100 COMMERCIAL ELECTRICAL TESTING 



that for no load at minimum input (unless full field is 

 specified), and the voltage across the armature brought 

 up gradually by increasing the field on the driving 

 generator, until the armature begins to revolve. The 

 voltage should then be steadily increased at that rate 

 which will bring the rotary to normal speed in approx- 

 imately one minute. This rate can be found by trial, 

 and when once found, the test should be repeated once 

 or twice to make certain that the results are correct. 



Phase Characteristics 



No LOAD. If the phase characteristic tests follow 

 a heat run in which an IRT regulator has been used, 

 it must be disconnected. The most satisfactory com- 

 bination is to run two converters for this test, the one 

 under test running as a rotary and driven by the other 

 running inverted with a direct current loss supply. 

 The speed and the direct current voltage are held 

 constant by varying, respectively, the field of the 

 inverted machine and the voltage of the loss supply. 

 It must be remembered that a lagging current will 

 increase the speed of the inverted rotary, and therefore 

 the inverted machine should be watched constantly 

 so long as the current lags. 



With the field excitation of the rotary reduced to 

 the lowest limit permitted by the inverted machine, 

 the alternating current amperes and volts line and the 

 direct current amperes and volts field should be read. 

 As stated above, the speed and the direct current volts 

 are held constant throughout the test. The field current 

 of the rotary is increased by small increments and 

 readings taken as above. The alternating current 

 amperes input will decrease rapidly until the minimum 

 input point is reached, when they will increase again. 

 The field excitation should then be increased until 

 the input current has a value of at least half the full 

 load current of the machine. 



FULL LOAD. The full load characteristic is taken 

 in exactly the same way as for no load. The direct 

 current volts are held constant at normal rating and 

 the amperes output constant at full load value. The 



