542 ELECTRICAL EQUIPMENT 



(4) The compensated type or ordinary type with line drop 

 compensator is useful to indicate at the power station the voltage 

 at any predetermined point of a feeder. 



Direct-current Field Ammeters. (1) They give an indication 

 of the heating in the fields of machines. 



(2) They assist in locating trouble in a machine. For in- 

 stance, in case the alternating current voltmeter on a generator, 

 which is supposedly operating normally, shows that there is no 

 voltage generated, a glance at the field ammeter may show no 

 reading, in which case it is evident immediately that the field 

 circuit is broken or the exciter system in trouble. 



(3) They give an indication of cross currents in generators. 

 For instance, consider a generator panel containing main alter- 

 nating-current ammeter, power-factor indicator, voltmeter, and 

 field ammeter. If the machine is up to speed, the amount of 

 field current in excess of normal which is required at a given power- 

 factor to hold normal voltage, shows proportionately the amount 

 of cross current. 



(4) They are of great value in the fields of synchronous 

 motors, because for any given load and power-factor the armature 

 current is a minimum for a certain value of the field current for 

 which the field can be adjusted with the aid of the field ammeter. 



Indicating Wattmeters. (1) They show the actual power in a 

 circuit no matter what the power-factor since they measure the 

 energy but not the reactive component. This makes them valu- 

 able in the circuits of alternating-current machines operated in 

 multiple since they show the division of load between machines, 

 something which ammeters alone do not indicate, except when 

 machines are operated at exactly the same power-factor and 

 voltage. 



(2) In the absence of curve-drawing instruments, they fur- 

 nish a means for obtaining the load curve of a station. 



(3) They indicate reversal of power in a circuit which an 

 ammeter will not do. 



Power-factor Indicators. (1) It is a well-understood fact that 

 it is most economical to operate power plants at as high a power- 

 factor, as possible in order to get maximum output from the 

 machines. The power-factor indicator is very useful in telling 

 directly what this power-factor is. Proper wiring arrangements 

 can be made to use only one instrument per board, plugging it to 



