The motor control device includes a current detecting portion for
detecting a phase current that flows in an armature winding of a stator
of a three-phase motor based on current that flows between an inverter
for driving the motor and a DC power supply. The motor control device
performs a position sensorless vector control for the motor based on a
control current that is obtained by a three-phase to two-phase conversion
of the phase current based on an estimated rotor position of the motor.
The motor control device farther includes a superposing portion for
superposing a superposed voltage having a predetermined frequency on a
drive voltage for driving the motor and an estimating portion for
deriving the estimated rotor position based on the superposed current
that is extracted from the control current and flows in the motor in
accordance with the superposed voltage. A voltage vector locus of the
superposed voltage from the superposing portion presents an ellipse.