CONVERTERS 



383 



(363) 



and this is the relative armature heating under the two conditions. 



To get the same loss in the armature of a converter and the same 



heating as in the direct-current generator, the armature current 



and the output may be increased in the ratio = 



The values of H and = for the various polyphase converters 

 are tabulated in Fig. 365. 



FIG. 365. 



1 



For a single-phase converter, n = 2, the value of =. is 0.85, 



and, therefore, the output of a machine as a single-phase converter 

 is only 85 per cent of its output as a direct-current generator for 

 the same temperature rise. 



For a three-phase converter == = 1.34 and therefore the output 



is 34 per cent greater than as a direct-current generator. For a 

 six-phase converter the output is increased 96 per cent. 



These values only hold if the alternating current is in phase with 

 the impressed e.m.f. When leading or lagging currents flow in 

 the armature the heating is very largely increased and the rating 

 must be decreased. The rating of a machine as a three-phase 



