162 



ELECTRICAL ENGINEERING 



This curve is the no-load saturation curve or magnetization 

 curve of the machine. 



Shunt-excited machines are operated at a point slightly above 

 the knee of the saturation curve to secure stability, that is, in 

 order to prevent large changes of e.m.f. being caused by slight 

 changes of field current. 



2001 



220 



40 



I 



5>t 



7.5xlO e 



6.0x10' 



4.5xl0 6 



aoxio 6 



1.5x10 





8 12 16 20 24 28 

 Field Current - Amperes 



82 



FIG. 127. No-load saturation curve. 



106. Magnetic Leakage. Since there is no material through 

 which magnetic flux cannot pass, it is not possible to confine all 

 the flux produced in a generator to the magnetic circuit. In prac- 

 tice the main circuit is made of so low a reluctance that only a 

 small portion of the flux leaves it. 



Fig. 128 shows the leakage flux about the magnetic circuit of a 

 bipolar generator and Fig. 129 shows it in the case of a multipolar 

 generator. 



The principal part of the leakage occurs between the pole tips 

 because the m.m.f. consumed between these points is from 60 to 

 80 per cent of the total m.m.f.; it includes the m.m.f. required to 

 drive the flux across the two gaps and through the teeth and 

 armature core. As a result the flux passing through the field poles 

 is greater than the flux crossing the gap into the armature by an 



