SYNCHRONOUS MACHINERY 267 



the remainder P is the loss in the armature due to the current 

 I 8C . The effective resistance of the armature is 



r=Y^. . . (256) 



It is greater than the true ohmic resistance as measured with 

 direct current since some extra losses called load losses occur in the 

 copper due to eddy currents set up by the unequal distribution 

 of flux throughout the volume of the conductor. 



The values of the synchronous reactance can be obtained by 

 separating the resistance from the synchronous impedance accord- 

 ing to the equation 



Xo = \/z 2 - r 2 . (257) 



The resistance r is, however, very small in comparison to the 

 synchronous reactance XQ and the rrdinates of curve (3), Fig. 244, 

 may be taken to represent either XQ or z . For purposes of calcu- 

 lation an average value of XQ is taken and it is assumed to remain 

 constant. 



Fig. 245 shows the results of a test on a 2500-kv.a., 5000-volt, 

 three-phase alternator. 



158. Voltage Characteristics. The relation between the ter- 

 minal e.m.f. and armature current of an alternator, with a fixed 

 value of field current and a given load power factor, is called 

 the " regulation curve" or " voltage characteristic" for the given 

 power factor. 



When the power factor is unity and the current is in phase with 

 the terminal e.m.f. E, it lags by an angle < (Fig. 240) behind the 

 no-load e.m.f. E and there is thus a small demagnetizing effect 

 proportional to 7 sin < , which decreases the flux and a large cross- 

 magnetizing effect proportional to I cos $ which changes the 

 distribution of the flux but only decreases it slightly due to satura- 

 tion. Thus even with non-inductive load the armature reaction 

 causes a decrease in the flux crossing the air gap, and the e.m.f. EI 

 generated by rotation is less than the no-load e.m.f. EQ. In 

 addition, the armature reactance x and the resistance r both con- 

 sume components of e.m.f. proportional to the current. 



Therefore, at non-inductive load the terminal e.m.f. falls with 

 increasing current as shown in curve (1), Fig. 246, which is the 

 voltage characteristic for unity power factor. 



