130 



ELECTRICAL MACHINERY 



IR drop from the generated voltage at that same load. 

 The generated voltage at any load is not easy to determine 

 because of the armature reaction effect. In the case of 

 a shunt generator, for example, knowing the value of the 

 shunt field current, we use the magnetization curve and so 

 expect to get the true generated e.m.f . for this field current. 

 But the generated e.m.f. with a certain field current and 



ignetizatio 



C G ^Armature I R drop 



D C=Armature reaction drop 



A G=Full load terminal voltage 



Field Current A 



Load Current 



FIG. 82. External Characteristic of a Separately Excited Generator. 



no load (under which condition the magnetization curve 

 is obtained) is different from the generated e.m.f. with the 

 same field current when the machine is carrying load. It 

 has been shown that the armature exerts a demagnetizing 

 effect on the main field and thus, even though the field 

 current of a generator is maintained constant, as the 

 machine is loaded the generated voltage falls because of 

 the field weakening due to the armature reaction. 



