xvi INTRODUCTION 



machines, we will give them the more distinctive name of Commutator 

 Motors. 



Synchronous Motors. A synchronous motor can be denned as 

 being, merely, an alternator used as a motor. The transmission of power 

 between an A.C. generator and an A.C. motor is, therefore, nothing 

 more than a particular case of the coupling of two alternators in syn- 

 chronous operation. Indeed, it is precisely through the study of the 

 features of the coupling of alternators in parallel that the occasion 

 presented itself of noting the phenomenon of the reversibility of alter- 

 nators, that is to say, the possibility of using the same machine both 

 as a motor and a generator, provided that it shall have been previously 

 brought to a speed absolutely equal to that of the generator which 

 supplies it with current. 



We can easily understand the possibility of operating such a motor 

 by comparing it to a motor with commutated current. It is known 

 that if the current of a shuttle armature of the Siemens ("H") type 

 is commutated at each half revolution, the motor-couple is always 

 in the same direction when the machine is supplied by direct current. 

 In an A.C. system, the same result is obtained without a commutator, 

 because the direction of the supply-current changes at each half revolu- 

 tion, and this effect occurs only when the motion of the motor is syn- 

 chronous, that is to say when the armature advances the distance of 

 one pole during one alternation of the supply-current. 



Although this property was noted as early as 1869 by Wilde, 1 

 it passed unnoticed during more than ten years, and it has really been 

 known only since the experiments of J. Hopkinson and Grylls- Adams, 

 at the South Foreland Lighthouse, in 1883. The Memoir of Hopkin- 

 son 2 (in which, without knowing the work of Wilde, he gives the 

 explanation to which reference will be made later), was epoch-making 

 in the history of alternating currents. 



In the South Foreland experiments, the alternators used were three 

 similar de Meritens singlephase alternating current machines, all 

 belt-driven from a common source of power. 



1 Wilde, On a Property of the Magneto-electric Current to Control and Render 

 Synchronous the Rotations of the Armature of a Number of Electromagnetic Induction 

 Machines, Philosophical Magazine, January, 1869. 



- Hopkins* in, On the. Theory of Alternative Currents, Particularly in Reference to 

 Two Alternate Current Machines Connected to the Same Circuit, Journal of the 

 Society of Teh-graph Engineers, 1884, p. 496, Vol. XIII. See also the paper on 

 The Alternate Current Machine as a Motor, by Grylls-Adams, presented at the 

 same meeting. 



