Abstract: Apparatus (103) suitable for determining the resistance and inductance of an electric motor (101) estimates the phase shift between a voltage applied to the motor and motor current. Estimation of the phase shift employs a heterodyne technique. The measured motor current is conditioned prior to heterodyning in a mixer 203 in order to reduce the effects of nonlinearities introduced by a voltage source inverter (102) which supplies the motor (101) with a voltage. A value for impedance may be calculated as a ratio of a voltage applied to the motor and the motor current. The resistance and inductance may then be calculated from the impedance and phase shift calculations. In cases where the voltage applied to the motor cannot be directly measured but only the voltage supply to the voltage source inverter 102 is known, a value for impedance may be determined based on a ratio of a reconstructed voltage signal having a phase angle equal to that of the motor current and the motor current.