GENERAL PRINCIPLES OF SYNCHRONOUS MOTORS 3 



must produce as many ampere-turns as in the case of large motors. 

 For this reason non-synchronous motors are more convenient for low 

 rotative speeds. 



The author has been able, however, to produce motors of low power 

 (a few hundred watts) which have moving iron and have a very high 

 number of poles (as many as 50 for example), by utilizing induction- 

 type excitation, the magnetic circuit being closed exteriorly, as shown in 

 Fig. 2, in such a way as to allow all the space needed for the exciting 

 coils. 



These coils can then be replaced by permanent magnets, thus 

 producing motors which run without excitation, at speeds sufficiently 



FIG. 3. 



low to be synchronized by hand, and which can render useful service, 

 in certain applications, such as for oscillographs. For this purpose 

 the author preferably employs a small horseshoe magnet that is made 

 to revolve around a stationary armature having a number of poles 

 which is a multiple of 6. It is possible, in this way, to obtain very 

 stable synchronous rotation of a revolving mirror without expending 

 more than i^ to 2 watts. 



Several firms made a specialty of synchronous motors, at an early 

 date, among which we may mention La Societe 1'Eclairage Electrique 

 in France, and the Fort Wayne Company in America. 



One form of motor constructed in France by the Societe 1'Eclairage 

 Electrique (Figs. 3 and 4), is constructed for polyphase currents or 

 for single-phase currents, for powers ranging from i to 130 H.P. The 

 table on page 5 gives the principal data referring to these matters. 



