CONVERSION OF ALTERNATING CURRENT. 183 



f =o. 5 oo 



(19) 



--- 0.354*0 



(20) 



These electromotive force relations are shown geometrically in 

 Fig. 152. 



96* 



60 



120 



i8o 



Actual electromotive force relations^ The relations given in equa- 

 tions (17), (18), (19) and (20) are not exactly realized in practice. 

 The departure of an actual machine from these ideal ratios depends 

 upon : I st, the wave shape of the alternating electromotive force 

 of supply ; 2d, the wave shape of the alternating electromotive 

 force of the converter itself, which depends chiefly upon the 

 percentage of the armature surface that is covered by the pole 

 faces; 3d, the resistance of the armature windings which causes 

 losses of voltage when the machine is loaded ; 4th, the position 

 of the direct-current brushes, and 5th, the manner in which the 

 machine is used, that is, whether it is used to convert from alter- 

 nating to direct current or vice versa. 



When the alternating electromotive force of supply is approxi- 

 mately harmonic, and the pole faces of the converter cover three- 

 quarters of the armature surface, and when the direct-current 

 brushes are in the neutral position the actual voltage ratios are 

 approximately as follows : 



