366 ELECTRICAL EQUIPMENT 



Cycle of Operation. The circuit shunting the exciter field 

 rheostat through the relay contacts is opened by means of a single- 

 pole switch at the bottom of the regulator panel and the rheostat 

 turned in until the alternating-current voltage is reduced 65 per 

 cent below normal. This weakens both of the control magnets 

 and the floating main contacts are closed. This closes the relay 

 circuit and demagnetizes the relay magnet, releasing the relay 

 armature, and the spring closes the relay contacts. The single- 

 pole switch is then closed and as the exciter field rheostat is short- 

 circuited the exciter voltage will at once rise and bring up the 

 voltage of the alternator. This will strengthen the alternating- 

 current and direct-current control magnets and at the voltage for 

 which the counterweight has been previously adjusted the main 

 contacts will open. The relay magnet will then attract its 

 armature and by opening the shunt circuit at the relay contacts 

 will throw the full resistance into the exciter field circuit tending 

 to lower the exciter and alternator voltage. The main contacts 

 will then be again closed, the exciter field rheostat short-circuited 

 through the relay contacts and the cycle repeated. This operation 

 is continued at a high rate of vibration, due to the sensitiveness of 

 the control magnets, and maintains not a constant, but a steady 

 exciter voltage. 



Regulator Arrangements. The most generally used regulator 

 arrangement consists of one common regulator for several exciters 

 operating in parallel. Such a regulator should have sufficient 

 capacity to take care of all the exciters, whether it is necessary to 

 operate them all at one time or not. Equalizing rheostats must 

 also be provided with such an arrangement in order that each 

 exciter shall carry its share of the load. The full field voltage of 

 one exciter may, for example, be considerably higher than another 

 and it may build up quicker when its rheostat is short-circuited 

 by the automatic regulator. Assuming that the field rheostats of 

 the two exciters are set so that, with the regulator contacts open, 

 the voltages are equal, the more sluggish exciter will tend to main- 

 tain its voltage at a lower point than the more active one. The 

 contacts, of course, open and close at the same speed on both. 

 The more active exciter would, therefore, tend to take more than 

 its share of the load. To cause proper division, the resistance in 

 the field circuit of the more active machine should be increased. 

 When an exciter requires more than one relay, the resistance of its 



