TESTS OF SYNCHRONOUS MOTORS 



133 



short-circuited on itself and is run at a speed near its normal speed. 

 The short-circuit current will be all the smaller, for a given excitation, 

 the greater the armature-reaction. Since a good stability of operation 

 is only obtained, as already seen, with low armature-reaction, it follows 

 that a good synchronous motor will be characterized by a short-cir- 

 cuit curve which is rapidly ascending such as that shown in Fig. 63, 

 which refers to a synchronous motor with revolving iron of Oerlikon 

 type, described by Kolben (Elektrotechnische Zeitschrift, 19 Dec. 



Amperes 



FIG. 64. Power input for 15 H.P. synchronous motor, run with variable field- 

 excitation. 



1895). A motor is good, according to this author, when a short-cir- 

 cuit-current equal to the normal operating current is obtained by an 

 excitation giving not more than one-third the normal voltage, for 

 the induced E.M.F. Finally, from what precedes, there exists, for 

 synchronous motors, a third kind of important characteristic curve, 

 the V-curves, of which the theory has already been given. These 

 curves are drawn for variable loads applied by a Prony-brake, by measur- 

 ing, for each value of the excitation, the current -values given by an 

 ampere-meter, and the electric-power values given by a watt-meter. 



