Induction motor loads and voltage stability assessment using PV curves

Knowledge of the load composition and characteristics, such as corresponding PV curves, are necessary tools for the voltage stability analysis. This paper investigates small industrial induction motors that are operated under different mechanical load conditions. Experimental tests were performed to determine the system´s PV characteristics and assess the voltage stability under different loading composition. The load characteristics of induction motor directly connected to the grid, as well as operated with a variable frequency drive (VFD) in open and closed loop speed control configurations were investigated experimentally. An equivalent exponential load model was identified for each configuration. The paper compares the constant power loads with the induction motor loads and shows that the VFD will have a pronounced effect on the load characteristic, which in turns affects the voltage stability.