Induction Motor Under Unbalanced Voltage Subharmonics

In addition to fundamental harmonics, the voltage waveform may contain various undesirable components, including subharmonics (subsynchronous interharmonics) and interharmonics (SaIs), which are components with frequencies less than or not an integer multiple of the fundamental frequency. Voltage SaIs are considered especially detrimental power quality disturbances, exerting negative impacts on rotating machinery. They are generated by various electrical equipment, including single-phase equipment. This work addresses the effects of the occurrence of subharmonics in one phase of the supply voltage on three-phase cage induction motors. The results of 2D finite element computations and experimental investigations show that subharmonics occurring in one phase cause high torque pulsations and unacceptable vibrations. For the investigated motor, the minimal value of the voltage subharmonic causing excessive vibration is more than twice that in the case of balanced three-phase subharmonics. It is concluded that power quality standards should contain separate limits for SaIs averaged for three line voltages and for SaIs occurring in each phase voltage.