480 FARM MOTORS 



be set so they close contact with each side respec- 

 tively and make contact with the other side at the instant 

 the current in the armature changes direction. 



641. Ring armature, direct-current dynamo. A ring 

 armature may be made for a direct-current dynamo by 

 winding on the iron ring a series of coils, the ending of 

 each coil being connected to the beginning of the next. 

 The junction of the two is connected to a section of the 

 commutator. As the number of groups of coils is in- 

 creased the number of sections of the commutator must 

 also be increased. An eight-coil ring armature is shown 

 in Fig. 361 ; the direction of current is indicated by the 

 arrows. The induced current from both halves of the 

 armature flows up toward the positive brush B, out over 

 the external circuit, back in through the negative brush C 

 and through each half of the armature to B again. As 

 each coil passes from the field of the N pole and enters 

 the field of the S pole, commutation takes place and the 

 direction of current is reversed. The brushes are located 

 at this point and the current from both sides is con- 

 ducted off on the same wire. When the brushes pass 

 from one of the commutator bars to another there is an 

 instant when the armature sections are short-circuited; 

 but this is at the instant when these coils are moving 

 parallel to the lines of force, hence there is no current 

 passing through them. 



642. Drum armature, direct-current dynamo. Instead 

 of winding the armature coils upon an iron ring some- 

 times they are wound upon a drum. Fig. 362 shows the 

 principle of the drum-wound armature suitable for a 

 bipolar field. Like the windings of the ring armature 

 the coils are in series and both halves are parallel with 

 the external circuit. 



643. Comparison of the drum and ring armature. By 



