DIRECT-CURRENT MACHINERY 



197 



switch Si and adjust the shunt field of (2) until the machines di- 

 vide the load in proportion to their capacities. 



From the above discussion it is seen that two compound-wound 

 generators connected in parallel form an unstable system unless 

 an equalizer connection is placed between their series windings. 



Load 



Equalizer Connection 



FIG. 165. Parallel operation of compound generators. 



120. Tirrill Regulator. The Tirrill regulator is an automatic 

 voltage regulator designed to maintain a steady voltage at the 

 terminals of a direct-current generator irrespective of ordinary 

 load fluctuations or changes in generator speed. It can also be 

 made to compensate for line drop by increasing the generator 

 voltage as the load increases. 



The regulator controls the voltage by rapidly opening and 

 closing a shunt circuit across the field rheostat of the generator. 

 The rheostat is so adjusted that when in circuit it tends to reduce 

 the voltage considerably below normal and when short circuited the 

 voltage tends to rise above normal. The relative lengths of 

 time during which the short circuit is closed or opened determines 

 the average value of the field current and therefore the value of 

 the terminal voltage. 



The method of operation of the regulator is illustrated in Fig. 

 166. The regulator consists essentially of two magnets con- 



