ROTARY CONVERTERS 105 



ture. This percentage varies for different machines 

 as follows: 



Single-phase 



Two-phase 



Three-phase 



Six-phase 



The calculation of the alternating current brush 

 contact resistance requires a measurement of the alter- 

 nating current flowing in the armature, which varies 

 in different types of machines. The following are the 

 constants by which the direct current should be multi- 

 plied to obtain the alternating current. 



For Single-phase 1.00 



Two-phase 72 



Three-phase 943 



Six-phase . . . . . . .472 



As with the direct current brush resistance a curve 

 of the alternating current contact resistance must be 

 referred to and no direct measurement of resistance 

 attempted. In every case the contact resistance per 

 ring should be calculated, the total loss being obtained 

 by multiplying by the number of rings. 



Brush contact area per ring = width of brush in 

 inches X arc of contact in inches X the number of brushes. 



rnt. , t, j ,. - alternating current 



The brush density per ring = - : 



brush contact area per ring 



The resistance obtained from the curve corresponding 

 to this value, divided by the brush area per ring, is 

 the contact resistance per ring. 



The alternating current brush friction should be 

 calculated in the same manner as that for direct cur- 

 rent measurements, the coefficient of friction being 

 taken from a curve. (See Fig. 31.) 



Table XII and Figs. 31-a and 31-b show the form used 

 in calculating and plotting rotary converter efficiencies. 



Normal Load Heat Runs 



When loading a rotary converter on a water rheostat, 

 see that all cables from the transformers to dyna- 



