What is armature reaction in DC machines and how is it compensated?

Armature reaction is the effect of armature flux on main field flux. Armature current creates its own magnetic field that distorts and weakens the main field. Effects include: magnetic neutral axis shift, flux weakening (reduces generated EMF), and potential commutation problems. Compensation methods include: interpoles (commutating poles) that neutralize armature field in the commutation zone, compensating windings in pole faces for large machines, and brush shift (older method, position-dependent).