Introduction
An MCM1 test was recently performed on a 3300 kW, 6300 V induction motor rated at 354 A, with a rotor built from 72 bars. Although no abnormalities were indicated by vibration measurements, a broken rotor bar was clearly identified through current analysis.
The Motor Under Test
How the Fault Was Detected
Vibration data were first collected, and no clear fault indicators were found. However, when current signals were analyzed using Motor Current Signature Analysis (MCSA), distinct sidebands associated with a broken rotor bar were observed around the fundamental frequency.
Current Signature Analysis Findings
An RMS current oscillation of approximately 23 Amps was recorded, and this fluctuation was linked directly to unequal current distribution caused by the broken rotor bar. Sidebands were also detected, and these were interpreted as strong evidence of rotor bar damage.
RMS Current Oscillation Due to Rotor Bar Breakage.
BRB Fault Related Frequency Components.
From an Electrical Fault to Mechanical Problems
It should be remembered that motor faults are rarely found in isolation. A broken rotor bar is classified as an electrical fault, but mechanical problems can eventually be created by it as well.
When one or more rotor bars are broken, the rotor current is no longer evenly distributed, and an asymmetric magnetic field is generated inside the motor as a result.
Unbalanced Magnetic Pull Due to BRB fault.
Consequently, Unbalanced Magnetic Pull (UMP) is experienced by the rotor. Instead of remaining centered, the rotor is continuously pulled toward one side of the stator by the magnetic force.
Initially, only a slight shaft deflection or a small increase in air-gap eccentricity may be produced. However, if the motor continues to be operated in this condition, the eccentricity is made more pronounced over time.
In severe cases, the stator may be rubbed against by the rotor. Excessive mechanical wear is caused by this contact, bearing loads are increased, and the rotor surface can be damaged.
As material is worn away or deposited unevenly, the rotor’s mass distribution is altered, and a secondary mechanical unbalance is created. At this stage, what began as a broken rotor bar has evolved into multiple interacting faults.
Air-Gap Eccentricity and Mass Unbalance Confirmed
Both static and dynamic eccentricity components were identified in the current spectrum, and mass unbalance components were also detected at corresponding frequencies.
Air-gap Eccentricity Components in the Current Spectrum.
Mass Unbalance Components in the Current Spectrum.
Motor Status Based on ISO 20958
According to ISO 20958 thresholds, several faults were flagged as failed, including the rotor condition, imbalance, and static air-gap eccentricity. These results were consistent with the broken rotor bar identified earlier in the current analysis.
Overall Motor Status Based on ISO 20958 (MCM1 Report)
The Importance of Combined Diagnostics
Reliance on a single diagnostic technique is not recommended by modern condition monitoring practices. By combining current signature analysis with vibration analysis, both the original broken rotor bar and its secondary mechanical effects can be identified by systems like MCM1.
Vibration Signal Sampling with MCM1.
Conclusion
A broken rotor bar was successfully diagnosed in this case study before catastrophic failure could occur. Through the use of MCM1, early warning was provided, and a much more complete picture of motor health was delivered to the maintenance team.
