204 ALTERNATORS. 



Approximate Determination of Armature Self-induction. The 



experiment just described suggests a simple approximate 

 method for determining the armature self-induction of an 

 alternator. 



It has been stated that when an alternator is connected 

 to a non-inductive circuit, the drop in voltage at its terminals 

 is principally due to the voltage required to overcome the 

 impedance of the armature. If it is assumed that the 

 difference between tke terminal voltage on load and on 

 open circuit is wholly due to the armature impedance, it is 

 easy to calculate the value of the self-induction. To make 

 the measurement, run the alternator at normal speed and 

 excitation and note the voltage on open circuit. Close the 

 circuit switch, so that the machine supplies current to a 

 non-inductive load, and read the current and voltage. The 

 second voltage is equal to C x E, when 

 C = measured current. 

 R = resistance in external circuit. 



This voltage will be in phase with the current, since the 

 circuit is non-inductive. The voltage spent in the armature 

 will be partly energy voltage in phase with the current, its 

 amount being C x r, when r is the resistance of the armature. 

 The remainder of the voltage lost in the armature will be 

 idle voltage 90 out of phase with the current, and due entirely 

 to armature self-induction. The total voltage generated 

 by the alternator is assumed to be the same as before closing 

 the switch and is therefore the first voltage noted. Hence 

 this total voltage must be the resultant of the energy and 

 idle voltages in the armature and external circuit together 

 after the closing of the switch. The relation between the 

 quantities is that indicated in the annexed diagram, Fig. 95, 

 where 



AB = E = no-load voltage. 



DB = ./!= terminal voltage with load = C R. 



AC = E^=id\e voltage due to armature self-induction. 



AD = E & = voltage overcoming armature impedance. 



CD = C x r = voltage overcoming armature resistance. 

 The base of the triangle is the sum of the energy voltages for 

 the whole circuit, which is partly in the armature and partly 

 in the external circuit. 



The value of E a is obtained by construction or by cal- 

 culation, since 



(E l + Crf. 



