ALTERNATING-CURRENT COMMUTATOR MOTORS 363 



carbon brushes of high contact resistance are used. The short- 

 circuit current must pass through two resistance leads in series and 

 is thus greatly reduced while the load current is carried by two or 

 more in multiple. The resistance of one of the leads must be very 

 much higher than that of an armature coil in order to reduce the 

 current sufficiently. 



There are losses in the leads due to the resultant of the two 

 currents flowing in them, but by increasing the resistance up to a 

 certain point the short-circuit current is reduced and the com- 

 bined loss is reduced. 



FIG. 343. 



The resistance leads are not made of large enough capacity to 

 carry the current continuously but under running conditions any 

 one lead would only be in circuit for a short time. If the motor 

 is stalled with power on the leads are likely to be destroyed. 



The torque of the motor is very much improved by the use of 



