ARMATURE WINDINGS 261 



the basis of an average size of slot, the actual overhang beyond 

 end of core would have a mean value of about J^ (k.v. + 3 + 7), 



where r is the pole pitch in inches. On this basis, and as a very 

 rough estimate, the mean length per turn in inches, would be 



2l a + 2.5r + 2 k.v. + 6 (97) 



The cross-section having been previously decided upon, the 

 resistance per phase of the armature winding can readily be 

 calculated. 



84. Ventilation. The gross length of the armature core (l a 

 in the last formula) will depend upon the space taken up by the 

 radial vent ducts and insulation between stampings. If radial 

 ducts are used, they are from % to % in. wide, spaced 2 to 4 

 in. apart, the closer spacing being used when the axial length 

 of the core is great and the peripheral velocity low. (See Fig. 35, 

 page 111.) 



In turbo-alternators, axial vent ducts are being used in place 

 of radial ducts. If there are no radial openings between the 

 armature plates, the length of the core can be reduced, and this 

 is always desirable in high-speed machines. The relation be- 

 tween net and gross lengths of armature core will then be 

 approximately l n = 0.92 a . Even when axial ducts are used, 

 one or more radial openings at the center of the core are some- 

 times provided so that the cool air may be drawn in at both ends 

 of the armature and discharged at the center. The fan for forced 

 ventilation may be inside or outside the generator. In large 

 units the external fan is generally to be preferred. The reader 

 is referred to Art. 33, Chap. VI, where the ventilation of dynamos 

 was discussed. 



85. Full-load Developed Voltage. The losses in the armature 

 core at full load will depend upon the developed e.m.f., which 

 is not quite so easily calculated as in the case of a D.C. dynamo. 

 The pressure that has to be generated in the armature windings 

 of an alternator for a given terminal voltage, will depend not 

 only upon the IR pressure drop, but also on the IX drop. In 

 other words, the inductance of the armature windings, and the 

 power factor of the load, must be taken into account when 

 calculating the developed voltage. 



The vector diagram, Fig. 99, refers to a machine working on a 

 load of unity power factor. The current is in phase with the 

 terminal voltage OE t ' } but the developed volts are OE g and not 



