SOLUTION

The purpose of compensating windings in DC machines is to compensate for harmful flux components created by armature windings. Flux components are harmful because they create an unfavorable air-gap flux distribution in DC machines. The dimensioning of compensating windings is based on the current linkage that has to be compensated by the compensating, winding. The conductors of a compensating winding have therefore to be placed close to the surface of the armature, and the current flowing in them has to be opposite to the armature current. In DC machines, the compensating winding is inserted in the slots of the pole shoes The compensating effect has to be created in the section of the pole pitch, as illustrated in Figure.

Another Explanation of the same question can be

Compensating windings are used to overcome the commutation and cross-magnetization of armature reaction, which causes high flux density in trailing pole tip. Also, the coil under this tip can develop the induced voltage that is high enough to cause a flashover between nearby commutator segments. Physically, this coil is much closer to the commutation zone in which the air temperature can be high due to the commutation process. Therefore, the compensating windings are located on pole shoes to avoid flashover.