Experimental Analysis and Mitigation of Soft Foot in Induction Motors

In a power plant pump and motors are the critical assets, pumps and their driving motors serve as vital components for efficient, continuous operation. Pumps transfer fluids across critical systems such as feedwater, condensate, cooling, lubrication, fuel, and ash/slurry handling. A major reliability concern in such high-power rotating machinery is excessive vibration, often stemming from misalignment and foundational issues this causes the secondary damage and catastrophic failure of the equipment. One prevalent cause is “soft foot” this is the condition where one or more motor feet do not sit flat on the base, different soft foot cause the different effects on machine like stator distortion and eccentricity, rotor issue like rotor bar loose, motor air gap issue and bearing misalignment leading to machine frame distortion. This can result in increased vibration, bearing wear, and even rotor-stator misalignment in electric motors. A detailed investigation into high vibration and noise from the service water pump motor revealed dominant 2X line frequency components, which is typical soft foot condition causing distortion in rotor and stator and its eccentricity indicative of electrical imbalance caused by soft footing. Blue matching and phase analysis confirmed soft foot presence which was found up to 0.65 mm during physical measurement through filler gauge, contributing to rotor-stator eccentricity and magnetic force imbalance. After corrective measures including proper blue matching, shimming, alignment, and foot surface correction, vibration levels reduced significantly from 12.8 mm/s to 0.8 mm/s. This case study highlights the critical role of soft foot detection and correction in ensuring machine reliability, especially in high-power electrical motors.