198 



ELECTRICAL ENGINEERING 



trolling two sets of contacts. The main control magnet has two 

 independent windings, one, the potential winding, connected across 

 the generator terminals and the other across a shunt in the load 

 circuit. The latter is the compensating winding and is only used 

 when a rise of voltage with load is required. The relay magnet 



Compensating Resistance Shunt 



Potential Winding 



Compensatmg'WindingX 

 Main Control Magnet 



FIG. 166. Automatic voltage regulator. 



is differentially wound and controls the circuit shunting the field 

 rheostat. The operation is as follows: When the short circuit 

 across the field rheostat is opened the voltage tends to fall below 

 normal. The main control magnet is weakened and allows the 

 spring to pull out the movable core until the main contacts are 

 closed. This closes the second circuit of the differential relay 

 and demagnetizes it. The relay spring then lifts the armature 

 and closes the relay contacts. The field rheostat is short cir- 

 cuited and the field current and terminal voltage tend to rise. 

 The main control magnet is strengthened and opens the main 

 contacts allowing the differential relay to open the short cir- 

 cuit across the field rheostat. The terminal voltage falls again 

 and this cycle of operations is repeated at a very rapid rate main- 

 taining a steady voltage at the generator terminals. When the 

 compensating winding is not used the terminal voltage is main- 

 tained constant. 



When it is necessary to compensate for line drop and maintain 

 a constant voltage at the receiver end of the line, the compensating 

 winding is connected across a shunt in the load circuit. The 

 resistance of the shunt is adjusted to give the required compound- 



