I’ve read Motor Failures due to Rotors is 5%, I think this is very misleading
I now have EmPower an AI driven Motor Current Signature Analysis instrument that is returning defect rates above 50% on new Aluminum Die Cast motors.
RCA Failed Motor:
I have been testing the questioned Motors each shutdown for several years with not too good results
This motor shows 18.7% impedance imbalance which is quite bad, but that imbalance can be either Stator or Rotor related.
Since an opportunity to conduct a Dynamic test is available the imbalance could be either. I am discounting the Stator imbalance as the Currents in each phase are relatively balance.
The Alarm Bells are ringing on Rotor.
Unfortunately the Motor failed to Ground and it was removed and dismantled for my inspection.
The Top Slot Insulation has been burnt away the full circumference of the Stator and there’s no evidence of the rotor rubbing the Stator core. The Bearings remained in good condition.
This indicates that the heat involved in the failure has been rotating. There’s no sign of overloading in the unaffected areas of the Stator Winding.
There has been substantial amount of heat and the grease has split which has been an issue from installation.
T he client has been using a high temperature grease but is hasn’t solved the issue.
I made an inspection of the rotor and found a heat mark which aligned with the Stator failure.
The temper colour of the Steel laminations was light blue indicating the Temperature has reached 330 degrees C well above the Class F insulation rating of 180 degrees C.
The Motor Test showing a conductive winding defect with the All TesPro7 shows the heat has been a progressive one and not a single event.
T he laminations of the Rotor have began to flake with the extreme heat.
The Number of rotor bars and stator slots were counted as this is needed for accurate analysis.
Stator Slots 48
RotorBars 40
The Ultrasonic thickness tester was calibrated to Aluminum sound speed at 4mm.
I tested the bearing raceway and got a reading that made sense, so I was confident that is was measuring correctly.
A sonic Gel was used to ensure good transmission of the measurement.
A measurement was taken along the line of the worst looking Rotorbar and in the defect area a measurement of 3.02mm was returned, and moving up the RotorBar a measurement of 13mm was returned.
This makes sense as the depth of the rotorbar is expected to be at least 13mm, but that may also have a void.
It is assumed the depth of 3.02mm is a coating of Aluminum at the top of the slot and below that an air pocket exists.
That assumption may be invalid but at least a large variation has been detected along the Rotorbar.
The client has a number of these Motors and previously has had a similar failure so this also set off Alarm bells and it was chosen to measure with EmPower the Motor in service.
Two locations on the End Ring shows suspect Casing void.
The failure mode is virtually identical to the current Motor under investigation.
Note: The balance weights are in line with the blow out, these are compensating for the loss in metal during the casing process.
EmPower is a device that takes High resolution current data sampling at 44.5kHz.
The software is an AI Model which conducts the analysis through a function called AUTOANALYZE.
This method is known to be able to grade Rotor quality as one of its analysis functions.
The Rotor Grade is a function of the Side bands of Pole Pass frequency when the data is pass through a Fast Fourier Transform, very similar to Vibration Analysis.
The result from EmPower is a Spectra showing the Pole Pass Frequency.
This is the Slip x Number of Poles.
The left hand side band is at -20dB down from the fundamental returning a Grade 7, multiple broken rotorbars and other severe rotor problems. The number of Side bands present is an indicator of the number of rotorbars in question. In this case there are severe problems.
All of the In service Motor were tested returning Rotor Grades of
Grade 7 4 Quantity
Grade 5 1 Quantity
Grade 4 1 Quantity
The defect rate is running at least 66%.
I have raised this issue with the Major EU Brands and none of these manufacturers conduct final QA testing for this type of defect.
The QA programs are centered around Stator tests only.
Conclusion: These results are not an isolated case, I have conducted the same analysis at a different site and got similar defect rates.
I can encourage that end users Acceptance Test Motors and return the defects, this is the only solution to improving reliability and Energy efficiency.
Motor manufacturers must meet Efficiency standards and this type of Rotor defect reduces that efficiency.
Mark Gurney – Motor Analyst
Bruce Lyle
wrote
Thursday, May 8, 2025, 12:24
We use a-lot of the same types of tests you do but not the ultrasound. That’s a great tip we will look into applying it. Are they going to give you the opportunity to destroy the rotor to visual inspect?
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