Online Estimation of Stator Resistance in Induction Machines Using the Zero-Sequence Component of the Stator Current: Investigating Two Implementations

Monitoring the energy consumption and efficiency of line-connected induction motors is an important task in industry to ensure economical operations and for condition-monitoring purposes. Motor efficiency, which is the intended application in this article, is conveniently obtained from airgap torque and rotational speed. Torque can easily be calculated from stator voltages and currents and depends only on one machine parameter, the stator resistance. Among the existing stator resistance estimation methods, this work is focused on the one that uses zero-sequence components of stator voltages and currents. It is very suitable for induction machines connected directly to the mains. To allow the zero-sequence current to flow from the source to the motor, the neutral terminal of the source must be connected to the wye-connected winding end of the motor, at least for a short period of time. This is done in this work in two different ways. First, the use of the zero-sequence component existing in the voltage supply as the voltage that causes the zero-sequence current to flow after a neutral connection is investigated, and then a simple pulse generator circuit is placed in the path of the neutral wire for this purpose. Implementation issues and performance characteristics of these methods are presented and discussed. Computer simulations and experimental results were obtained on a 2-hp motor test bench and used to validate the method.