404 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Volts 



Amperes 



ing circuit containing resistance only, and the point P represents 

 the operating conditions of the generator when supplying current 

 to the given receiving circuit. When the resistance R r of the 



. receiving circuit is increased 

 the angle 6 increases, becom- 

 ing 90 for an infinite value of 

 R r , that is, when the receiv- 

 ing circuit is open. When R r 

 is decreased more and more 

 the current at first increases to 

 an excessive value and the 

 terminal voltage falls off be- 

 cause of armature drop R a f a , 

 because of the decreased field 

 excitation which accompanies 

 the decrease of terminal volt- 

 age, and because of the demagnetizing action of the current in the 

 armature. When the field excitation has been reduced consider- 

 ably, the excessive demagnetizing action of the armature current 

 begins to overpower it and ^ ______ __ __ __ _ 



the terminal voltage and cur- 

 rent both fall in value. 



When R r is zero, that is 

 when the brushes of the 

 shunt generator are con- 

 nected by a short thick wire, 

 then the voltage between the 

 brushes is zero, the field cur- 

 rent is zero, and the only 

 current flowing through the 

 armature and the short cir- 

 cuit between the brushes is 

 the current which is due to the residual magnetism of the field 

 magnet of the machine. 



Amperes 



Fig. 38. 



