GENEB 



GENERAL PRINCIPLES OF SYNCHRONOUS MOTORS 27 



brought by two double brushes bb', bib', which can be shifted to a 

 suitable extent by means of the collar supporting them, the object 

 being to reduce sparking to a minimum. The use of double brushes 



FIG. 15. 



FIG. 16. 



has for its object to reduce the sparking by short-circuiting the field 

 .windings at the time when the induced current is reversing; the extra 

 current which is then produced in the exciting coils prolongs the cur- 

 rent in the field-circuits and diminishes 

 the undulations of the inducing mag- 

 netic flux. Nevertheless, these fluctu- 

 ations are quite perceptible and 

 interfere very much with the efficiency 

 and the stability of the machine. 

 These commutating arrangements 

 can only be used for low voltages. 

 For this reason, when the voltage at 

 the terminals of the motor exceeds no 

 volts, the commutator is supplied by the 

 secondary winding of a transformer 

 which is connected in multiple with the 



mains or else by a special low-voltage circuit wound on the same arma- 

 ture, as in the case of the old Westinghouse alternators (Fig. 17). 



Commutated currents can, theoretically, be used for excitation 

 either in series or in shunt (even compound) as indicated in the dia- 

 gram of Figs. 1 8 and 19. The advantages and disadvantages of 

 these various systems are the following: l 



Series-excitation allows the use of large wires for the field-coils, 

 and insures coicidence in phase between the currents of the inducing 



1 A more detailed study of these " commutated " excitations will be found in 

 an article on synchronous motors by the author, in La Lumiere Electrique, 1892, 

 Vol. Ill, pp. 465, 466, etc. 



FIG. 17. 



