1. Field of the Invention
The present invention relates to a motor control apparatus and a motor control method.
2. Description of Related Art
A brushless polyphase electric motor (hereinafter, simply referred to as a brushless motor) is known as a motor having no brush and no commutator. Unlike a brush motor, in which brushes slidably engage a commutator, the brushless motor does not substantially have a frictional wearing and is thereby suitable for a long term use. Therefore, the brushless motor is widely used as a drive motor, which is frequently used and is disposed in an environment where a periodic maintenance work is not performed or is difficult to perform.
In this brushless motor, the voltage is sequentially applied to windings of multiple phases on a phase-by-phase basis (i.e., applying the voltage to the windings of one phase and then applying the voltage to the windings of a next phase, and so on) to sequentially flow the current through the windings of the multiple phases and thereby to generate a rotating magnetic field. In this way, a magnet rotor having permanent magnets is rotated by the rotating magnetic field.
In an ideal brushless motor, an induced voltage, which is generated in each of the windings upon the rotation of the magnet rotor, has a sine waveform. Therefore, when a voltage of a sine waveform is applied to each of the windings in conformity with the sine waveform of the induced voltage, it is possible to reduce the oscillations (vibrations) and noises at the brushless motor.
However, in a real brushless motor, a distortion may occur in the waveform of the induced voltage, which is generated in each of the windings, due to magnetic saturation when an electric current flows under application of a load. Also, a distortion may occur in the waveform when harmonic components of the induced voltages are overlapped in an interior permanent magnet (IPM) motor, in which permanent magnets are embedded in a rotor. Furthermore, a distortion may occur in the waveform of the induced voltage due to a fluctuation in the load of the motor. Therefore, in the brushless motor, when an electric current is supplied to the windings based on the induced voltage, a periodic oscillation may possibly be generated due to a torque ripple, a cogging and/or a load fluctuation in response to the distortion component of the induced voltage.
WO 2005/035333 (corresponding to U.S. Pat. No. 7,474,067) recites a motor control apparatus, which reduces generation of the cogging and the torque ripple. In this motor control apparatus, a sine waveform of an induced voltage, which is generated in each of the windings, is coordinate transformed into a dq coordinate system. Then, the torque ripple or cogging, which is generated due to the presence of the harmonic component contained in the sine waveform after the coordinate transformation, is computed. Thereafter, a harmonic current command value (a command value, i.e., a specified value of the harmonic current), which causes generation of a torque that has a phase opposite to a phase of the computed torque ripple or cogging torque, is computed. Then, the voltage, which is applied to the respective windings, is controlled based on the harmonic current command value to reduce the torque ripple.
Furthermore, a resonance may occur in the motor at a specific frequency due to, for example, a structural factor. Thereby, even when the technique of WO 2005/035333 (corresponding to U.S. Pat. No. 7,474,067) is used, the resonance may possibly be generated in the motor at a specific frequency to cause generation of the resonance or noise.