(57) A PWM modulation unit (7) modulates a three-phase voltage instruction, which is input,
on the basis of a PWM method, and outputs a gate signal to each phase switching element
of an inverter (4). A high-frequency component arithmetic unit (8) calculates, at
each time of switching of the inverter (4), a high-frequency component of a current
occurring due to a voltage which is determined by the PWM modulation unit (7) and
is output from the inverter (4). An index arithmetic unit (9) calculates, as an index
R proportional to a rotational phase angle estimation error, from the high-frequency
component of the current in a γδ coordinate system which rotates in sync with rotation
of a synchronous motor (6). A rotational phase angle estimation unit (10) executes
an estimation arithmetic operation of the rotational phase angle by using the index
R. The invention provides a synchronous motor sensorless control apparatus which enables
stable driving with simple adjustment, and does not cause an extreme increase in amount
of arithmetic calculations.
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