COMMUTATING ALTERNATING CURRENT MOTORS 359 



primary. This transformer e.m.f. will set up a large current 

 in the short-circuited coils and the rupturing of this cur- 

 rent as the coil moves from under the brushes causes heavy 

 sparking at the commutator. 



Commutating Poles of no Use on an A-C. Motor. Corn- 

 mutating poles cannot be used to overcome the difficulty 

 as in the c-c. motor because this transformer e.m.f. in the 

 short-circuited coil is greatest when the motor current is passing 

 through its zero value. Hence, if commutating poles were 

 used in series with the armature as in the c-c. motor, the 



FIG. 232. Sketch to Show Reason for High Current in a Coil During 

 Commutation, due to " Transformer " e.m.f. 



flux from the commutating poles would be zero when it 

 should be a maximum. The method of eliminating the 

 trouble is not by commutating poles but by increasing the 

 resistance of the circuit through which the transformer 

 current flows. 



107. Resistance Leads. The resistance of the coil 

 itself is not increased but an extra resistance is introduced 

 in the leads connecting the coils to the commutator bars. 

 These leads generally consist of a piece of resistance wire, 

 which is placed in the bottom of the same armature slot 

 in which the coil is to be placed. These leads have a 

 resistance of about twice that of the coil itself and are 



