88 ELEMENTS OF ELECTRICAL ENGINEERING. 



48. Voltage regulation of generators. A generator which is 

 designed for supplying current at constant voltage is called a 

 "constant voltage" generator. Thus a shunt or compound 

 generator is called a " constant voltage " generator, although the 

 maintenance of a strictly constant voltage by such a machine is 

 accomplished by the manipulation of a field rheostat. The range 

 of variation of the terminal voltage of such a machine with varia- 

 tion of current output, when the field rheostat is left wholly 

 unchanged, is an important practical matter. The change of volt- 

 age from full load to no load, expressed as a percentage of full load 

 voltage, is called the "voltage regulation" of the generator, speed 

 of driving and resistance of field circuit being kept constant. 



Example. The field rheostat of a certain shunt generator is 

 adjusted so that its terminal voltage is 110 volts when it is 

 delivering its rated full load current of 250 amperes. When the 

 main switch is opened so as to reduce the current output to zero, 

 the terminal voltage of the generator is observed to rise to 126.5 

 volts. The regulation of this machine is : 



126.5 ! I0 



I 10 



= o. 1 5 or 15 per cent. 



It is to be noted that the word regulation is applied to changes 

 of voltage inherent in the machine itself, while the word control 

 is applied to changes of voltage due to deliberate adjustments by 

 an attendant. 



49. Control of voltage of shunt or compound generator by manip- 

 ulation of field rheostat. The "field rheostat" is the rheostat 

 which is in circuit with the shunt field winding, as indicated in 

 Fig. 69. If for any reason the voltage between the terminals of 

 the generator is greater or less than the desired value, resistance 

 may be added to or taken from the shunt field circuit by manipu- 

 lating the field rheostat, thus decreasing or increasing the current 

 in the shunt field winding, and thereby decreasing or increasing 

 the field excitation, so as to bring the terminal voltage to the 

 desired value. 



