380 



ELECTRICAL ENGINEERING 



The worst condition of local heating occurs in the coil next the 

 lead of a single-phase converter, Fig. 363. The alternating current 

 has its maximum value when the direct current reverses. 



When the power factor is not unity the minimum resultant 

 current will not occur in the centre coil of a phase but in a coil 

 displaced from it by the angle of lag or lead of the current. 



(I) Direct Current 

 Resultant Current 



FIG. 363. Current in the coil next to the lead of a single- 

 phase converter at unity power factor. 



Alternating Current 

 Resultant Current 



Direct CurrentV \ 



FIG. 364. Current in coil c, Fig. 360, at 70 per cent power factor. 



Fig. 364 shows the current in coil c when the power factor is 

 70 per cent and a component of lagging current equal to the power 

 current flows in the armature. 



240. Heating Due to Armature Copper Loss. Take the case 

 of a two-pole, n-phase armature, Fig. 360, and use the same 

 notation as before. In the centre coil c of the phase, the direct 



