Thermal Protection of Vector-Controlled IM Drive Based on DC Current Injection

A technique for stator-resistance-based thermal protection suitable for a low-cost vector-controlled induction machine (IM) drive is suggested in this paper. In order to provide accurate stator resistance monitoring at medium and high speeds, the dc signal injection-based method is selected. The main property of the scheme is that small regulated dc current is periodically injected in the stator windings without interrupting the flux and torque control loops. In the proposed scheme, inverter nonlinearities are properly compensated and accurate resistance estimation is achieved with minimal level of injected dc current. This results in consistent and controllable torque ripple, which is minimal and constant for any actual stator resistance value. Estimated stator resistance can be also used for control algorithm tuning. The method is implemented in a fixed-point microprocessor and tested on a low-cost shaft-sensorless IM drive under various operation modes.