DIRECT-CURRENT MACHINERY 185 



of copper. If this condition is allowed to continue the commu- 

 tator will become roughened and the brushes will be gradually 

 destroyed. 



If the current in coil c has just reached zero when the brush 

 breaks contact, there will be no spark due to energy stored in the 

 field of c but that due to the current from b will remain. 



If an e.m.f. had been present in the coil in the direction neces- 

 sary to reverse the current and of such strength that the current 

 had reached a value i greater than I c , then at the instant of break- 



i z I c 2 

 ing contact the excess energy L -^ % would tend to produce a 



Z z 



spark. 



113. Voltage Characteristic or Regulation Curve. The voltage 

 characteristic of a direct-current generator is the relation between 

 the terminal e.m.f. and the current output. 



The e.m.f. generated in the armature is 



6 = Zn$^- 10~ 8 = Kn$ volts. (Art. 103.) 



Take the case of a separately excited generator where the speed 

 n and the field current // are both kept constant. 



At no load the flux crossing the air gap under each pole is < and 

 the e.m.f. generated is 



this is also the terminal e.m.f. at no load. 



As the generator is loaded the terminal e.m.f. decreases due to 

 two causes, (a) armature reaction and (b) armature resistance. 



(a) When current flows in the armature, the armature m.m.f. 

 decreases the flux crossing the air gap and therefore decreases the 

 generated e.m.f. 



This has been shown in Art. 109. If the brushes are moved 

 ahead in a generator or back in a motor in order to obtain satis- 

 factory commutation, the m.m.f. of the armature turns between 

 the poles opposes the field m.m.f. and therefore decreases the flux. 

 These are the demagnetizing ampere turns. The m.m.f. of the 

 turns under the poles distorts the flux and causes a slight de- 

 crease due to the high saturation of one half of the pole tips. They 

 are called the cross-magnetizing ampere turns. These two effects 

 are combined under the term armature reaction. 



