DIRECT-CURRENT MACHINERY 



169 



Total ampere turns per pair of poles = 16,548. 



FIG. 133. 



Similar calculations have been made for voltages from 25 to 175 

 volts and the results are tabulated in Fig. 134. 



The curve in Fig. 135 is plotted with the voltages as ordinates 

 and the ampere turns per pair of poles as abscissae; it is the sat- 

 uration or magnetization curve of the machine when running 

 without load. 



FIG. 134. 



108. Building up of E. M. F. in a Self-excited Generator. In 



a self-excited generator at rest there is no flux crossing the gap 

 except the residual magnetism. When the armature is rotated 

 only a small e.m.f. is generated in it and a very small current is 

 produced in the field winding. If the magnetomotive force of 

 this current is in the direction of the residual magnetism, it will 

 increase the flux and the e.m.f. will increase and gradually build 

 up to its full value. 



If, however, the current opposes the residual magnetism, it will 

 cause it to decrease and the e.m.f. will not build up until the field 

 winding is reversed. 



