Discrete-time current control of synchronous motor drives

Abstract

The aim of this thesis is to analyze discrete-time models and current control of synchronous motors with a magnetically anisotropic rotor structure, such as interior permanent-magnet synchronous motors (IPMSMs) and synchronous reluctance motors (SyRMs). Current regulators in most modern electrical drives are implemented in digital processors. Discretization of continuous-time controllers using the Euler and Tustin approximations, also known as the emulation-based design, is the most common approach. This design gives satisfactory results when the ratio between the sampling and fundamental frequencies remains high. The performance of the emulation-based design deteriorates as the frequency ratio becomes small. For this reason, the controller based on the exact discrete-time model of the machine is preferred. If the exact expressions are computationally too demanding, approximate expressions (series expansions) could be used instead. A hold equivalent discrete-time model with the effects of the zero-order hold (ZOH) and a sampler is studied in both the stator and rotor coordinates. A two-degrees-of-freedom (2DOF) state-space controller is used with the gains based on the exact discrete-time model of the motor. The results are compared with the emulation- and series expansions (of the exact discrete-time model) based controllers. The robustness of these methods against parameter errors is analyzed and the current controllers are also investigated by performing simulations and experiments on a 6.7-kW SyRM drive.