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ELECTRICAL MACHINERY 



In a well designed machine normal voltage is obtained 

 with that value of field current which gives a voltage 

 somewhere near the knee of the magnetization curve, as 

 it is called; this point is shown at C in Fig. 81. It is seen 

 that for an increase in field current over the normal field, 

 OB, the increase in generated e.m.f. (hence in flux) becomes 



B 



Field Current 



FIG. 81. Magnetization Curve of a Generator. 



smaller and smaller. This condition indicates that the 

 magnetic circuit is becoming saturated. 



Residual Magnetism. When the field current is zero 

 the generated e.m.f. is not zero but has some small value, 

 perhaps 3% of the normal voltage; this is due to the 

 residual magnetism of the machine. After the field of a 

 generator has once been magnetized the iron pole pieces 

 and yoke stay magnetized to a slight extent and this magnet- 

 ism which stays in the frame after the magnetizing current 



