336 PRINCIPLES OF ELECTRICAL DESIGN 



The thickness of wedge for keeping the field windings in posi- 

 tion might be about 1% in. as shown in Fig. 135; but as the cen- 

 trifugal force exerted upon it by the copper in the slot may be 

 very great on account of the high peripheral velocity, careful 

 calculations should be made to determine the compression and 

 bending stresses in the wedge. The allowable working stress for 

 manganese-bronze or phosphor-bronze wedges is about 14,000 Ib. 

 per square inch. 



Although we are not designing a single-phase turbo-alternator, 

 it may be stated here that a convenient means of providing 

 amortisseur or damping windings on the rotors of single-phase 

 machines (see Art. 113) is to use copper wedges in the slots and 

 connect them all together at the ends by means of substantial 

 copper end rings. 



While discussing the matter of rotor slot design, the question 

 of stresses in the rotor teeth should be mentioned. After the 

 slot depth has been decided upon, the centrifugal pull on the 

 rotor tooth should be calculated and the maximum stress in the 

 steel determined, the slot proportions being modified if this stress 

 exceeds 14,000 Ib. per square inch for cast steel or 16,000 Ib. per 

 square inch for mild steel. The total centrifugal pull at the root 

 of one tooth is due to the weight of the tooth plus the contents 

 of one slot, including the wedge, while the pull at the narrow 

 section near the top of tooth (the width W in Fig. 135) is due to 

 the contents of one slot plus the wedge and the portion of the 

 tooth above the section considered. 



Items (45) and (46) . The calculation of the average permeance 

 of the air gap between rotor and stator is carried out as explained 

 in Art. 93 of Chap. XIII. The approximate paths of the flux 

 lines are shown in Fig. 135 which is a " developed" section through 

 the stator and rotor teeth; that is to say, no account is taken of 

 the curvature of the air gap, the tooth pitch on the rotor being 

 made exactly equal to 1.5 times X, namely 3.93 in., or 10 cm. 

 The actual air gap from tooth top to tooth top (item (9)) is 5 = 

 0.875 in., and if we neglect the slightly increased reluctance due 

 to the angle of the tooth sides under the wedge, the component 

 flux paths may be thought of as made up of straight lines, or of 

 straight lines terminating in quadrants of circles. The perme- 

 ance of each section of the flux path between stator and rotor 

 over a space equal to the rotor slot pitch is easily calculated as ex- 

 plained in Art. 5, Chap. II (cases a and c). The calculated nu- 



