EXAMPLE OF ALTERNATOR DESIGN 



351 



amp. per square inch. This is a high, but not necessarily an 

 impossible figure. 



The mean length per turn of the rotor winding should be meas- 

 ured off a drawing showing the method of bending and securing the 

 end connections. We shall assume this length to be 156 in. All 

 the turns will be in series, and the mean length per turn for the 

 four poles in series will be 156 X 4 = 624 in. Assuming the 

 potential difference at the slip rings to be 120 volts, the cross- 

 section of the winding, by formula (26), is 



(m) 



624 X 37,000 

 120 



192,500 or 0.1512 sq. in. 



If we use a copper strip 0.12 in. thick, the number of conductors 



2 875 

 in each slot will be -^-^ = 24, making the turns per pole 24 X 3 



= 72. 



O*7 Q(V) 



The current per conductor at full load must be ' = 514 



514 

 amp., whence the current density is A = ., , = 3,430 



amp. per square inch. 



The total length of copper strip is 



X 



72 V 4 V 

 - 



La 



= 3,740 ft. 



The resistance (hot) will be about 0.250 ohm, and the required 

 pressure at slip rings will be 0.25 X 514 = 128.5 volts. The 

 PR loss is therefore 128.5 X 514 = 66 kw., or 0.825 per cent. 

 of the rated output. This is rather on the high side for so large 

 a machine, and it may be accounted for by the fact that the air 

 gap is perhaps somewhat greater than it need be; but the ef- 

 ficiency will not be affected appreciably. 



Item (69). The cooling air, which enters at one end of the 

 machine, is supposed to travel through the longitudinal vent 

 ducts to the other end of the machine, no radial ducts being 

 provided. Such an arrangement leads to the temperature of 

 one end of the machine being higher than the other end; but 

 systems of ventilation designed to obviate this are usually less 

 simple, and the straight-through arrangement of ducts has much 

 to recommend it. In machines larger than the one under con- 

 sideration, it might be necessary to have the cold air enter at 

 both ends, in which case one or more radial outlets would be 

 provided at the center. 



In addition to the ducts of which mention has already been 



