DIRECT-CURRENT MACHINERY 157 



The self-inductance L of the coil opposes any change of current 

 by generating a back e.m.f. L -r volts; when the current is large 



and the time of commutation short this back e.m.f. is large and 

 the current will not be reversed when the brush breaks contact 

 with bar 1 and sparking will occur. 



To counteract the effect of self -inductance the brushes in a gen- 

 erator are moved ahead of the neutral in the direction of rotation 

 and back in a motor. The short-circuited coil is then hi a field 

 which generates in it an e.m.f. due to rotation which opposes the 

 back e.m.f. of self-inductance, or, as it is usually called, "the re- 

 actance voltage of the coil, " and assists commutation. The prob- 

 lem of commutation is discussed fully in Art. 110. 



100. Brushes and Brush Holders. The brushes collect the 

 current from the moving commutator and from them it passes to 

 the receiver circuit. 



Brushes were at first made of copper because it had a low re- 

 sistance and large current-carrying capacity but commutation of 

 large currents was not satisfactory. Carbon brushes were then 

 introduced and commutation was greatly improved due to the 

 action of the high-resistance contact film between the brush and 

 commutator. A much better contact surface was also obtained 

 and the wear on the commutator was reduced. But since carbon 

 will only carry about 40 amperes per square inch while copper will 

 carry 300 amperes per square inch a much larger brush area is 

 required and a larger commutator. 



In order to maintain a good contact between the brush and 

 commutator a spring is used exerting on the brush a pressure of 

 about 2^ pounds per square inch of contact area. 



The brush holders are made of brass and carry part of the cur- 

 rent but leads are connected directly from the brushes to the main 

 leads of the machine to prevent any drop of voltage which might 

 occur due to poor contact between brushes and holders. 



101. Field Windings. The field winding is a stationary 

 electric circuit consisting of one or more coils of wire placed on 

 each of the field poles. They are supplied with current and pro- 

 vide the magnetomotive force necessary to drive the magnetic 

 flux through the machine. The method used in calculating the 

 number of ampere turns required to produce the flux in a machine 

 is worked out in Art. 107. 



