Patent ID: 8330402

Claim:
An alternating-current motor control apparatus comprising: a voltage controller configured to output a command voltage vector so that the command voltage vector is time-averaged for time periods; a square-wave voltage generator configured to control, using a square-wave voltage command that is alternately changed in positive and negative directions every time period, amplitudes and phases of voltages to be applied to an alternating-current motor; a current detector configured to detect motor currents at a timing that is synchronized with periods which are 1/N-th of the time periods, where N is equal to or larger than one; a coordinate transformer configured to perform coordinate transformation to transform the motor currents detected by the current detector into input two-phase currents in a coordinate system at rest; an envelope extractor comprising: a first extractor configured to generate output two-phase currents including waveforms having amplitudes that periodically change based on the input two-phase currents; and a second extractor configured to perform one of differential processing and pseudo differential processing with respect to the output two-phase currents to extract envelopes connecting vertices of the amplitudes of the waveforms; a magnetic-pole-position computer configured to compute, using the envelopes extracted by the second extractor, a magnetic-pole-position computation value to obtain a magnetic-pole position; and a speed computer configured to compute a speed of the alternating-current motor using the magnetic-pole-position computation value.