THE CONTINUOUS CURRENT GENERATOR 103 



capacity of 12X55 = 660 amperes. The length per turn 

 is 4 feet, therefore the length per coil is 16 feet. There 

 are 20 coils in series in one path, therefore the length per 

 path = 320 ft. 



The resistance of 320 ft. of No. 8 wire is (at 50 C.) 0.224 ohm. 



The resistance of 320 ft. of double No. 8 is therefore 0.112 

 ohm. 



Hence the armature resistance is 0.112/12 = .00934 ohm; 

 Full load IR drop = 660 X. 00934 = 6. 16 volts. 



Allowing 2 volts for the drop at the brush contacts gives a 

 full load IR drop in the armature of about 8 volts, so that 

 the full load terminal voltage = 285 8 = 277 volts. 



In these two sample problems the data was not taken 

 from actual machines. The voltages obtained are not 

 those of commercial machines. Certain voltages have been 

 more or less standardized, by usage, for certain classes of 

 service. The voltages ordinarily used are 125 and 250 

 volts for lighting generators, and for railway generators 

 550-600 volts. Very special machines may be built for 

 voltages as high as 10,000 volts, but these are seldom used. 

 Generators intended for electroplating are generally built 

 for 10 volts or less. 



20. Field Excitation. The field coils of a dynamo- 

 electric machine may be supplied with current from the 

 armature of the machine itself in which case it is called a 

 self-excited machine, or the power for the field coils may 

 be furnished from some other electric circuit, in which case 

 the machine is said to be separately excited. In general 

 we may say that all c-c. machines use self-excitation while 

 practically all alternating current generators have to be 

 separately excited. 



The field current of any generator, whether c-c. or 

 a-c., must be continuous; the poles must be continuously 

 excited in the same direction. Now the e,m,f. of an a-c. 



