Interpoles OR Compoles
These are small poles fixed to the yoke, having few heavy gauge turns connected in series with the armature so that they carry full armature current. Their polarity in case of a generator, is the same as that of the main pole ahead in the direction of rotation as show in fig.
The function of
interpoles is two-fold:
The interpoles induce an e.m.f. in the short-circuited coil, which neutralizes the reactance e.m.f. there by making commutation sparkles. The e.m.f. induced by the interpoles is know as commutation or reversing e.m.f. induced by the interpoles is known as commutating or reversing e.m.f. and this e.m.f. is proportional to armature current. This ensures automatic neutralization of reactance voltage because both e.m.f’s are due to same armature current.
Another function of interpoles is to neutralize the cross-magnetizing effect of armature reaction. In fig. 2.9, OFm and OFe represents main flux and cross magnetisting flux (m.m.f.) respectively. OFi represents the flux (m.m.f.) due to interpoles, is opposite to OFc, hence they cancel each other. This cancellation of cross-magnetising is automatic and for all loads because, both are produced by the same armature current.
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