260 PRINCIPLES OF ELECTRICAL DESIGN 



temperatures and high inductance and a large number of teeth, 

 involving more space taken up by insulation, and a higher cost 

 generally. Although larger slots are permissible in A.C. than 

 in D.C. machines, a slot pitch (X) greater than 2.5 in. is not 

 recommended. The upper limit might be 2.75 in. if the air 

 gap is large, while the lower limit is determined by considera- 

 tions of space available for conductors and insulation, bearing in 

 mind the higher cost of a large number of coils. In large turbo- 

 alternators, the slot pitch may be as large as 3 in. or even 

 3J^> in. but in such cases a slot wedge built up of laminated 

 iron plates is generally used, thus virtually reducing the slot 

 opening and equalizing the flux distribution over the slot pitch. 

 In a three-phase machine, the number of slots per pole per phase 

 is usually from 1 to 4; but in turbo-alternators, with large pole 

 pitch, the number of slots may greatly exceed these figures. 



The conductors must be so arranged that the width of slot 

 is not such as to reduce the tooth section beyond the limit 

 corresponding to a reasonable flux density in the iron of the 

 tooth (see Art. 76 of the preceding chapter); but, on the other 

 hand, a deep slot is sometimes objectionable because it leads 

 to a high value of slot leakage flux. The depth of the slot 

 should preferably not exceed three times the width, although 

 deeper slots can be used, and may, indeed, be desirable in cases 

 where poor inherent regulation is deliberately sought. 



83. Length and Resistance of Armature Winding. Apart 

 from the pitch, T, and the gross length of armature core, l a , the 

 length per turn of the winding will depend upon the voltage and 

 also upon the slot dimensions. The voltage will determine the 

 amount by which the slot insulation should project beyond the 

 end of the armature core, and the cross-section of the coil will 

 be a factor in determining the length taken up in bends at the 

 corners of the coil. A rough sketch of the coil should be made, 

 and the length of a mean turn estimated as closely as possible 

 for the purpose of calculating the resistance and weight. With 

 the high pressures generated in some machines, it is necessary 

 to carry the slot insulation a considerable distance beyond the 

 end of the slot in order to guard against surface leakage, and 

 although no definite rules can be laid down to cover all styles 

 of winding, the straight projection of the coil-side (and insula- 

 tion) outside the slot would be at least % (k.v. + 1) in.; where 

 k.v. stands for the pressure between terminals in kilovolts. On 



