Decoupling Network of Field-Oriented Control in Variable-Frequency Drives

A synchronous current controller for field-oriented control is a popular control scheme that is used in industry to separately control the synchronous current of an induction motor in steady state. However, in some transient states such as during start-up and variation of the mechanical load, the synchronous current along the d-axis remains coupled to the q-axis current. Conventional decoupling current control considers only the model of an induction motor to compensate these coupling terms, but the motor is often powered by an inverter, which also influences the current, voltage, and torque behavior. Thus, a novel decoupling current control method is proposed to consider the coupling terms of both the motor and power converter in a synchronous current controller. Existing approaches assume that the motor is directly connected to the power supply, so a model is also suggested for a pulse width modulation (PWM) inverter. Analytical equations with simulation and experimental results prove that the coupling terms are still present when not considering the PWM inverter in the decoupling network. The proposed method shows good performance, and the d-axis current is always separated from the q-axis current during variation of the mechanical load.