Digital Exclusive: Mitigating structural resonance in an electric motor support: A field case study

11/24/2025 12:00:00 PM Authors: S. AL DOSSARY and W.

KHALAF ALLAH, Saudi Aramco, Dhahran, Saudi Arabia Rotating equipment operating near structural natural frequencies is prone to resonance, resulting in elevated vibration levels and potential long-term damage. This article discusses a field case involving persistent axial vibration on an electric motor despite acceptable performance during decoupled workshop testing.

The discrepancy prompted a full diagnostic investigation, including bump testing, modal analysis and structural reinforcement. Motor and system description. The equipment in question is a 460-V, 150-horsepower (hp) (112-kW), 4-pole, 60-Hz, 1,800-rpm electric motor operating a lime transfer blower within a petrochemical plant (TABLE 1). Initial data was collected in April 2015, and later follow-ups were conducted throughout 2015 and 2016.

TABLE 1. Details of the operating electric motor

Initial observations and measurements. Initially, the motor (FIG.

1) exhibited significantly higher axial vibration that fluctuated from 6.6 mm/sec to a peak of 9.8 mm/sec. The vibration spectrum revealed that the operating frequency (30 Hz) was close to the system's natural frequency (32 Hz), confirming the presence of structural resonance (FIG.

2). The resonance was attributed to insufficient stiffness in the motor support.

FIG.

1. Motor measurement points (click image to enlarge).

FIG.

2. Motor DE axial spectrum indicating dominant 1X rpm (click image to enlarge). During solo-run testing at the vendor's workshop without the coupling hub, the vibration levels at all points were within acceptable limits (