ARMATURE WINDINGS 267 



flux distribution may be assumed, making the maximum air- 



7T 



gap density ~ times the average value over the pole pitch. The 

 



calculation of slot leakage flux will be explained later, and its 

 effect may for the present be neglected. 1 



As a check on the calculated core loss, the figures of Art. 32 

 (page 104) may be used; but these values will depend upon 

 whether the copper or the iron losses are the more important, 

 i.e., on the relative proportions of iron and copper in the machine. 

 Iron losses 50 per cent, in excess of the average values given on 

 page 104 would not necessarily betoken inefficiency or a high 

 temperature rise. 



When computing the total losses to be carried away in the 

 form of heat from the surface of the armature core, the whole of 

 the copper loss should not be added to the iron loss, but only 

 the portion of the total PR loss which occurs in the buried part 

 of the winding, i.e., in the "active" conductors of length l a . 

 In the case of large machines, it may be necessary to make some 

 allowance for eddy-current loss in the armature conductors. 

 This loss might be considerable if solid conductors were used; 

 but it is usual to laminate the copper in the slot so that the 

 eddy-current loss due to the slot flux is very small. This point 

 must not, however, be overlooked in large units; and special 

 means may have to be adopted to avoid eddy-current loss in the 

 armature conductors. 



89. Temperature Rise of Armature. The probable tem- 

 perature rise of the armature is estimated as explained in Art. 

 34 of Chap VI, in connection with the design of D.C. dynamos. 

 The cooling surfaces are calculated in a similar manner ; but with 

 the stationary armature and internal rotating field magnets, the 

 belt of active conductors is the inside cylindrical surface of the 

 armature; and this is cooled by the air thrown against it by the 

 fanning action of the rotor. The cooling coefficient, contain- 

 ing the factor v (the peripheral velocity), may be used, just as if 

 the armature were rotating instead of the field magnets. The 

 external cylindrical surface of the armature core will have no 

 air blown against it (in the self- ventilating machine), and the 

 value of v in the formula will be zero. In regard to the radial 



1 The amount of the slot leakage flux, expressed as a percentage of the 

 total flux per pole, becomes of importance in well-designed machines only 

 when the pole pitch is very small. 





