378 



ELECTRICAL MACHINERY 



Heating of Coils when the Power Factor is not Unity. 

 Fig. 247 shows the conditions for a converter having normal 

 excitation, that is, with the power factor of the machine 

 unity. If the excitation is changed so that the converter 

 is operating at a power factor less than unity (due either 

 to a lagging or a leading current) the relative heating in 

 the different coils is yet more varied; there is not only more 

 difference between the hottest and the coolest coil than 

 when the machine has normal excitation, but, in addition, 



Slip ring 



t 



Slip ring 



Slip ring 



FIG. 248. Relative Heating a Different Power Factor; the Full Line 

 is for cos cj> = 1 and the Dotted Line for a Lagging Current in the 

 Converter Armature. 



nearly all coils are hotter. This is shown in Fig. 248, where 

 the full-line curve shows the relative heating in the dif- 

 ferent coils for cos cj> = l and the dotted curve shows the 

 same results when the converter is run with an underexcited 

 field. 



116. Capacity of a Synchronous Converter. It is seen 

 from Fig. 247 how the heating of the coils is diminished 

 as the number of phases is increased and hence it is evident 

 that a given armature will have more capacity as a six-phase 

 converter than as a three-phase, or greater as a three-phase 

 than as a single-phase, etc. If we have a certain armature 

 which has a safe capacity of 100 kw. used as a continuous- 



