THE CONTINUOUS CURRENT GENERATOR 129 



has been reduced to zero is called the residual magnetism 

 of the machine. The amount of this magnetism depends 

 upon the quality of iron used in the magnetic circuit; if 

 all the iron of the magnetic path, poles and yoke as well as 

 armature core was soft and well annealed there would be 

 practically no residual magnetism. 



This residual magnetism plays a very important part 

 in the operation of a self-excited generator. When such 

 a machine is starting, there is no voltage developed in its 

 armature, but as the speed increases the residual magnetism 

 gives a small e.m.f. in the armature and as the shunt field is 

 connected across the armature this small e.m.f. produces a small 

 current through the shunt field of the machine. This current, 

 even though it is small, increases somewhat the field strength 

 of the machine. As a result the e.m.f. generated in the 

 armature increases and hence more current flows in the 

 shunt field circuit. 



" Building Up." This action and reaction between 

 armature and shunt field continues until the generator is 

 operating at normal voltage; it is called the " building up " 

 of the generator. It is evident that if there was no residual 

 magnetism in the field this building up operation could 

 not take place; it would be necessary to excite the fields 

 from some outside source every time the generator was 

 started and this would much complicate the operation of 

 a generating station. If a shunt generator, by some chance, 

 loses its residual magnetism (the jarring it receives during 

 shipment might possibly effect such a result) it" is necessary 

 to connect its field circuit to some source of electric power 

 and so re-establish the residual magnetism. In doing this 

 precautions must be observed as outlined in Chapter XIV. 



The efficiency curve of a generator will not be taken 

 up here as Chapter V will treat this subject in detail. 



25. External Characteristics of Series, Shunt, and Com- 

 pound Generators. The terminal voltage of any machine 

 can be obtained for any load by subtracting the armature 



