Power Balance and Power Factors of Distorted Electrical Systems and Variable Speed Asynchronous Electric Drives

Abstract

In this study, power quality coefficients that determine the distortion of electrical systems are established. The power factor is chosen as a universal criterion for electromagnetic and electro-energetic compatibility of distorted sources and distorting loads. Mathematical expressions are obtained for its calculation in various cases of electrical systems distortion. As an application example, power balances and power factors of a variable frequency asynchronous drive system are considered. The criterion: the power factor not only determines the energy and electromagnetic compatibility of the drive with power supply, but it also evaluates drive efficiency. Experimental studies were carried out under usage of inexpensive, commonly used measuring devices both on the grid side of the frequency converter and on the asynchronous motor side. The studied coefficients were determined in two ways: measuring instruments and analysis of oscillograms. The second method for determining the above coefficients from oscillograms of currents and voltages also requires finding the total harmonic distortion (THD) of currents and voltages for the corresponding load adjusting points. The similarity of the obtained results shows that both of these methods can be applied. According to the operating mode of the load, the criterion is calculated as an average range in a certain range of speed control, or it is determined by considering a given tachogram of speed changes. The results of experimental studies confirm the correctness of the theoretical provisions.