ALTERNATING-CURRENT WINDING 191 



rotating and stator end windings, of the insulation on the lowest 

 range becoming damaged, which may make it necessary to rewind 

 that range. 



We are now able to make the following general statements, the 

 pertinence of which it will not be difficult to see from consideration 

 of the above points. 



Properties of Spiral Coil Windings. 



1. All single-phase windings can be laid up in one range, 

 irrespective of whether they are whole-coiled or half-coiled. 



2. All two-phase windings can be laid up in two ranges, irre- 

 spective of whether there is one coil per phase per pole or per pair 

 of poles, i.e. whether whole-coiled or half -coiled. 



3. Three-phase windings can be laid up in either two or three 

 ranges, according to the following conditions : 



(a) Three-phase two-range windings must have one coil per 



pair of poles, i.e. must be half -coiled. 

 (6) Three-phase whole-coiled windings (having one coil per 



phase per pole), must be three-range. 



4. Any winding having a total number of coils which is 

 even, can be laid up in two ranges irrespective of the number of 



5. Any winding having a total number of coils which is odd, 

 may be laid up in two ranges if one of the coils is bent so that it 

 is half in one range and half in the other (see fig. 231). 



6. Divisibility of armature. If the armature is to be divisible 

 into sections without disturbing the coils, the winding must have 

 one coil per phase per pair of poles (i. e. must be half -coiled). 



Of the above, No. 5, perhaps, needs further comment. 

 Suppose we have a three-phase 12-pole half -coiled winding. There 

 is one coil per phase per pair of poles. 



12 

 No. of pairs of poles = = 6. 



No. of coils per phase = Q f 

 No. of phases = 3. 



Total number of coils =3x6 = 18. 



Now if we lay this winding up in two ranges, we shall have 

 1 8 

 -g- = 9 coils in each range, which is quite practicable. If instead 



of twelve poles we have only six, then with one coil per phase 

 per pair of poles we should have only three coils per phase, 



