VOLTAGE REGULATION OF THE ALTERNATOR. 131 



of the total induced electromotive force which is lost in overcoming 

 the electromotive force produced by <' is 90 ahead of the current 

 in phase. This lost electromotive force is, furthermore, proportional 

 to the armature current / so that it may be expressed as XI 

 where X is a reactance exactly like an inductance-reactance. This 

 quantity X is called the " synchronous reactance " of the armature. 

 The general relation between the total induced voltage (due to 

 field flux <) and the terminal voltage of an alternator is shown 

 in Fig. 1 1 6. The line 01 rep- 

 resents the current in one of 

 the armature windings, the line 

 OE represents the total induced 

 electromotive force, the line E x 

 represents the terminal voltage, 0* >/ 



Fig. 116. 



RI represents the electromotive 



force lost in overcoming armature resistance, and the line XI 

 represents the electromotive force which is lost on account of 

 armature magnetizing action. 



64. Calculation of voltage regulation of an alternator. Nearly 

 all alternating current generating stations deliver current at an 

 approximately constant voltage, and the various lamps and motors 

 are connected in parallel across the mains. It would be desirable 

 therefore if an alternating current generator could be made to 

 give an exactly constant voltage, however its current output might 

 vary, and in practice alternators are constructed to give as small 

 a variation of voltage as possible from zero load to full load. 

 Purchasers generally require of a manufacturer a certain guarantee 

 as to the voltage regulation of an alternator. In view of this guar- 

 antee, it is always necessary for the manufacturer to determine 

 the voltage regulation of an alternator by test, so as to be sure 

 that it comes within the guarantee. In the case of a small 

 machine, this test can be made directly by driving the machine, 

 adjusting its field excitation to give normal voltage at full load, 

 and observing the rise of voltage when the load is thrown off, 

 speed and field excitation being unchanged. In the case of large 



