LOSSES IN ARMATURES 



113 



ture and field coils of the machine. This will result in a dif- 

 ference of about 20C. between the average temperatures of the 

 outgoing and incoming air. 



In order to ensure that there shall be no unduly high local 

 temperatures in the machine, the air ducts or passages must be 

 suitably proportioned, and provided at frequent intervals. When 

 the paths followed by the air through the machine, and the 

 cross-section of these air channels, are known, the average velocity 

 of the air over the heated surfaces can be calculated. Formula 

 (56) can then be used for determining approximately the dif- 

 ference in temperature between the cooling surfaces and the 

 air; but it should not be relied upon for accurate results. There 

 is a need of further experimental work upon which more accurate 

 formulas can be based. 



35. Summary, and Syllabus of Following Chapters. All 

 necessary particulars have been given in this and the fore- 

 going chapters for determining approximately the dimensions 

 and windings of an armature suitable for a given output at a 

 given speed. A suggested method of procedure in design 

 will be explained later; but, so far as the preliminary design 

 of the armature is concerned, the dimensions are determined 

 by using an output formula (Art. 19, Chap. IV) and deciding 

 upon the diameter D and the length l a of the armature core. 

 When proportioning the slots to accommodate the winding, the 

 diameter D should be definitely decided upon, but slight altera- 

 tions in the length l a can readily be made later if it is found 

 necessary to modify the amount of the flux per pole ($) or the 

 air-gap density (B g ). Once the calculations for temperature rise 

 have been made, and the design so modified if necessary 

 as to keep this within 40 to 45C., the dimensions of the arma- 

 ture will require no further modification. The question of 

 commutator heating will be taken up later; but, in designing 

 the armature for a given temperature rise, the assumption is 

 made that no appreciable amount of heat will be conducted to 

 or from the commutator through the copper lugs connecting the 

 armature winding to the commutator bars. 



The flux per pole necessary to generate the required voltage 

 being known, the remainder of the problem consists in designing 

 a field system of electromagnets capable of providing the re- 

 quired flux in the air gap. This problem is similar to that of 

 designing an electromagnet for any other purpose, and it has 



