30 



SYNCHRONOUS MOTORS 



winding a continuous-current commutator from which the exciting 

 current is collected by means of two brushes. Such is the system of 

 the Fort Wayne Company in America, and of the Societe 1'Eclairage 



Electrique (Labour system) in France. The 



commutator need be only of small dimensions, 

 proportional to the current which it delivers, 

 unless it is also used in starting the motor. 

 When the voltage of the armature is too 

 high or when it is desirable to simplify the 

 connections of the main circuit, it is suffi- 

 cient to wind, on the armature, a small exciter 

 winding connecting with a small commutator, 

 which serves solely for the purpose of excitation. 

 This method of excitation, which is the 

 simplest of all, unfortunately still presents 

 several objections. It is applicable only 

 to motors having continuous current wind- 

 ings and a rotating armature; it cannot be 



adopted in the case of high voltage machines or alternators having 

 a stationary induced winding; it does not lend itself to automatic 

 regulation of the inducing field; finally, (and this is the most serious 

 objection), when there are wattless demagnetizing currents in the motor, 

 from any cause, the E.M.F. at the brushes and the excitation are both 

 weakened in consequence. Now, that is precisely what occurs when 

 the motor experiences a reduction in speed in consequence of an over- 

 load. The stability of operation, which decreases with the excitation, 

 will, therefore, be diminished when the toad increases. The oscilla- 

 tions due to improper operation are therefore exaggerated by this 

 reaction. 



The self -excitation of synchronous motors must, therefore, be 

 studied with care, and its application is limited to small motors. For 

 larger machines the use of a separate exciter is the most satisfactory 

 plan. 



