Background: Repeated Gearbox Failures Prompt Investigation
In the month prior to testing, the gearbox coupled to this motor broke down twice, prompting the maintenance team to look beyond the gearbox itself for a possible root cause. To investigate whether a condition on the motor side was contributing to the repeated failures, an MCM1 test was carried out on the motor. The test detected a broken rotor bar together with severe mass unbalance, giving the team a likely explanation for the recurring gearbox breakdowns and a clear direction for corrective action.
Damage at the Gearbox Coupling Following Repeated Failures
Introduction
An MCM1 test was performed on a 110 kW, 400 V induction motor rated at 220 A. A broken rotor bar fault was identified through current analysis, and severe mass unbalance was confirmed by vibration testing, demonstrating why both diagnostic methods should be applied together.
The Motor Under Test
Motor Specifications
The motor examined in this case study was rated at 110 kW and 400 V, with a nominal current of 220 Amps. Both current and vibration signals were sampled during normal operation, and the data were processed automatically by MCM1.
Motor Specifications
Background: Why Rotor Bars Break
Rotor bars are typically subjected to repeated thermal and mechanical stresses during motor operation. Over time, cracks can be initiated at the bar-to-end-ring joints, and these cracks are gradually widened by thermal cycling, starting duty, and mechanical vibration.
Once a bar is fully broken, current can no longer flow through it. As a result, current is forced to redistribute among the remaining bars, and this redistribution is what produces the diagnostic signatures analyzed in this case study.
The Motor Under Test
Current Analysis
The stator current was found to exhibit periodic oscillations, and these were attributed to a broken rotor bar fault. An RMS current oscillation of approximately 33 Amps was recorded, and this fluctuation was linked directly to the rotor fault.
RMS Current Oscillation Due to Rotor Bar Breakage.
When FFT analysis was applied to the current signal, characteristic sidebands associated with the broken rotor bar were revealed around the fundamental frequency. These sidebands are produced because the broken bar modulates the rotor’s magnetic field at twice the slip frequency, and this modulation is reflected back into the stator current.
BRB Fault Related Frequency Components.
Early and reliable fault detection was enabled by these sidebands, and their amplitude relative to the fundamental frequency component was used to assess fault severity. The clarity of these sidebands made the fault straightforward to confirm without requiring the motor to be stopped or disassembled.
Vibration Analysis
Vibration data were also collected, and severe mass unbalance was confirmed as a consequence of the broken rotor bar fault. In the Time Domain Waveform of Vibration, a clear sinusoidal pattern was observed, and this shape was recognized as typical evidence of mechanical unbalance.
Time Domain Waveform of Vibration.
In the Frequency Spectrum of Vibration, a distinct 1X running speed peak of 7.5193 mm/s was identified. This amplitude was considered significant when compared to typical baseline levels, and it was interpreted as confirmation that the electrical fault had progressed into mechanical unbalance.
Frequency Spectrum of Vibration (Velocity).
It should be noted that vibration alone would not have revealed the rotor bar’s electrical origin. Only when the vibration data were interpreted alongside the current spectrum was the full fault mechanism understood.
From Electrical Fault to Mechanical Unbalance
As demonstrated in this case, a broken rotor bar is rarely found as an isolated electrical issue. Once the rotor current was unevenly distributed, an asymmetric magnetic field was generated inside the motor as a direct result.
Consequently, Unbalanced Magnetic Pull (UMP) was experienced by the rotor. Instead of remaining centered within the stator bore, the rotor was continuously pulled toward one side by the resulting magnetic force.
Over time, this pull was translated into measurable mechanical unbalance, and this progression was confirmed by the dominant 1X vibration peak recorded during testing. If left unaddressed, this condition could have led to increased bearing loading, accelerated wear, and eventual rotor-to-stator contact.
The close relationship between electrical and mechanical fault modes was clearly illustrated by this case, and this relationship is precisely why a purely mechanical interpretation of the vibration data would have been incomplete.
Motor Status Based on ISO 20958
According to ISO 20958 thresholds, several fault categories were flagged as failed, including the rotor condition, imbalance, and static air-gap eccentricity. In each case, the amplitude and jump-over-noise-floor values were found to exceed the 20.1 dB threshold defined by the standard.
Overall Motor Status Based on ISO 20958 (MCM1 Report)
Bearing-related faults, misalignment, and looseness were all evaluated as passing, and this selective failure pattern strengthened confidence in the diagnosis. The combination of a failed rotor result and a failed imbalance result was considered highly consistent with the fault mechanism described above.
These results were consistent with the fault identified earlier in the current analysis, and they provided quantitative, standard-based confirmation of what had already been suspected from the waveform and spectral data.
The Importance of Combined Diagnostics
Reliance on a single diagnostic technique is not recommended by modern condition monitoring practices. Had only vibration data been reviewed, the mass unbalance might have been treated as a standalone mechanical issue, and its electrical root cause could have been overlooked entirely.
By combining current signature analysis with vibration analysis, both the original broken rotor bar fault and its secondary mechanical effects were identified by MCM1. This integrated approach is what allows maintenance teams to address the true root cause rather than only its downstream symptoms.
Conclusion
A broken rotor bar was successfully diagnosed in this case study, and its resulting mass unbalance was clearly confirmed through complementary vibration testing. Through the use of MCM1, a complete diagnostic picture was delivered, and early intervention was made possible before further mechanical degradation could occur.
