INTRODUCTION 



Classification of Singlephase and Polyphase A.C. Motors. An 



alternating current motor comprises, like any dynamo-electric machine, 

 an inducing magnetic field and its induced circuit, the one turning with 

 respect to the other. But, while the field of B.C. motors is always 

 constant, that of A.C. motors may be either constant, alternating, or 

 revolving, according to whether it be produced by a direct current, 

 an alternating current, or a system of polyphase alternating currents 

 serving to excite windings suitably interlaced. Therefore, as was 

 proposed quite logically, in 1891, by E. Hospitalier, 1 A.C. singlephase 

 and polyphase motors can be classified according to the nature of their 

 magnetic fields, into three classes: 



(1) Constant field motors. 



(2) Alternating field motors. 



(3) Revolving field motors. 



The first class constitutes the subject-matter of the present book, 

 the second and third classes being reserved for another volume. As 

 will be seen, these three classes each contain singlephase and poly- 

 phase motors. 



Constant field motors can also be characterized by the fact that the 

 armature-rotation can be maintained at a single speed only, which is 

 synchronous with the alternations of the currents employed. We 

 will therefore call them, more frequently, in accordance with common 

 custom, synchronous motors, in contradistinction to the two other 

 classes, which may be characterized as asynchronous or non-synchronous. 

 For the sake of greater precision, we will apply the latter qualification 

 to motors of the last class only; and since alternating-field motors 

 (with one single exception, which is of little importance) are char- 

 acterized by the use of a commutator similar to that used with D.C. 



1 Bulletin de la Societe francaise de physique, 17. Juillet, 1891. 



