226 PRINCIPLES OF ELECTRICAL DESIGN 



to obtain ideal commutation, together with the permissible 

 variation from this value, are indicated on the same diagram. 

 It will be seen that there should be no difficulty in obtaining 

 sparkless commutation at full load with the brushes in the selected 

 position (over the point k) ; but the brush might be moved with 

 advantage 4 or 5 nearer to the leading pole tip; and, al- 

 though the final flux curve C would be slightly modified, it 

 would not depart materially from the line drawn in Fig. 85. 

 The angular degrees referred to are so-called "electrical" de- 

 grees, because the pole pitch has been divided into 180 parts; the 

 displacement referred to therefore corresponds, in a four-pole 

 machine, to a movement of 2 to 2J^ actual space degrees. 



In connection with Fig. 85, it should be observed that it is 

 only in the case of a full-pitch winding that both coil-sides will 

 be moving through a field of the same density at the same 

 instant of time. In the design under consideration the pole 

 pitch is equal to 11J4 times the slot pitch, while the two sides 

 of the coil would probably be spaced exactly 11 slot pitches 

 apart. This is very little short of a full-pitch winding, and the 

 flux cut by the two sides of the coil is very nearly the same 

 at any given instant; but the method illustrated by Fig. 85 can, 

 of course, be used for determining the proper brush position with 

 short-pitch as well as with full-pitch windings. 



Item (108) : Brush Pressure. Refer Arts. 53 and 54. Assume 

 1M lb. per square inch. 



Items (109) to (112): Brush Resistance and Losses. Refer 

 Arts. 53 and 54. From Fig. 68 (page 179) we find the 

 surface resistance of hard carbon brushes to be about 0.025 ohms 

 per square inch for a current density of 36.2 (item (93)). The 

 area of all brushes of the same sign is 9 sq. in. (item (94)) ; and the 

 total brush resistance is therefore 



- 2 Q 5 X 2 = 0.00556 ohm. 

 y 



The calculated IR drop is 0.00556 X 326 = 1.81 volts, which 

 should be increased by about 25 per cent, as suggested on page 

 181, making this item 2.25 volts. The PR loss is 326 X 2.25 = 

 730 watts. In these, .and some previous, calculations, the value 

 of the line current (item (12)) has been used in place of the total 

 current passing through the armature windings. It is true 

 that an allowance should have been made for the shunt exciting 



