SWITCHING EQUIPMENT 



557 



common, one of the wheels being for the exciter field rheostat 

 and the other for the main generator field rheostat. Such arrange- 

 ments permit of quite a saving in the space required. 



For larger sizes it becomes necessary to mount the rheostats 

 remote from the switchboard, in the basement or otherwise. 

 The operating mechanism may then consist of a sprocket-wheel 

 chain drive, operated by a handwheel on the front of the board, 

 or it may be electrical, either in the form of ratchets or motors 

 controlled from the main board. 

 A typical arrangement of a 

 sprocket-wheel chain drive is 

 shown in Fig. 347, but it is, of 

 course, evident that the rheostat 

 proper can be located in many 

 different positions than what is 

 shown. This class of control is 

 generally limited to rheostat 

 capacities of up to about 350 

 amperes. 



In many installations it is, 

 however, not possible to locate 

 the rheostat so that the dial 

 switch can be operated by means 

 of chain drive from a hand- 

 wheel on the panel. For such 

 conditions the rheostat can be 

 equipped with an electrically 

 operated ratchet switch (Fig. 

 348), which can readily be con- 

 trolled from the main board, and 

 the rheostat proper can be located in any part of the station. 

 The capacity is limited to the same as the chain-operated type, 

 i.e., about 350 amperes, and the operation is as follows: 



The switch arm is carried around by pawls which engage 

 the knurled rim of a wheel to which the switch arm is rigidly 

 fastened. These pawls are controlled by a core actuated in com- 

 mon by the solenoids AA. When the solenoids are de-energized 

 the pawls are disengaged and in their normal position rest equi- 

 distant from the solenoids. To cut resistance into the field, it 

 is necessary to close to the left the single-pole switch B. This 



FIG. 347. Sprocket-wheel Chain 

 Drive for Field Rheostats. 



