122 



ELEMENTS OF ELECTRICAL ENGINEERING. 



dust and moisture. The field poles, usually four in number, pro- 

 ject inwardly from this steel case which serves as a field magnet 

 yoke. The figure shows the lower half of the motor case 

 dropped, thus exposing the armature and commutator to view. 

 The motor case is supported at one side by the car axle, the two 

 axle bearings being shown in Fig. 91, and on the opposite side 

 the case is supported by a spring which rests on the truck frame. 

 This gives the heavy motor structure that freedom of motion 



Fig. 9 1 . 



which is necessary when the car is running over a rough track at 

 high speed, but at the same time the car axle and the armature shaft 

 are at a fixed distance apart, so as to accommodate the spur gears 

 which transmit power from the armature to the axle. The spur 

 gear shown in Fig. 91 is attached to the projecting end of the 

 armature shaft. The wire-leads from the brushes and from the 

 series field windings are brought out of the steel case through 

 rubber bushings and connect through flexible insulated cables to 



