318 PRINCIPLES OF ELECTRICAL DESIGN 



The power required to drive the ventilating fan for turbo- 

 alternators will generally be from 0.35 to 0.5 per cent, of the 

 rated full-load output of the generator. 



The brush-friction loss is usually small. If A is the total area 

 of contact, in square inches, between brushes and slip rings, and 

 v is 1 the peripheral velocity of the slip rings in feet per minute, 



vA 

 the brush-friction loss will be approximately ^-^ watts. 



1UU 



The hysteresis and eddy-current losses in the iron can be cal- 

 culated for any given load because the required developed voltage, 

 and therefore the total flux per pole, are known. The losses in 

 the teeth can now be calculated with greater accuracy than 

 before the full-load flux curves were drawn, because the maximum 

 value of the tooth density will depend upon the maximum value 

 of the air-gap flux density as obtained from the flux distribution 

 curve for the loaded machine, as explained in Art. 60 (page 

 196). Unless the density in the teeth is high, it is usually un- 

 necessary to calculate the actual tooth density because the 

 "apparent" tooth density may be used in estimating tooth losses; 

 but with low frequency machines the density in the teeth may 

 be carried well above the "knee" of the B-H curve, and it would 

 then be necessary to determine the actual tooth density as ex- 

 plained in Art. 37 of Chap. VII under dynamo design. 



Eddy Currents in Armature Conductors. The I 2 R loss in the 

 armature copper may be calculated when the cross-section and 

 length of the winding are known; but in the case of large machines 

 with heavy conductors, the eddy-current loss in the "active" 

 conductors may be considerable, and an allowance should then 

 be made to cover this. The eddy currents in the buried portions 

 of the winding are due to two causes: 



1. The flux entering the sides of the teeth through the top of 

 the slot. 



2. The slot leakage flux which the armature conductors them- 

 selves produce when the machine is -delivering current to the 

 circuit. 



The loss due to (1) is independent of the load, and would be of 

 importance in the case of solid conductors of large cross-section 

 in wide open slots. With narrow, or partially closed, slots, it is 

 negligible; but occasion arises when it is advisable to laminate 

 the conductors in the upper part of the slot to avoid appreciable 

 loss due to this cause. 



