ARMATURE WINDINGS AND SLOT INSULATION 97 



perature rise. The hottest accessible part of the armature, 

 after a full-load run of sufficient duration to attain very nearly 

 the maximum temperature, should not be more than 40 or 45C. 

 above the room temperature. No definite rules can be laid 

 down in the matter of armature conductor section because the 

 ventilation will be better in some designs than in others, and a 

 large amount of the heat to be dissipated from the armature core 

 is caused by the iron loss which, in turn, depends upon the flux 

 density in teeth and core. 



The current density in the armature windings generally lies 

 between the limits of 1,500 and 3,000 amp. per square inch. If 

 the armature were at rest, the permissible current density would 

 be approximately inversely proportional to the specific loading, 

 <7, i.e., to the ampere-conductors per inch of armature periphery. 

 When the armature is rotating, the additional cooling effect due 

 to the movement through the air will be some function of the 

 peripheral velocity, and, for speeds up to about a mile a minute 

 or, say, 6,000 ft. per minute the permissible increase of current 

 density will be approximately proportional to the increase in 

 speed. The constants for use in an empirical formula expressing 

 these relations are determined from tests on actual machines, 

 and the writer proposes the following formula for use in deciding 

 upon a suitable current density in the armature winding: 

 A = 600,000 + i 

 q 6 



where A stands for amperes per square inch of copper cross- 

 section, and v is the peripheral velocity in feet per minute. 



30. Length and Resistance of Armature Winding. Before the 

 resistance drop and the PR losses in the armature can be calcu- 

 lated, it is necessary to estimate the length of wire in a coil. 

 This length may be considered as made up of two parts: (1) the 

 "active" part, being the straight portion in the slots, and (2) 

 the end connections. 



The appearance of the end connection is generally as shown in 

 Fig. 33, and since the pitch of the coil is measured on the cir- 

 cumference of the armature core, the sketch actually represents 

 the coils laid out flat, before springing into the slots. 



The angle a which the straight portion of the end connections 



makes with the edge of the armature core is sin" 1 ; where 

 X is the slot pitch; s, the slot width; and 6, any necessary clear- 



7 



