146 



ELECTRICAL ENGINEERING 



91. Armature Core. The armature core carries the rotating 

 electric circuit in slots punched out on its periphery. It is built 

 up of sheets of steel about 0.014 inch in thickness. Alternate 

 sheets are coated with an insulating varnish to increase the re- 

 sistance in the path of the induced eddy currents. Open spaces 

 are left in the core, called vent ducts (v.v. Fig. 109), which allow 

 air to circulate through the armature and carry off the heat gen- 

 erated due to the iron and copper losses. The number of vent 

 ducts required depends on the length of the armature. 



The armature punchings are carried on a spider s and are kept 

 in place by heavy end plates which have projections on their outer 

 edges to support the end connections of the armature coils. 



92. Armature Winding. The armature winding is the seat 

 of the generated electromotive force. It must be tapped at cer- 

 tain points by brushes, in order that the machine may supply 

 power to an external receiver circuit. The winding consists of a 

 number of coils of one or more turns, connected together to form 

 a continuous winding; leads are run from their junctions to the 

 commutator bars from which the current is collected by the 

 brushes. The coils forming the winding must be so connected 

 together that the e.m.f.'s generated in coils between brushes of 

 opposite polarity will all act in the same direction. 



The earliest type of armature winding was the ring winding, 



FIG. 110. Bipolar ring winding. 



Figs. 110 and 111, but this has been replaced by the various forms 

 of drum windings, a few of which are illustrated in Figs. 113 to 118. 

 93. Ring Windings. In the bipolar ring winding, Fig. 110, 

 all the conductors on each half of the armature are connected in 

 series between the brushes. When the brushes are placed on the 

 neutral line, that is, in such a position that the coil being com- 



