CHAP. IX] ARMATURE REACTION IN D.C. MACHINES 175 



is 1200 amp., and in addition to the interpoles the machine is to be 

 provided with a compensating winding. Show that each interpole 

 should have at least 8 turns, and each main pole be provided with 10 

 bars for the compensating winding, in order to neutralize the armature 

 distortion under the main poles and provide the proper commutating 

 field. 



Prob. 15. Machines provided with interpoles are very sensitive 

 as to their brush position. By shifting the brushes even slightly from 

 the geometrical neutral the terminal voltage of such a generator can be 

 varied to a considerable extent. In the case of a motor the speed can 

 be regulated by this method, without adjusting the field rheostat. Give 

 an explanation of this " compounding" effect of the brush shift. 



55. Armature Reaction in a Rotary Converter. The actual 

 currents of irregular form which flow in the armature winding of a 

 rotary converter may be considered as the resultants of the direct 

 current taken from the machine and of the alternating currents 

 taken in by the machine. The resultant magnetizing action upon 

 the field is the same as if these two kinds of currents were flowing 

 through two separate windings. Therefore, the armature reaction 

 in a rotary converter can be calculated by properly combining the 

 armature reactions of a synchronous motor and of a direct-current 

 generator. 



The alternating-current input into a rotary converter may be 

 either at a power factor of unity, if the field excitation is properly 

 adjusted, or the input may have a lagging or a leading component, 

 the same as in the case of a synchronous motor. The armature 

 reaction due to the energy component of the input consists chiefly 

 in the distortion of the field, against the direction of rotation of the 

 armature. But the action of the direct current is to distort the 

 field in the direction of rotation, and since the two m.m.fs. are not 

 much different from one another, the resultant transverse arma- 

 ture reaction is very small. The direct reaction of the direct cur- 

 rent depends upon the position of the brushes, and the direct 

 reaction due to the alternating currents is determined by the 

 reactive component of the input, which component may vary 

 within wide limits. Thus, the resultant direct reaction of a rotary 

 converter may be adjusted to almost any desired value. 



The ohmic drop in the armature of a rotary converter has a 

 different expression than in either a direct-current <r a synchro- 

 nous machine, because the i*r loss must be calculated for the actual 

 shape of the superimposed currents. Rotary converters are some- 



