INTRODUCTION XVH 



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These machines could be easily coupled in parallel, as generators, 

 by bringing them to the same speed before coupling them. The belt 

 being then removed from one of them, it was observed that it con- 

 tinued to run synchronously by the action of the current of its neighbors, 

 and that it could even develop a considerable amount of power, as 

 measured by a friction brake, before losing its synchronism. These 

 experiments were repeated a few years later by Mordey, on a much 

 larger scale, with machines of low inductance presenting a much greater 

 stability of operation and driven by independent prime movers. He 

 was thus able to demonstrate the synchronizing power of the alternators 

 on the motors or engines driving them, and even to cause one of the 

 latter, with the power shut off, to be dragged by one of the alternators 

 which it was driving. This gives the key to the principles involved 

 in parallel working. He also showed, later, the possibility of accom- 

 plishing this coupling with machines connected by means of long lines 

 of high resistance. 



Synchronous singlephase motors have two great disadvantages: 

 they are not self-exciting, and they cannot start alone, even without 

 load. Zipernowsky was the first to overcome this difficulty by the 

 expedient of adding a commutator to his motors, which enables them 

 to be started with alternating current, and, after they have attained 

 synchronous speed, to be excited by a portion of the alternating current 

 which they consume. 



These motors, manufactured by the firm of Ganz & Co., had a 

 certain vogue, in consequence of the tests made of them at Frank- 

 fort, in 1899, by a Technical Commission. The efficiency was satis- 

 factory, being 77 per cent for motors of 15 H.P., and 86 per cent for 

 motors of 30 H.P. 1 This system is no longer used at the present time, 

 except for small powers (i to 5 H.P.). 



When the invention of polyphase currents became known, it led 

 naturally to the idea of utilizing them for the transmission of power 

 between two synchronous machines of the same type. Bradley, in 

 America, and Haselwander, in Germany, took out patents, as early 

 as 1887, tne former on a two-phase synchronous motor, and the latter 

 on a three-phase synchronous motor. In both cases the motor was 

 produced by making taps on a Gramme ring and connecting these 

 with insulated rings mounted on the armature-shaft. Non-synchronous 

 motors were only invented in the following year, by Ferraris and by 

 Tesla. 



1 La Lumiere Electrique. Vol. XXXII (1889), P- 328. 



