Load modeling of an induction motor operated with a variable frequency drive

Knowledge of load characteristics and load modeling is a necessary tool for adequate voltage stability analysis. This paper presents experimental tests of an off-the-shelf variable frequency drive feeding an induction motor and shows how this load draws power from the grid and its effect on the voltage stability. A description and simulation of the drive is provided to understand the input and output waveforms observed. The results show that commonly used exponential load models of the induction motor, compared to the drive-fed motor, are completely different for reactive power, but very similar for real power. Calculation of power factor for the loads with high harmonic distortions, like the motor drives, and its application to the stability analysis are discussed. It is also shown that the motor-drive increases the maximum power transfer limit for voltage stability and reduces the voltage drop, compared to the induction motor alone.