io6 



NATURE 



[December 3, 1891 



such a resistance can be introduced without extra rubbing 

 contacts; and it is for the purpose of introducing this 

 resistance into these stationary circuits that the three I 

 wires, traihng on the ground in Fig. 32, are seen attached 

 to the conductors attached inside the stationary part of ' 

 the motor. ^ 



The necessity, at starting the motor, for increasing the 

 resistance of the conductors carrying the induced currents | 

 will appear from the following consideration. When the : 

 motor (Fig. 32) is running at full speed under a light I 

 load, the interior part rotates at such a rate — relatively to , 

 the frequency of alternation of the currents in the main \ 

 wires — that the magnetic field is practically stationary, I 

 just as it is in an ordinary direct current motor. But at ; 



into the circuits of the stationary conductors of his large 

 motors while the motor is getting up speed. 



We have hitherto spoken of the conductors on the 

 rotating part as being wound on the outside of a laminated 

 iron drum, and those on the stationary part as being 

 wound on the inner surface of a laminated iron ring ; but, 

 as a matter of fact, in the large Frankfort motor both sets 

 are composed of copper rods, insulated in asbestos tubes, 

 and slipped into holes punched out of the iron close to the 

 periphery. This burying of the copper bars to a small 

 depth inside the iron has been adopted because it has 

 been found that the generation of Foucault currents in 

 the thick bars can in this way be more effectively prevented 

 than by following the method usually adopted with bar 



.—The Laiiffen three-fhase alternate current dynamo and exciter. 



the start, the interior laminated iron drum is only moving 

 slowly, while the currents flowing in the conductors at- 

 tached to it are alternating rapidly, hence the magnetic 

 field is rotating rapidly, and powerful currents are induced 

 in the stationary conductors, so powerful, in fact, as to 

 produce a magnetic field which seriously distorts that 

 produced by the main alternating currents. In fact, there 

 is the same antagonism of magnetic fields that occurs 

 with a direct current motor, if the armature field be very 

 powerful in comparison with that of the field magnet, 

 and if the lead of the brushes be adjusted so as to cause 

 the fields to oppose one another ; and it is to avoid this 

 result that M. Dobrowolski introduces a liquid resistance 



NO. I I 53, VOL. 45] 



armatures, which consists in moulding each conductor out 



of stranded copper wire with the various wires composing 



the strand partially insulated from one another. 



' No tests have yet been published of the power and 



j efficiency of this machine, but the smoothness with which 



I it ran, pumping up water for the artificial waterfall in the 



j Frankfort Exhibition, and the absence of the roar audible 



! with some alternate current machines, and even of the 



rhythmical hum noticeable with the best alternate current 



'.. motors, were very striking. 



In the last article it was proved that if three harmonic 

 alternating currents of the same periodic time and maxi- 

 mum amplitude, but differing by 120° in phase, flowed in 



