The Effect of Tapered Foot on Uneven Air Gap in Induction Motors

by Steve McConnell


This short paper describes the phenomena of “soft foot” in electrical motors and the effect this problem has on vibration data. The author describes the difference between a simple soft foot from a tapered or angular foot and how a tapered soft foot can be difficult to impossible to detect by using a dial indicator. A case history is provided showing how the author identified and solved a soft foot using vibration analysis.


“A tapered foot is detected and corrected by monitoring vibration. The problem cannot be detected with a dial indicator. Vibration frequency and amplitude are monitored as base bolts are loosened one at a time. It is not possible to determine excessive tapered foot in terms of linear measurement.
A condition of soft foot is assessed by placing a dial indicator on top of each foot and loosening the base bolt. When the indicator does not move, the foot is satisfactory. Depending on the type of taper in the foot, however, the dial indicator may not move. A change in vibration amplitude at a specified frequency is the only way to determine tapered foot. An uneven air gap always exists at two times the electrical line frequency (7,200 CPM).
Because the close side of the rotor will first be attracted to the north and then to the south poles, the force itself varies at two times the frequency of the magnetic field relative to the eccentricity. When the rotor is not centered within the stator, it will always induce excitation at 7,200 CPM, whether or not it is a two-pole motor. If the motor has four or six poles, each pole pulses as it passes by the air gap to generate four or six pulses per revolution. Given the 1,800 or 1,200 CPM synchronous frequency of the four- or six-pole motor, vibration at 7,200 CPM will be present. Unless the analyst is aware of this phenomenon in a two-pole motor, it may be perceived as misalignment (two times operating speed). A high-resolution spectrum is required to separate two times operating speed and two times line frequency on two-pole motors.”

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