Referring to FIG. 6, a signal in a predetermined high frequency region of the motor current (i_dqs^{{circumflex over (r)}}) obtained from the motor stator may be filtered through a filter 160 to generate the fundamental wave current (i_dqsf^{{circumflex over (r)}}). The filter 160 may be a notch filter or a low-pass filter.

A subtractor 20 may generate a high frequency current (i_dqsh^{{circumflex over (r)}}) by subtracting the fundamental wave current (i_dqsf^{{circumflex over (r)}}) from the motor current (i_dqs^{{circumflex over (r)}}). In this specification, the “motor current” may include a fundamental wave current or a harmonic wave current, and the “motor voltage” may include a fundamental wave voltage or a harmonic wave voltage.

The current signal (i_sig) is a result obtained by dividing the estimated q-axis current ripple of the high frequency current (i_dqsh^{{circumflex over (r)}}) by the clock signal.

As described above, the current signal (i_sig) changes according to the angular error. More specifically, the current signal (i_sig) is a value changing according to the torque command (T_e*), the position (θ_r) of the rotor and a first angular error (a difference between the estimated angle and the position (angle) of the rotor, {tilde over (θ)}_r). Thus, the current signal (i_sig) may be expressed as a function of the torque command (T_e*), the position (θ_r) of the rotor and the first angular error ({tilde over (θ)}_r) as in Equations 4 and 5.