258 ELECTRICAL MACHINERY 



each other, the exciter voltage rises (due to cutting out 

 the field rheostat) and when they separate the exciter 

 voltage falls. When the regulator is in operation, the 

 armature S is continually vibrating (due to actions described 

 later) so that the exciter voltage is continually oscillating 

 above and below its proper normal value. 



Action of the Exciter Solenoid. Suppose now that con- 

 tact C" remains in a fixed position; we will examine the 

 action of the solenoid A, which is connected across the 

 exciter terminals. The plunger P is carried on a lever, 

 L, and is held in its highest position, when A is not excited, 

 by the spring B. The magnetic pull of A is about equal to 

 that of the spring B. As the exciter voltage rises and falls 

 therefore, the plunger P, falls and rises accordingly and s 

 the contacts C and C f are correspondingly opened and 

 closed. 



Combined Action of Relay Magnet and Exciter Magnet. 

 We have seen that the operation of the contacts E } E f is 

 effected by the opening and closing of the contacts C, C'. 

 Hence the combined action of the relay magnet and exciter 

 magnet is to maintain the exciter voltage constant (and 

 hence maintain the generator field current constant). 

 For suppose (after the regulation has been properly 

 adjusted) that the exciter voltage rises above normal. 

 The plunger P is pulled down opening the contacts 

 C, C', thus (by the action of the relay magnet) opening 

 the contacts E, E f . This inserts the rheostat in the 

 exciter field, by opening the short-circuit path in parallel 

 with the rheostat. 



This action will decrease the field current of the exciter, 

 and therefore will decrease its voltage. The magnetic 

 pull on the plunger P is thus lessened, and it rises due to 

 the action of the spring B. This closes the contacts (7, 

 C', closing the contacts E, E f and so short-circuiting the 

 field rheostat once more. The exciter voltage again rises and 

 the process is repeated. This action keeps the armature 



