuited conductors, 

 in which currents are induced by the rotating magnetic 

 field, to the inside of the stationary laminated iron ring, so 

 as to form a kind of short-circuited Siemens armature 

 turned inside out. 



The preceding conclusion is undoubtedly correct if it be 



' Co.ncluded from p. 60. 



NO. I I 53. VOL. 45 J 



(Fig. 32). The great simplicity of the original Ferraris' 

 motor is thus abandoned, but, although this would be 

 disadvantageous in the case of small motors, where sim- 

 plicity and freedom from sparking are all-important, the 

 change is not so serious in a large motor, like that shown 

 in Fig. 32, since high efficiency, and therefore small waste 

 of power, combined with relatively small first cost, arefthe 

 main things tq be aimed at in large motors. 3 . -^T ' 



Another reason why the application of rubbing con- 

 tacts to a large rotatory-field motor is less objectionable 

 than might at first sight appear arises from the fact that, 

 even if the motor were constructed in one of the original 

 forms shown in the preceding article, it would be necessary 

 to employ rubbing contacts for a totally different reason, 

 viz. to introduce resistance, at the moment of starting the 

 motor, into the circuits which carry the currents induced 

 by the rotating magnetic field. Whereas, if these currents 

 be induced in conductors attached to the stationary ring, 



