ALTERNATOR REGULATION AND OPERATION 141 



This differs from equation (39) only in the sign of IX, which 



i< now negative. 



nple. Repeat the foregoing problem when the power-factor is 

 irmit leading. 



cos = 0.70 IR = 4.37 volts 



sin fi = 0.711 IX m li.l volts 



E' = V(220 X 0.70 + 4.4) 1 + (220 X 0.714 - 19. 1) 1 = 207 volts. Ans. 



The induced emf. in the armature is now less numerically 

 than the terminal voltage. This is a condition which cannot 

 in a direct-current generator. It results from the phase 

 position of the IZ drop with respect to V. 



65. Alternator Regulation. The voltage E', as determined in 

 the preceding paragraph, is the voltage induced in the alternator 

 armature under load conditions. In practice it is a quantity 

 difficult to measure and can be calculated only approximately. 

 There is no simple method of making a direct measurement of 

 the armature reactance X. As a mattertrf fact, it is seldom neces- 

 sary to know either the value of E' or of the armature reactance X. 



A knowledge of the voltage regulation is very important 

 because it shows how closely a machine will maintain its voltage 

 from no load to full load, under the various conditions of load. 



If there were no armature reaction, E' would be the no-load 

 voltage of the machine, just as in a separately excited direct- 

 current generator the induced voltage underload would be equal 

 to the no-load voltage if there were no armature reaction. The 

 of armature reaction is to change the value of the magnetic 

 flux, and this i- accompanied by a corresponding change in the 

 value of the induced emf. The effect of armature reaction on 

 peration of the machine is analyzed in the methods for 

 determining n-gulat ion. 



It i- usually impossible to find the regulation of an alternator 

 by actual load i nil. particularly in the lamer BiieS, until after the 

 machine has been put in BBTVioe, and even then it may he difficult 

 to secure the dr-in-d adjustment of the load. To set up a 



a load test requires ' machine for driving purposes, 

 and considerable power may have to be -upplied and absorbed. 

 With polypha-uu> t lie add<'d dillicult y of ui 



ing a balanced load. 



