400 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Volts E a 



Lay off a scale of values of R s on the vertical line CD, Fig. 

 33, taking the actual given value of R s as unity. 



The finding of the value 

 of E a for a speed ;/', differ- 

 ent from n, may be most 

 easily explained by taking a 

 numerical example. Thus, 

 let it be required to find the 

 value of E a for a speed n' 

 equal to n/i.2. As a first 

 step imagine the value of R a 

 to be increased to 1.2 with- 

 out change of speed. Under 

 these conditions the ordi- 

 0.4 nate Q' P , Fig. 33, would 

 be the value of E a . As a 

 second step imagine this in- 

 creased value of R s and the 

 original speed n to be both 

 decreased in the ratio 1.2 



E' 



D 



0.8 



0.6 



Q' 



Fig. 33. 



to i.o. This would bring 

 R s back to its actual value, 

 and it would reduce the value of E a , namely Q' P , in the ratio 

 1.2 to i.o. Therefore the required value of E a at speed n' is 

 i/i. 2 times Q'P. 



Fig. 34 shows a typical voltage-speed characteristic of a 

 shunt generator at zero current output. If it were not for the 

 effect of the residual magnetism this voltage-speed characteristic 

 would cut the speed axis at 5, Fig. 34. The speed correspond- 

 ing to the abscissa of 5 is called the critical speed of the given 

 shunt generator. When driven at a speed less than this critical 

 speed the shunt generator cannot build up at all. 



The straight line OB r , which is tangent to the magnetization 

 curve (speed n) OA, Fig. 33, at the origin, intersects the line 



