BIPOLAR DIAGRAM 



159 



*ing motor current, while that above the line OD and to the left 



[X corresponds to leading motor current. 



The region below OD and between LL and 55 corresponds to 

 leading generator current, while that below OD and to the left of LL 

 corresponds to lagging generator current. 



Extreme Cases 



The assumptions (involved in the above-described diagrams) that 

 the flux <& has the same direction as R, and that constant $ means 

 constant R, irrespective of the direction or space phase of <, become 



FIG. 80. 



less and less warrantable as the conditions of operation depart farther 

 and farther from the normal. 



E.g., consider the case of light load and very low excitation 

 (Fig. 80). R makes such a large angle with F (the axis of the field 

 magnets) that < will not only be less in proportion to R, owing to 

 the greater reluctance of the magnetic circuit in this direction, but 

 it will also be pulled down more nearly into the direction of F, because 

 that is the direction of minimum reluctance. Then E\ ( = E\} will 

 also be shifted by an equal angle, since it must be in quadrature 

 with <. This is shown by the light lines of Fig. 80. Thus R will 

 lag behind E\ by less than 90, and if F be reduced to zero, A (=R) 

 will still have a torque component and the motor will operate under 

 very light load without any excitation. 



