286 ELECTRICAL ENGINEERING 



squirrel-cage winding in the pole faces. At synchronous speed the 

 armature-reaction flux is stationary relative to the fields and, 

 therefore, does not produce any current in the grids but if the 

 machine falls below or runs above synchronous speed, the flux 

 sweeps across the grids and produces e.m.fs. in them and large 

 currents flow which react on the field and tend to hold the machine 

 exactly in synchronism. 



Hunting may sometimes be reduced or eliminated by changing 

 the field excitation and thus changing the natural period of oscilla- 

 tion of the machine. 



Maohines with high armature reaction are much less liable to 

 hunt than machines with low armature reaction since the high 

 armature reaction reduces the circulating currents produced by 

 changes in phase. 



175. Automatic Voltage Regulator for Alternating-current 

 Generators. The Tirrill regulator described in Art. 120 and 

 manufactured by the General Electric Co., may, with slight 

 changes, be applied to regulate the voltage of alternators. The 

 desired voltage is maintained by opening and closing a short circuit 

 across the exciter field rheostat. 



The method of operation of the regulator can be understood 

 from the diagram of connections shown in Fig. 262. The direct- 

 current control magnet is connected to the exciter bus bars and has 

 a fixed core in the bottom and a movable core in the top attached 

 to a pivoted lever, at the other end of which is a spring which closes 

 the main contacts. The alternating-current control magnet has 

 a potential winding connected across one phase of the alternator 

 and it may also have a compensating winding connected through 

 a current transformer to one of the feeders. The core is movable 

 and is connected to a pivoted lever controlled by a counterweight. 

 The relay magnet is differentially wound and is connected as shown. 



Operation. The direct- and alternating-current control magnets 

 are adjusted for the required .voltage by means of the counter- 

 weight. The exciter field rheostat is then set to reduce the voltage 

 about 65 per cent below normal. This weakens both of the control 

 magnets and the spring closes the main contacts and demagnetizes 

 the relay magnet. The pivoted armature is released and the relay 

 contacts are closed and thus short circuit the exciter field rheostat 

 and immediately raise the exciter voltage and the alternator 

 voltage. When the alternator voltage reaches the value for 



