Analysis of current-regulated voltage-source inverters fur permanenet magnet synchronous mutor drives in normal and extended speed ranges

Permanent magnet synchronous motors driven by voltage-source inverters are used in a wide range of servo-drive applications such as machine tools and industrial robots. The current-regulated voltage-source inverter (CR-VSI) has the advantage of permitting direct torque control by controlling the amplitude of the currents in the machine armature and their phase with respect to the back-emf. The torque-speed characteristic of the permanent magnet synchronous motor is limited at high speeds by the saturation of the current regulator caused by the finite inverter dc source voltage and the increase of the back-emf. In this paper, the generation of a smooth torque at low speeds and the system operating limits in the high and extended speed ranges are investigated. A numerical simulation of tile drive is used to study various control strategies. Simulation results for ramp, hysteresis and space vector type current controllers are presented to illustrate important performance characteristics of each. The nature of current regulator saturation is described and a flux weakening algorithm is discussed in order to achieve extended torque-speed characteristics above the rated speed.