PURPOSE: To enable highly accurate arithmetic operation of magnetic flux achieved by calculating by a magnetic flux arithmetic means, armature-reaction reactance in an arbitrary operational region, in which direct-axis and quadrature-axis currents are superposed, by the use of an arithmetic expression of a stage means according to the saturation characteristic of direct-axis and quadrature-axis armature-reaction reactances.

CONSTITUTION: In a microcomputer 6, the saturation characteristic of direct-axis individual or quadrature-axis individual armature-reaction reactance is obtained by a direct-axis or quadrature-axis armature-reaction reactance detecting means 4 or 5. Further in the microcomputer 6, by substituting the saturation characteristic of direct-axis or quadrature-axis armature-reaction reactance for an arithmetic expression of a storage part 7, the armature-reaction reactance in an arbitrary operational region, in which direct-axis and quadrature-axis currents are superposed, is calculated.