ALTERNATORS. 



241 



EXPERIMENT XXXVIII. DETERMINATION OF SHORT- 

 CIRCUIT MAGNETISATION CURVE, OR SHORT-CIRCUIT CHARAC- 

 TERISTIC. 



DIAGRAM OF CONNECTIONS. 



FIG. 114. 



M v M 2 Source of direct current. 



G Alternator armature. 



F Alternator field windings. 



R Field regulating resistance. 



A v A z Ammeters. 



V Voltmeter. 



S, S Switches. 



Instructions. Excite the alternator from a source of 

 direct current through a regulating resistance, switch and 

 ammeter. Connect the armature terminals together through 

 an ammeter and switch. Connect a voltmeter to the 

 alternator terminals. Run the alternator at normal 

 speed, close the switch in the armature circuit, and 

 excite the field at first with a small current. Read 

 the ammeters, and then open the switch, and after 

 adjusting the speed to its previous value, read the voltmeter. 

 Maintaining the speed fairly constant,* gradually increase 

 the excitation and observe the readings of the ammeters 

 with the main circuit switch closed, and the voltmeter after 

 opening the switch at each value. Take a similar set of 

 readings for decreasing values of the excitation. 



In the case of any but small machines, it will be inadvisable 

 to suddenly open or close the switch, and a resistance which 



* From the results of the next Experiment, shown in Fig. 120, it will be 

 seen that when running at approximately normal speed, the short-circuit 

 current remains constant for considerable speed variations. The importance 

 of exact speed regulation does not therefore exist in this experiment, except 

 that, of course, the corresponding ammeter and voltmeter readings at a given 

 excitation must be taken at identical speeds. 



