276 METHODS OF CALCULATION 



HA gives, at once, the value of the transverse inductance L'. The 

 same determination can therefore be made over again with increasing 

 values of <f>, by simply increasing the excitation each time. The 

 greater the lag-angle, <j>, the more accurately L' will be determined. 



It may be noted, moreover, (Fig. 250), that if a line (BD) is drawn 

 from B perpendicular to OA, the portion intercepted (OD) corre- 

 sponds to the e.m.f. e obtained with no-load. The difference 

 DA'=Ee therefore gives, in volts, the value of the armature- 

 reaction produced by the reactive component of the output current. 



When the angle at A (Fig. 250) is very small, we have substantially, 



DA=ZJ cosA = 

 Let Z t = V 



If we let La = the direct reaction, then, for a very small variation 

 of the excitation we can write 



DA' = v>LdTd, 

 whence AA' = to(L t La) Id- 



Instead of evaluating in the preceding test the active current I v 

 in order to deduce the reactive current Id, it would be easy, if a 

 measuring motor were at hand, to make this deliver directly to the 

 shaft of the alternator operating as a synchronous motor the necessary 

 power for driving the motor, in such a manner that AB on the V-curve 

 (Fig. 23) becomes nil. But the same result may be obtained yet more 

 easily when a steam alternator unit has to be tested, by admitting 

 to the engine just enough steam to satisfy the losses both of the engine 

 and alternator, so that the alternator only receives a reactive current. 

 With this object, the steam admission may be regulated in such a 

 manner that the alternator, excited so as to give on open circuit the 

 normal e.m.f. U of the system, runs idly in synchronism; then no 

 change is made either in steam admission or in the pressure, and the 

 operations are conducted entirely on the electric side of the alternator, 

 connecting this with the system and varying its excitation so as to 

 develop the V-curve. It is possible to measure in advance the elec- 

 tric power necessary to drive the alternator and its steam engine on 

 open circuit, and thus to deduce the total losses on open circuit. 

 The power wasted may thus be measured by the steam-engine 

 indicator-diagram, which permits of determining the constant of the 

 curve of steam consumption as a function of the power produced 



