354 



ALTERNATING CURRENTS 



The converter rating is determined by the allowable tempera- 

 ture of the hottest part of its armature. Although the conduct <>rs 

 midway between slip-ring taps are operating at temperatures 

 lower than the allowable safe values, the converter rating must 

 be adjusted to conform to the safe temperature limits of the con- 

 ductors whose temperature is highest. 



The greater the number of phases, the greater will be the 

 number of slip-ring taps. This will produce a lesser temperature 



(2, 



A.C. Curj-ent 



(3) 



i 



(f) 

 I 



(1) 



\ 



(a) D.C. and A.C. Currents in 

 Conductor 6, P.F. - 1.0 



(6) Resultant Current 

 FIG. 325. Current at unity power-factor in a conductor at slip-ring tap. 



range due to difference in position of the various armature con- 

 ductors, because the resultant of the direct and the alternating 

 current for conductors located near the slip-ring taps, which 

 conductors operate at the highest temperature, is decreased in 

 magnitude. The average heating for all the conductors will be 

 reduced, which will permit an increase in rating for the converter. 

 The rating of a given converter increases rapidly with increase 

 in the number of phases, as shown in Table 144. 



Table 144 gives the rating of a converter for different numbers 

 of phases, the output as a direct-current generator being taken 

 as unity. 



