why dc machines have double layer winding?
The double layer windings are the most widely used class of windings. Though both lap and wave types are possible, because of inherent problems of a wave windings, it is now an accepted practice to use the lap type for double layer ac windings. Double layer windings fall into two main classes depending upon the value of SSP-integral slot winding when SPP is an integer and fractional slot winding when SPP is a fractional number. To meet the requirement of symmetry among phases, the number of slots phase (S/3) must be a whole number.

Double layer windings in open slots have the following advantages over single layer windings in semi-closed slots.


Ease in manufacture of coils and lower cost of winding
Less number of coils are required as spare in the case of winding repairs
Fractional slot winding can be employed
Fractional pitch coil can be used