94 COMMERCIAL ELECTRICAL TESTING 



transformers, and two direct current voltmeters should 

 always be used. During the test the direct current 

 voltage is held constant and the alternating current 

 voltage read between rings 1 and 3 on a two-phase, 

 and 1 and 4 on a six-phase machine. 



The ratio is taken at no load and at full load, and 

 should be as follows when the machine is running from 

 the alternating current end : 



RATIO A-C. TO D-C. VOLTAGE 



The amount of pole face arc will change the ratio. 



An easy and approximately correct method of telling 

 whether a rotary is running with the proper shunt 

 field excitation is to note the ratio of the alternating 

 current to the direct current, which should be as follows: 



Three-phase alternating current and direct current 

 practically the same. 



Two-phase alternating current equal to three-quarters 

 of the direct current. 



Six-phase alternating current equal to one-half the 

 direct current. 



Equalizer Taps 



As soon as a rotary is assembled and before any 

 running tests have been started, the spacing of the 

 equalizer taps and the taps to the collector rings must 

 be carefully checked. Occasionally a wrong connection 

 is made and, if it is not corrected before the running 

 tests are started, one or more equalizer leads may 

 become badly overheated or be burned off. 



