Robust Control of an Induction Motor Drive

This thesis considers robust control of an induction motor drive, consisting of an input filter, a voltage source inverter and one or several induction motors in parallel. The motor torque is here controlled by using the method Indirect Self Control (ISC), and power oscillations between the inverter and the input filter are damped by means of a stabilization controller in an outer feedback loop. Closed-loop performance with ISC is analyzed under the assumption of a stiff inverter input voltage. It is shown how parameter errors influence the torque loop and the conclusion is that the motor leakage inductance should not be overestimated, especially not with a large desired control bandwidth. It is also shown that model errors enhance cross coupling, but that the performance is quite insensitive even to large parametric errors. Based on the closed-loop model, expressions for the controller parameters are derived to obtain required stability margins. For design of the stabilization controller to suppress oscillations between the inverter and the input filter, it is shown that the effects of time delays and limited torque (or current) control bandwidth are important and cannot be neglected. From models including these non-ideal properties of the control system, explicit expressions for stabilization controllers to use with ISC as well as field-oriented control (FOC) are derived. This is valuable as stabilization often is designed through costly and time-consuming manual tuning. In the controller design, the trade-off between tight torque control and stability of the DC-link is also explicitly considered. In this way reasonable stability margins are obtained, while minimizing the negative effects on torque control. Stability of the closed-loop drive with the proposed stabilization is validated through realistic hardware-in-the-loop simulations using real control HW and SW. Using models obtained from frequency domain system identification, stability of the non-linear closed-loop drive is verified by combining stability results for linear systems with the small gain theorem for the non-linear model errors. This thesis considers the input filter dynamics in connection with torque control of an induction motor. The key result is a model-based framework for simultaneous treatment of DC-link stability and efficient torque control.