These are small poles fixed to the yoke and spaced in b/w the main poles. They are wound with comparatively few heavy gauge Cu wire turns and are connected in series with the armature so that they carry full armature current. Their polarity, in the case of a generator, is the same as that of the main pole ahead in the direction of rotation.
The function of interpoles is follows :
(i) As their polarity is the same as that of the main pole ahead, they induce an e.m.f. in the coil (under commutation) which helps the reversalof current. The e.m.f. induced by the compoles is known as commutating or reversing e.m.f. The commutating e.m.f. neutralizes the reactance e.m.f. therby making commutating sparkles. With interpoles, sparkles commutation can be obtained up to 20 to 30% overload with fixed brush position. In fact, interpoles raise sparking limit of a machine to almost the same value as heating limit. Hence, for a given output, an interpole machine can be made smaller and, therefore, cheaper than a non - interpolar machine.
As interpoles carry armature current their commutating e.m.f. is proportional to the armature current. This ensures automatic neutralization of reactance voltage which is also due to armature current. Connections for a shunt generator with interpoles.
(ii) Another function of the inerpoles is to neutralize the cross - magnetizing sffect of armature reaction. Hence, brushes are not to be shifted from the original position. As before, represent the m.m.f. due to main poles. OA represent the cross - magnetizing m.m.f. due to armature. BC which represent m.m.f. due to interpoles, is obviously in opposition to Oa, hence they cancelled out each other. This cancellation cross - magnetizing is automatic and for all loads because both are produced by the same armature current.
The distinction b/w the interpoles and compensating windings should be clealy understood. Both are connected in series and their m.m.fs. are such as to neutralize armature reaction. But compoles additionally supply m.m.f. for counteracting the reaction voltage induced in the coil undergoing commutation. Moreover, the action of the compoles is localized, they have negligible effect on the armature reaction occurring on the reminder of the armature periphery.