1 88 ELEMENTS OF ELECTRICAL ENGINEERING. 



For a four-ring converter f r = u.-/u-/jQ 

 For a six-ring converter I r = 0.471/0 



92. The armature heating of the synchronous converter. 

 Synchronous converter ratings. Inasmuch as the armature re- 

 action of a synchronous converter is negligible, the power rating 

 of such a machine is determined by armature heating. This 

 power rating is always expressed in terms of the rating the same 

 machine would have if used as an engine-driven direct-current 

 generator, and the difference in rating of a machine when used 

 as an engine-driven direct-current generator and when used as a 

 synchronous converter is determined wholly by the RI* losses 

 in the armature windings, inasmuch as the eddy current and 

 hysteresis loss in the armature core is the same (nearly) in the 

 two cases. 



The following table gives the ratings of synchronous converters 

 expressed in terms of the rating of the same machine when used 

 as an engine-driven direct-current generator. It will be noted, for 

 example, that the rating of a six-ring converter is nearly twice as 

 great as the rating of the same machine when used as an engine 

 driven direct-current generator, and it must be remembered that 

 this increased rating refers to the increase of permissible direct- 



TABLE. 



POWER RATINGS OF SYNCHRONOUS CONVERTERS. 



current output, the voltage of the machine being fixed. Thus a 

 six-ring converter rated as a 1000 ampere direct-current gen- 

 erator would be rated as a 1920 ampere synchronous converter 

 and this increased current rating necessitates a much larger com- 

 mutator so as to permit of a greatly increased contact surface be- 

 tween the direct-current brushes and the commutator. In fact a 

 striking feature of the synchronous converter is the large size of 



