CONVERTERS 



387 



The resultant m.m.f. of the n phases in the direction at right 

 angles to OB or in line with the field m.m.f. is zero. 



Therefore the resultant m.m.f. of the alternating current in 

 the converter armature has a constant value 



_ 

 Ma ~ m 2~ 



ZI 



. . (368) 



and is hi quadrature ahead of the field m.m.f. It is thus equal to 

 the m.m.f. of the direct current and is opposed to it and the re- 

 sultant armature reaction of the direct current and of the cor- 

 responding power component of the alternating current is zero. 



FIG. 369. Synchronous motor. 



The armature reaction due to the power current required to 

 supply the losses remains but it is very small and produces only 

 a slight distortion of the flux in the air gap. 



When the power factor is not unity the wattless currents in 

 the armature exert m.m.fs., as in the synchronous motor, which 

 act in line with the field m.m.f. and are either magnetizing or 

 demagnetizing but are not distorting. 



Thus in the rotary converter there is very little field distortion 

 or very little shifting of the neutral points under load. As a 

 result the limit of overload set by commutation is much higher 

 than in the direct-current generator. This is very important in 

 the case of converters supplying railway loads where the load 

 factor is usually below 50 per cent. The overload capacities for 



