254 ELECTRICAL MACHINERY 



design of the machine; ordinarily it is about five times the 

 rated full-load current. 



If the armature impedance is known of course the short- 

 circuit current can be approximately calculated. The gen- 

 erated voltage of a certain machine is 122 volts when the 

 machine is excited to give its rated voltage, 110 volts, when 

 carrying the full-load current of 50 amperes. The armature 

 impedance is measured and found to be .40 ohm. Now 

 if the external circuit resistance is reduced to zero (i.e., the 

 machine is short circuited) all of the generated voltage must 

 be used up in overcoming the impedance drop in the arma- 

 ture. So that we may put 



or 



110 = /X.40 

 from which 



7 = 275 amperes. 



The short-circuit current of this machine is, therefore, 

 five and one-half times the full-load, rated current. 



Factors Affecting the Form of the External Characteristic. 

 The factors which act to change the terminal voltage of an 

 alternator as the load is varied are : the armature resistance, 

 the armature inductance, and the armature reaction. In 

 obtaining the external characteristic the field current is 

 maintained constant and hence, if the armature current did 

 not affect the field strength, the generated e.m.f. would 

 remain constant as the load varied. But, as previously 

 shown, the armature reacts to affect the strength of the 

 main field; a lagging current demagnetizing the field and a 

 leading current magnetizing the field. Thus, as a lagging 

 load is increased, the generated e.m.f. of the armature decreases 

 even though the field current and speed are held constant. 

 With a load of leading current, the generator e.m.f. increases 

 with an increase of load. 



