152 ELEMENTS OF ELECTRICAL ENGINEERING. 



the speed of the synchronous motor increases and decreases. This 

 device sometimes takes the form of a hollow fly-wheel rigidly 

 fixed to the motor shaft and filled with liquid, and (&) The use 

 of damping frames which consist of copper bars set into the pole 

 faces of the synchronous motor parallel to the armature shaft, the 

 ends of these bars being short-circuited by end-bars of copper. 



Fig. 130. 



Figure 130 shows the field magnet of a synchronous motor, 

 or rotary converter, the pole faces of which are provided with 

 copper damping frames or grids. This arrangement is seldom 

 used on belted synchronous motors, it is however very frequently 

 used on rotary converters and upon synchronous motors which 

 are coupled to direct-current generators. The action of these 

 damping frames is as follows : As the speed of a hunting syn- 



