THE CONTINUOUS CURRENT GENERATOR 127 



called the armature characteristic (or field compounding 

 curve) of the generator. Many other curves of similar 

 nature may be constructed and they are all grouped under 

 the general name of characteristic curves. By inspection 

 of these curves it may easily be seen how one quantity 

 varies with respect to another, the rest of the variables 

 involved in the operation of the machine being maintained 

 constant. 



The most important curves of a generator are the 

 external characteristic) the efficiency, and the magnetiza- 

 tion curves. The first two are plotted with terminal 

 volts and efficiency respectively as ordinates and load 

 current as abscissae in both cases. 



24. Magnetization Curve. The magnetization curve 

 is plotted between terminal volts and field current, the 

 machine being operated at no load and normal speed while 

 the data for the curve is being obtained. When there is 

 no load on a generator, the terminal volts and generated 

 e.m.f. are the same. The generated e.m.f. is directly 

 proportional to the flux through the armature if the speed 

 is held constant; the curve plotted between no-load ter- 

 minal volts and field current shows therefore the relation 

 between the field current and the flux which this field 

 current produces, hence its name of magnetization curve. 



If the reluctance of the complete magnetic circuit of 

 the machine were constant, no matter how much the flux 

 might be, the magnetization curve would be a straight 

 line. The reluctance of the air gap (which, as shown 

 before, constitutes the principal reluctance of the magnetic 

 circuit) is independent of flux density; the reluctance of 

 the iron part of the path, however, increases as the flux 

 density increases, especially at the higher densities. Hence 

 the magnetization curve of a machine is nearly a straight 

 line but tends to bend over slightly at the higher values 

 of the field current. The amount of bending shows how 

 nearly the iron part of the magnetic circuit is saturated. 



