COMPENSATORS 221 



current voltage, the alternating current voltage must 

 also be increased. In large systems, with a number of 

 substations receiving current from the same generating 

 station, any given substation voltage must be varied inde- 

 pendently of that of the others. Several methods are 

 possible: for lighting systems an induction regulator 

 is often employed, or the step-down transformers may be 

 provided with a dial switch to vary the ratio of trans- 

 formation. These devices do not operate automatically, 

 whereas the compounding of a rotary converter is auto- 

 matic. 



The excitation of the shunt field is adjusted at no load 

 to a value which causes the machine to take a small 

 lagging current. This lagging current, flowing in the 

 reactive coil, reduces the voltage at the collector rings 

 below that at the transformer terminals. As the rotary 

 takes load the current through the series field first 

 reduces the wattless current through the reactive coils and 

 at higher loads forces a leading wattless current through 

 them. When the current becomes leading, the voltage 

 at the collector rings is higher than at the terminals of 

 the transformers. 



Rotaries may be made to over compound; that is, 

 increase the continuous current voltage as the load 

 increases. 



Reactive coils are often placed in multiple with long 

 distance, high voltage transmission lines to compensate 

 for capacity; they are also used as dimmers, for the 

 lighting of theaters, etc. 



Complete tests on reactive coils for rotaries consist 

 of measurements of resistance, reactive drop, and heat 

 runs at normal and overload, polarity and insulation 

 tests. Reactive drop is usually taken during the heat 

 run. These coils have the same heating guarantees as 

 the transformers with which they operate. 



For the heat run, connect the coils in Y and supply full 

 current at proper frequency, taking precautions to sec 

 that the meters are protected from stray fields. The 

 transformer cables should be kept close together, to 

 prevent high impedance and unbalancing. Heat runs 

 on air blast reactances should be started without air at 



