Evaluation and Procedure for Estimation of Interharmonics on the Example of Non-Sinusoidal Current of an Induction Motor With Variable Periodic Load

The paper describes problem of isolating, evaluating and eliminating interharmonics of current using the example of the operation of an asynchronous electric motor operating at a variable periodic load. The complexity of the problem lies in the fact that the value of the current changes depending on the load of the electric motor, which can change by a multiple of the frequency of the supply network, thereby introducing an interharmonic distortion of the current waveform. The main indicators of harmonic distortion of currents and voltages are the individual harmonic coefficient and total harmonic distortion, which must not exceed the IEC requirements, but at the same time it worsens the operation of the entire network due to interharmonics. The paper applies simulation modeling in the Matlab/Simulink mathematical package. The model consists of an asynchronous motor with a short-circuited rotor, a variable periodic load unit with a three-phase voltage source, and a current spectral analysis subsystem. The parameters of the main current harmonic were determined. An algorithm for the selection of interharmonics of current arising as a result of a variable periodic load is given. By representing the difference between the current curve and the fundamental harmonic as a series of Fourier harmonics, the frequencies, amplitudes and phases of the interharmonics were determined. To reduce the current interharmonics for devices operating with variable periodic load, it is proposed to use active harmonic filters of fixed filtration or with automatic adjustment of filtration parameters. Experimental studies of the active filter with the proposed operation algorithm have been carried out. The results of the experiment justified the expediency of using this for suppressing interharomics in electric drive circuits.