Modeling of Voltage Characteristics of Variable Frequency Motors and Comparison with Asynchronous Motors

Due to the rectification and inversion links of frequency conversion module, the variable frequency motor has different load characteristics from the traditional motor, and with massive grid-connection of variable frequency motors, the grid is presenting new load characteristics. In this paper, specific models of variable frequency motor and the traditional asynchronous motor were built based on the MATLAB/Simulink simulation platform, and on that basis, we compared and analyzed the influence of voltage sag on their respective load characteristics, and their tolerance of voltage sag. During simulation of the load characteristics of variable frequency motor, the magnitude of voltage sag was used as the control variable to obtain and analyze the active power and reactive power curves under different voltages levels. Then, the grid system model was built to study the stability of power system under two different loads of variable frequency motor and asynchronous motor respectively, and discuss the influence of variable frequency motor on the security and stability of power grid. The simulation results show that under voltage sag, the active power and reactive power demands of variable frequency motor and asynchronous motor were different; after the voltage was recovered from the voltage sag, the active power demand of variable frequency motor was significantly higher than that of asynchronous motor. If the common asynchronous motor model is solely used for calculation of grid stability in the environment having large percentage of electronically connected loads, the results would be too optimistic.