PROCEDURE IN DESIGN OF D.C. GENERATOR 223 



the ratio 



The new resultant m.m.f. curve ob- 



ampere-turns. This must be added to the open-circuit m.m.f. 

 curve of Fig. 83, all the ordinates of which must be increased in 

 5,050 + 900 

 5,050 ' 



tained by adding this full-load field m.m.f. to the armature 

 m.m.f. may now be used to plot the final full-load flux curve 

 C of Fig. 84. 



Items (84) and (85) : Diameter of Commutator. Refer Art. 53, 

 page 181. 



A diameter of commutator not exceeding three-quarters of 

 the armature diameter will be suitable. Let us try a diameter 



t Q pr 



D c = 13.5 in., making v c = 3,070 X -j^g = 2,130ft. per minute. 



This dimension is subject to correction if the thickness of the 

 individual bar does not work out satisfactorily. 



Items (86) to (88) : Number of Commutator Bars. Refer Art. 

 27, page 93. On a 220-volt machine, the potential difference 

 between adjacent commutator segments might be anything 

 between 2.5 and 10 volts (page 94). If we provide the same 

 number of commutator bars as there are slots on the armature, 

 the average voltage between the segments at full load would 



230 X 4 



16.1, which is not within the limits obtained 



be about 





in practice. In order to improve communication it will therefore 

 be necessary to have a smaller number of turns between the 

 tappings, and since the number of turns in each coil is not 

 divisible by 2, we shall have to provide 57 X 3 = 171 commu- 

 tator segments. 



Items (93) to (99): Dimensions of Brushes. Unless a very 

 soft quality of carbon is used, the current density over brush- 

 contact surface does not usually exceed 40 amp. per square 

 inch. Taking 35 as a suitable value, the contact surface of all 



OOC 



brushes of the same sign will be -5^- = 9.32 sq. in., or 4.66 sq. 



OO 



in. per brush set. If the brush covers three bars, the width 

 will be W = 0.247 X 3 = 0.741 in. Let us make this dimen- 

 sion % in. The total length of brushes per set, measured in 

 a direction parallel to the axis of the machine, will then be 

 l c = 4.66 -r- 0.75 = 6.23, or (say) 6 in., which can be made up of 

 six brushes each 1 in. by % in. 



Item (100) : Length of Commutator. The spaces between brushes 

 (which will depend upon the type of brush holder) and the 



