Diminution of current measurement error for vector controlled AC motor drives

The errors generated from current measurement path are inevitable, and they could be divided into two categories: offset error and scaling error. The current data including these errors cause periodic speed ripples which are one and two times of stator electrical frequency respectively. Since these undesirable ripples bring about bad influences to the motor drive system, a compensation algorithm must be included in the control algorithm of the motor drive. In this paper, a new compensation algorithm is proposed. The signal of the integrator output of the d-axis PI current regulator is used to compensate the current measurement errors. Usually the d-axis current command is zero or constant to acquire the maximum torque or unity power factor in the AC drive system, and the integral output of the d-axis PI current regulator is nearly zero or constant as well. If the stator currents include the offset and scaling errors, the motor speed has the ripple related to one and two times of the stator electrical frequency respectively, and the integrator output of the d-axis current regulator also has the ripple. The compensation of the current measurement errors is easily implemented by subtracting the DC offset value or rescaling the input measurement gain of the stator currents. Therefore, the proposed algorithm has several features: the robustness to the variation of the mechanical parameters, the application of the steady and transient state, the easy implementation, and less computation time. The MATLAB simulation and experimental results are shown in order to verify the usefulness of the proposed current compensating algorithm.