Experimental Analysis and Mitigation of Soft Foot in Induction Motors

Abstract

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.

Introduction

1. Background

• Hindalco Mahan operates a captive power facility with six 150 MW units for smelter operations.

• A high vibration and noise issue was reported in the service water pump motor, even during no-load conditions (12.8 mm/s vibration).

2. Root Cause: Soft Foot

Soft foot occurs when a machine does not sit evenly on all feet, causing frame distortion and misalignment. It is a critical factor in shaft alignment and vibration problems.

Common causes:

• Twisted/damaged frames or feet

• Uneven foundations or excessive/poor shimming

• Debris or dirt under feet

3. Types of Soft Foot

4. Effects on Motors

Soft foot in motors can lead to:

• Stator distortion & magnetic imbalance → 2x line frequency vibrations (e.g., 100 Hz)

• Rotor-stator contact and air gap inconsistencies

• Bearing misalignment → overheating

• Structural damage: cracked feet, bolt fatigue, shim displacement

5. Case Analysis: Hindalco Motor Issue

• Symptoms: High vibration (12.8 mm/s) and noise during no-load; drops to 1.5 mm/s on shutdown → electrical origin suspected.

• Vibration spectrum: Shows 2x line frequency (100 Hz), typical of soft foot or rotor-stator air gap issues.

• Inspections found:

  • Poor blue matching at two feet (gap up to 0.65 mm).

  • Misalignment at base and mounting surfaces (0.8 mm level variation).

Conclusion

The reliability issues at Hindalco’s Mahan plant clearly show that soft foot—a condition where one or more motor feet fail to fully contact the base—poses a major threat to high-power rotating machinery. Symptoms such as increased 2× line frequency vibration, stator and rotor distortion, bearing misalignment, and frame deformation are all classic indicators of soft foot Blue matching, phase analysis, and filler-gauge measurements (showing up to 0.65?mm foot lift) confirmed the presence of soft foot. These were strongly correlated with diagnostic signals: notably a dominant 2X electrical frequency peak and mechanical vibration—hallmarks of eccentric stator–rotor interaction and magnetic force imbalance Following corrective alignment—blue matching, shimming, bolt torque control, and foot surface correction—vibration dropped dramatically from 12.8?mm/s to 0.8?mm/s (According to ISO10816), underscoring the effectiveness of the intervention. By addressing the root mechanical issue of soft-footremoving foot imperfections, ensuring proper blue-matching, and aligning the shafthe root cause of vibration and noise in the service water pump motor was eliminated. This restored motor health, extended bearing and rotor life, and prevented costly downtime. Ensuring proper footing and alignment is critical to reliable motor operation in industrial power plants.

References

[1] Mechanical vibration / by J.P. Den Hartog book [2] Vibration Monitoring on Induction motors case study/William T. Thomson, EM Diagnostics Ltd., Scotland [3] Mobius VCAT-III vibration analysis literature. [4] Advancement Vibration Analysis/ Mechanalysis literature. [5] Vibration standards ISO10816 and ISO7919, ISO 20816-3:2022.

Copyright

Copyright © 2025 Sushil Sharma. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.