A periodic disturbance observer determines real part I^An and imaginary part I^Bn of an estimated current including a periodic disturbance, from value of identification identifying a system transfer function of an nth order torque ripple frequency component from a command torque to a detected torque value, with a one-dimensional complex vector having a real part P^An and an imaginary part P^Bn, a cosine coefficient TAn, a sine coefficient TBn, and the real part P^An and imaginary part P^Bn of the system transfer function; subtracts command compensating current IAn* and IBn* obtained through pulsation extracting filter GF, respectively, from the real part I^An and imaginary part I^Bn of the estimated current, and thereby determines estimated periodic disturbance current real part dI^An and imaginary part dI^Bn to cancel the periodic disturbance current.
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