28 



SYNCHRONOUS MOTORS 



and induced circuits during the operation of starting, which operation 

 is thereby facilitated. It has, however, the great objection of making 

 the induced E.M.F. vary with the load, whereas, according to theory, 

 the most advantageous E.M.F. is fixed a priori, and should remain 

 practically independent of the load. With series-excitation, strong 

 wattless currents occur at very light and very heavy loads (which may, 

 it is true, be reduced by shunting the field-coils, as shown in Fig. 19), 

 and, besides, there is a lack of stability of operation at light loads 



FIG. 18. 



FIG. 19. 



because the field is then too weak. Morevoer series-excitation causes 

 very bad sparking at the brushes. 



Shunt-excitation has the advantage of being constant and easily 

 regulated; but it has another objection, that of necessitating change 

 of position of the diameter of commutation with respect to the phase 

 of the current to be commutated. With normal load, the position of 

 the brushes can be easily regulated in such a way that the commutation 

 may occur exactly when the alternating current to be commutated is 

 passing through the zero-point; but if the load increases, the lagging 

 of the armature behind the field increases, without being followed by 

 the phase of the current to be commutated. The same thing occurs 

 during any accidental oscillation of the speed of the armature (rotor). 



