134 



ELEMENTS OF ELECTRICAL ENGINEERING. 



(current) is the " total induced electromotive force " required to 

 force the given current through the short-circuited armature, and, 

 since the resistance of the short-circuited armature is negligibly 

 small, it is evident that the only thing which opposes the flow of 

 this short-circuited current is the armature inductance or magnet- 

 izing action, whichever one may wish to call it. The reactance 

 value of this inductance depends of course upon the frequency, 

 and the term synchronous reactance refers to the reactance value 

 of the armature inductance or magnetizing action at the normal 

 speed of the machine. 



Electromotive force method of calculating voltage regulation. 

 This method may be best explained by giving numerical ex- 

 amples. Let it be required to calculate the voltage regulation 

 (full load to zero load) of the 2,ooo-volt, 2,ooo-kilowatt, 3 -phase 

 alternator referred to in connection with Figs. 1 1 7 and 1 1 8 

 (a) for receiving circuits of unity power factor and (b) for receiv- 

 ing circuits of 0.85 power factor, the resistance of one of the 

 armature windings (one phase) being 0.07 ohm. 



(a) For unity power factor. The full-load current in one 

 armature winding is 333 amperes, so that the RI drop in the 

 armature at full load is 23.3 volts. This RI drop is in phase 

 with the current and therefore in phase with the terminal voltage, 



2000 volts 



1150 volts = XI 



^23-3 volts = RI 

 Fig. 119. 



since the power factor of the receiving circuit is assumed to be 

 unity, so that the sum of the RI drop and the terminal voltage 

 is 2023.3 volts. From Fig. 118 we find that 1150 volts corre- 

 spond to 333 amperes, that is to say, the value of XI in Fig. 1 19 



