CH. XII ARMATURE REACTION 269 



Fig. 66 gives the general dimensions of a railway motor 

 with a slotted armature, designed to carry 24 amperes at 

 full load. 



The outside diameter of the armature is 16", the inside 

 diameter is 10", the length of the armature 10f". The 

 armature has 60 slots, each 5/8" deep and 13/32" across ; 

 the teeth are 14/32" across. The single gap from iron to 

 iron is "9/64", the bore of the poles is 16" and 9/32" ; the 

 angular breadths of the poles are 9" and 5/8" and 8" and 

 3/4" respectively, and the length parallel to the shaft 11^". 



The armature is ring-wound ; there are 120 com- 

 mutator bars, 6 turns per bar ; hence A = 720. Each turn 

 consists of two wires, No. IG B and 8., wires in parallel, 

 thus making 12 wires per bar; the wires of two bars are 

 placed in one slot, giving 24 wires per slot. The armature 

 is series-connected with two brushes, giving p=2. The 

 resistance of the armature with carbon brushes is 1'08 

 ohms when hot. 



The two magnets have 810 turns each, and these are 

 connected three in parallel, 270 in series, so that the 

 current per wire in the magnets is one-third of the main 

 current. The resistance of the magnet winding thus 

 connected is 0*69 ohm, making the whole resistance 

 of the motor with magnets and armature in series 1*77 

 ohms. 



Figs. 67, 68, and 69 give the results of experiments 

 made in the Electrical Engineering Laboratory of McGill 

 University to ascertain the amount of the 

 armature reaction in this motor. 



The magnets were first excited with 4 amperes per 

 turn, and the armature rotated by means of a belt 

 on a pulley keyed to the shaft, but no current allowed 



