358 ALTERNATING CURRENTS 



poles. This counter electromotive force assists the current in 

 the coils undergoing commutation to reverse, and better coin mu- 

 tation results. This same electromotive force of self-induction 

 exists in the converter coils which are undergoing commutation. 

 Therefore, commutating poles are used in converters, particularly 

 in those of large capacity, in order to improve commutation. 

 The commutating poles need not be as strong as those which are 

 required for a direct-current machine of the same rating, as there 

 is little or no cross-magnetization to be neutralized. 



The resultant current in the armature conductors of a con- 

 verter, under ordinary conditions of operation, is considerably 

 less than either the alternating or the direct current. Therefore, 

 a much larger commutator, in proportion to the armature, is 

 required, than would be necessary for a direct-current generator 

 having an armature of the same size. Converter armatures 

 have abnormally large commutators, as shown in Fig. 327. 



147. Voltage Control. The ratio of the direct-current electro- 

 motive force to the alternating-current electromotive force in a 

 converter armature is fixed, regardless of field excitation. How- 

 ever, the ratio of brush voltage to slip-ring voltage may be changed 

 a limited amount by varying the field excitation. The brush 

 voltage and the diametrical slip-ring voltage, increased by\/2, 

 differ from each other by the impedance drop through the con- 

 verter armature. If this impedance drop changes either in 

 phase or in magnitude, the ratio of brush voltage to slip-ring 

 voltage changes. The impedance drop may be varied in phase 

 and in magnitude by changing the excitation. Weakening the 

 field below the value which gives unity power-factor makes the 

 current lag, increases its value and lowers the induced electro- 

 motive force. (See page 313, Fig. 290.) Strengthening the field 

 above the value which gives unity power-factor makes the current 

 lead, increases its value and raises the induced electromotive 

 force. (See page 311, Fig. 288.) The effect of changing the field 

 excitation is therefore to change the power-factor, which in turn 

 changes the magnitude and phase of the impedance drop in the 

 armature, as has already been explained in connection with the 

 synchronous motor (see pages 311 and 313). The ratio of brush 

 voltage to slip-ring voltage can therefore be changed in this man- 

 ner. This ratio can be varied by only 2 or 3 per cent, above and 



