356 ALTERNATING CURRENTS 



decrease in power-factor than is the case with other types of 

 alternating-current machinery. This results from the rapid 

 increase in the resultant current in the converter armature with 

 phase displacement between the alternating and the direct-cur- 

 rent waves. Assume that in Fig. 325 the alternating current 

 lags the induced electromotive force by 45. This corresponds 

 to a power-factor of 0.71. For the same power and electromotive 

 force, the alternating-current wave must be increased to 1/0.71, 

 or 1.41, times the value shown in Fig. 325. This current wave 

 is shown in Fig. 326 (a). It is to be noted that the resultant 

 wave shown in Fig. 326(6) has been increased considerably in 

 magnitude over the value shown in Fig. 325(6). Hence, for the 

 same heating in the two cases, it would be necessary to lower 

 by a considerable amount the output of the converter operating 

 at a power-factor of 0.71. Table 144 shows the large reduction 

 in rating caused by lowering the power-factor from unity to 0.9. 



At values of power-factor other than those near unity, the 

 synchronous converter loses most of its advantages over the 

 motor-generator set. Therefore, a converter should be operated 

 at a power-factor which is very nearly unity. 



146. Armature Reaction in a Converter. At unity power- 

 factor, the resultant current in a converter armature is compara- 

 tively small, as shown in Fig. 324(6). Therefore, the armature 

 reaction is correspondingly small and there is practically no 

 distortion of the field. As a result, the machine commutates 

 very much better than when operating as a direct-current gener- 

 ator carrying the same load. When the power-factor decreases, 

 the resultant armature current increases, as shown in Fig. 326(6). 

 As the rotational losses do not change to any great extent with 

 change of power-factor, the power necessary to overcome these 

 losses changes only a small amount with change of power-factor. 

 Hence the energy component of the fundamental of the resultant 

 current changes only a small amount with change of power- 

 factor, since the power necessary to rotate the armature is equal to 

 this energy component multiplied by the back electromotive 

 force. Therefore, at power-factors less than unity, practically 

 the only current which is added to the energy current existing at 

 unity power-factor, is a quadrature current, lagging or leading 

 the induced electromotive force by 90 time-degrees. Only the 



