336 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Volts 



increasing field current and for decreasing field current, as shown 

 in Fig. 20. This effect of magnetic hysteresis upon the magnet- 

 ization curve of a dynamo is usually ignored in discussing the 

 ^____ ^^^^^^^^^ relation of the magnetization curve to 



the characteristic curve of the dynamo 

 as generator or as motor. In fact the 

 effect of hysteresis is nearly zero under 

 practical operating conditions, inasmuch 

 as the mechanical vibrations of the 

 dynamo and the slight pulsations of ar- 

 mature and field currents cause the ar- 

 mature flux <E> to settle to the normal 

 value, as explained in Art. 12, Appen- 

 dix A. 



Amperes Is 



Fig. 20. 



CHARACTERISTIC CURVES OF GEN- 

 ERATORS. 



26. Characteristics of the series gener- 

 ator. The characteristic curve of a 

 series generator may be determined experimentally by driving 

 the machine at constant speed and observing corresponding val- 

 ues of current output and terminal voltage for different resist- 

 ances of the receiving circuit. 



External characteristic. Internal characteristic. The curve 

 OA, Fig. 21, showing the relation between terminal volts E x and 

 current output, is often called the external characteristic to distin- 

 guish it from the curve OC, showing the relation between total 

 induced electromotive force E a and armature current. (In the 

 series generator the armature current is of course equal to the 

 current output.) This curve, OC, is called the internal charac- 

 teristic, or the total characteristic. When the external character- 

 istic OA is given, the internal or total characteristic OC may 

 be plotted as follows : 



The difference between E a and E x is equal to the electromo- 

 tive force RI y which is used to overcome the combined resistance, 



