360 



ELECTRICAL MACHINERY 



introduced into each commutator connection as shown by 

 r,r,r, in Fig. 233. 



The Resistance Leads do not Materially Increase the 

 Armature Resistance. The path for the transformer cur- 

 rent is made up of the coil, two resistance leads, two com- 

 mutator bars and the brush, so that the use of these leads 

 cuts down the transformer current in a coil to about one- 

 fifth the value it would have without them. 



The addition of these resistance leads does not materi- 

 ally affect the resistance of the armature as a whole 

 because the total armature resistance between brushes is 



FIG. 233. To Show where Resistance Leads are Used. 



many times that of one coil and the load current has 

 to flow through only two of the resistance leads in 

 addition to the coils. Thus if the armature had 100 

 coils and each resistance lead (sometimes called preven- 

 tive lead) had twice the resistance of a coil, the resistance 

 of the armature between brushes would be 54/50 as great 

 as though no resistance leads were used, while the resist- 

 ance of the path of the transformer current would be 

 increased five times. It is claimed that the use of such 

 leads makes sparkless commutation possible at all loads 

 on the a-c. series motor and also that the leads show no 

 deterioration after much service. 



108. Operating Characteristics. The operating, or run- 

 ning, characteristics of the motor resemble very much those 



