338 THE ROTARY CONVERTER. 



armature drop at low excitations, both on account of the 

 comparatively high armature current and also on account 

 of the low power factor and lag in current. 



Referring to Fig. 167, the power factor is at its maximum 

 value at an excitation of '61 amps , the power factor then 

 being '99. Under these conditions the current taken by 

 the machine is 2 '8 amps. The armature drop is then 

 almost entirely due to armature resistance, and will be 

 nearly in phase with the applied volts. Thus the drop 

 will be 2 '8 x '145 volts = '41 volts, the armature resistance 

 being '145 ohms. The a.c. volts actually converted are 

 therefore 58 - '41 = 57 '59, and the ratio of conversion 



'u;).o = '1 (approx.), 82'3 being the actual voltage on the 



d.c. side. 



In calculating this ratio we have not taken account of the 

 fact that cos is not exactly unity. The difference between 

 '7 and the ratio of transformation '675 previously obtained 

 is to be attributed to this, to armature magnetic reactions, 

 and the fact that the wave form of the alternating current 

 supplied was considerably more " peaky " than the form 

 of wave given by the converter when driven as a 

 generator. 



It is important to notice that the direct voltage 

 continues to rise with increase of excitation after the 

 point of maximum power factor is reached. This is due 

 to the impedance voltage in the armature now becoming 

 more nearly in phase with the impressed voltage, instead 

 of the generator back voltage, as the current now leads 

 the voltage in the supply circuit. 



In Fig. 169 are shown the primary and secondary 

 watts and primary current observed while the converter 

 was supplying a constant direct current of 4 amps. 



The very great variation of primary current and con- 

 siderable variation in primary power taken are shown 

 very strikingly in contrast to the nearly constant power 

 output. The secondary watts increase slightly on 

 account of the rise in the ratio of voltage transformation. 



Compounding Rotary Converters. It has been shown (see 

 Fig. 166) that the most constant ratio of transformation is 

 not obtained by maintaining the power factor of the 

 circuit constant, as the fall in pressure on the secondary 



