558 



ELECTRICAL EQUIPMENT 



energizes the left-hand solenoid, engages the left-hand pav/1 and 

 moves the dial switch in a clockwise direction. When the solenoid 

 core has reached its extreme point of travel, the winding of the 

 solenoid is automatically open-circuited by the small switch C, 

 and the pawl is immediately pulled to its neutral position by a 

 spring, automatically closing the circuit of the solenoid switch by 

 the small switch C. The same cycle of operation is then repeated 

 until the switch B is opened. If it be desired to cut resistance 

 out of the field circuit the single-pole switch B is closed to the right 

 when the same cycle of operation is performed and the dial switch 



Looking at Face of Switch 



FIG. 348. Connections of Solenoid-operated Ratchet-driven Field Rheostat 



Switch. 



moves in a Counter-clockwise instead of a clockwise direction. 

 Each end of the switch dial is provided with a limit switch, D, 

 which is automatically operated by the switch arm to open the 

 circuit of the solenoid when the resistance is entirely cut in or 

 out. The purpose of the limit switch, D, is simply to protect the 

 apparatus in case the controlling circuit is left closed when the 

 dial switch has reached its extreme point of travel in either 

 direction. 



For circuits above 300 to 350 amperes the motor-operated type 

 of rheostat (Fig. 349) is the most practical, as the heavy contact 



