In a power system, the voltage waveform usually contains harmonics and sometimes interharmonics, often defined as components of frequency greater than the fundamental voltage component but not of its integer multiple. Previous studies have reported a minor effect of voltage interharmonics on a cage induction machine. This paper reveals their extraordinary harmfulness for induction motors. Namely, voltage interharmonics may cause high vibration, which can result in machine damage. In addition, interharmonics can lead to torque pulsations corresponding to the natural frequency of the first elastic mode. Consequently, possible torsional resonance may cause destruction of a power train. In this study, the results of investigations on undesirable phenomena due to interharmonics are presented for seven motors with a rated power 3 kW–5.6 MW.

Sinusoidal voltage fluctuations can be considered a specific result of the occurrence of voltage subharmonics and interharmonics, which are components of low frequency or not being an integer multiple of the frequency of the fundamental voltage harmonic. These components—symmetrical subharmonics and interharmonics—are of the same magnitude, while their frequencies are symmetrical with respect to the fundamental frequency. Depending on their phase angles, various kinds of voltage fluctuations can be distinguished: amplitude modulation, phase modulation and intermediate modulation. In this study, the effect of phase angles on noxious phenomena in induction motors was analyzed. Additionally, torque pulsations and vibrations of an induction motor under sinusoidal voltage fluctuation and a single voltage subharmonic or interharmonic were compared. The investigations were performed with the finite element method and an experimental method. Among other findings, it was found that for some phase angles torque pulsations could be about ten times higher than for other angles, roughly corresponding to the amplitude modulation.

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In many power systems, voltage parameters change over time. Variation of power quality disturbances causes thermal transients in electrical machines. Under volatile power quality disturbances, the windings temperature can reach an extremely high level. This problem might be especially important in the case of low-power motors, as they have comparatively short thermal time constants. In this study, thermal transients of an induction machine are analysed for time-varying power quality disturbances. The machine's thermal response to voltage deviation, voltage unbalance, and voltage waveform distortions is compared. The effect of fluctuating voltage unbalance on the heating of the windings is examined. The results of experimental investigations and numerical calculations are presented for two low-power, totally enclosed fan-cooled induction cage machines. It was found that voltage unbalance causes much faster heating of the windings than do other power quality disturbances and that the current power quality standards require revision.