i-A] SERIES GENERATOR. 5 



ential wound) motors for more or less constant speed service 

 (Exp. 2-A). 



PART II. CHARACTERISTICS OF A SERIES GENERATOR. 



The characteristic curves to be obtained are: the magnetisa- 

 tion curve, with the machine separately excited; the external 

 series characteristic, with the machine self excited ; and the total 

 characteristic, which is computed. 



6. Magnetization Curve. This curve shows the armature 

 voltage (on open circuit) corresponding to different field currents 

 when the generator is separately excited from an outside source, 

 as in Fig. I. No load is put upon the machine. Means for vary- 

 ing the field current must be provided ; see Appendix, 14. 



7. Data. Readings are 

 taken of field current, ar- ^ ^ A 



mature voltage and speed; 

 the first reading is taken 

 with field current zero, 



showing the voltage due to 



. , , , . rpv, FIG. i. Connections for magnetization 



residual magnetism. The ^ , 



curve, separately excited. 



field current is then in- 

 creased by steps from zero to the maximum* rating of the 

 machine, the readings taken at each step giving the " ascending " 

 curve. The descending curve is then obtained by decreasing the 

 field current by steps again to zero. In Fig. 3, only the ascend- 

 ing curve is shown ; see also Fig. 2, of Exp. i-B. 



To obtain a smooth curve, the field current must be increased 

 or decreased continuously; there will be a break in the curve if 

 a step is taken backwards or if the field circuit is broken during 



*(7a). Current Density. For field windings an allowable current 

 density is 800-1,000 amp. per sq. in. (1,600-1,275 circ. mils per amp.) ; for 

 armatures, 2,000-3,000 amp. per sq. in. (640-425 c. mils per amp.). For a 

 short time these limits can be much exceeded. The sectional area of a 

 wire in circular mils is the square of its diameter in thousandths of an 

 inch. 



