328 PRINCIPLES OF ELECTRICAL DESIGN 



(corresponding to the higher critical speed) is easily attainable, 

 the diameter of the shaft being of the order of 13 in. near the 

 rotor body, and 10}^ in. in the bearings. It is not proposed to 

 go further into details of mechanical design; but attention may 

 be called to the fact that a rotor forged solid with the shaft, 

 or a solid rotor with the shaft projections bolted to the two ends, 

 (i.e., without a through shaft), is stiff er than a laminated rotor 

 with through shaft. We shall assume a solid rotor in this design, 

 although the length of the rotor body (about 49J^ in.), being 

 less than one and one-half times the diameter, would indicate 

 the feasibility of a rotor built up of steel plates. 



Items (21) to (27). On account of our having an odd number 

 of conductors per slot, we shall decide upon a single layer winding 

 (see Art. 78, Chap. XII). The current density in the armature 

 windings cannot be determined by the empirical formula (96) of 

 Art. 81, because this is not applicable to speeds higher than 8,000 

 ft. per minute, and in any case, the conditions of cooling in an 

 enclosed machine with forced ventilation are not the same as 

 for a self-ventilating generator. In a turbo-alternator there is 

 usually plenty of room for the armature conductors, the chief 

 trouble being with the rotor winding, which may have to be 

 worked at a high current density. There is no definite rule for 

 the most suitable current density in the armature conductors, 

 the permissible copper cross-section being dependent on the 

 length of armature core, the position and area of the vent ducts, 

 and the supply of air that can economically be passed through the 

 machine. The specific loading will obviously have some effect 

 on the allowable current density in the copper; and, as a guide 

 in making a preliminary estimate, we may use the formula 



1,600,000 

 A = - 



q 



which gives us for item (22) a current density of 2,000 amp. per 

 square inch of armature copper. 



It is well to laminate the conductors in a direction parallel 

 to the slot leakage flux (see Art. 88, page 267, and Art. Ill, 

 page 318), and we may build up each conductor of four flat 

 strips each % by 0.14 in., giving a total cross-section of 0.35 

 sq. in. per conductor. 



There will be 12 copper strips in each slot, the total thickness, 

 including the cotton insulation, being about 1.92 in. The slot 



