Steven
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In a salient pole machine (none cylindrical rotor with poles facing outwards) the air gap is not uniform. To model the machine, the stator reactance is broken down into two components; one parallel to the rotor axis (direct d-axis) and one perpendicular (quadrature q-axis) displaced by 90 degrees to each other. Search for salient pole equivalent circuit and you should get more technical explanation.

The idea was first proposed by Andre Blondel as the two armature reaction theory.