Analysis of WEG 3-Phase AC Motor: Noise and Rotating Looseness Patterns

Question:

I am presenting an attached analysis of a 3-phase AC Motor by the brand WEG, with specifications at 1200kw, 1630 HP, operating at 50 Hz with 6 poles and a speed of 1000 rpm. The signatures provided reveal pole pass frequency side bands around 1x and its harmonics in high resolution. Additionally, a normal signature at 3000 Hz suggests a pattern of rotating looseness. To diagnose the issue of cracked or broken rotor bars, further confirmation is needed. Evaluating the severity of the defect and determining if noise is evidence of the fault are crucial steps in the process. Understanding the probable causes of the rotating looseness pattern is essential for effective troubleshooting. Please refer to the attached Spectrum Motor.pdf for more detailed information.

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Based on the analysis of your spectrum, it appears that your grid frequency is 60 Hz and the rotor speed is 1000 rpm, equivalent to 16.67 Hz. The presence of bearing issues is indicated by anomalies in the waveform and the absence of rotor bar frequencies. A crucial indicator of rotor bar defects is the rotor bar frequency, calculated by multiplying the number of rotor bars by the RPM. While velocity readings are within normal range, the acceleration value of around 2 Gs points to a severe issue, as indicated by the unique signature of the values. Look for any bearing defect frequencies in the spectrum, as their presence would confirm a bearing problem. The 1X harmonics and elevated noise floor often signify looseness, but the root cause seems to be related to the bearings. Feel free to share your findings for further discussion.

Dear Yasaswy, Thank you for your thorough investigation and valuable insights. In Egypt, the supply frequency is 50 Hz, different from the 60 Hz frequency in other countries. Although pole pass frequency is not visible in the spectrum, there are side bands around 1x and its harmonics 2x, 3x, 4x. A current signature analysis also reveals side bands around the line frequency. There is agreement on the presence of rotating looseness, but the source of the looseness, such as bearing frequency, remains unclear. Any additional recommendations for further investigation would be greatly appreciated. Attachment: Microsoft Word - id2current.doc.pdf (16 KB) - Version 1

When the pole pass frequency around 1x and 2xLF is present, it typically indicates an eccentric rotor. In this case, multiple issues are at play. The primary concern is significant looseness within the system, leading to impacts in the waveforms and pole pass frequency sidebands. It is likely that the bearing has sustained damage, causing looseness in the shaft and resulting in eccentric movement of the rotor. If there is noticeable noise, it is advisable to first inspect the bearing before proceeding with a solo run to address any potential issues.

Based on the data provided, it appears that there is evidence of looseness as mentioned before. This could be due to various factors such as a loose machine base, motor feet, bearing fit to shaft or housing, or coupling components. It is important to ensure the accuracy of the numbers in your data, which shows a calculated rpm of 997.2 based on a frequency of 16.62Hz. By subtracting this from 1000 rpm, we find a slip of 2.8. Multiplying this slip by the number of poles in the motor (assumed to be 6), we get 16.8 cpm or 0.28Hz pole pass sidebands. It seems unlikely that this issue is related to rotor bar problems, as the data shows a frequency of 0.43Hz. In my experience, sidebands resembling yours can be caused by a bearing outer race spinning in the housing. The presence of the 1x rpm and sidebands indicates the speed at which the bearing is rotating on the shaft or housing. While the impact in the time waveform is not excessive, it is clear that the looseness is a concern.

Hello Dave and Yasaswy, I want to express my gratitude for your thorough investigation and insightful response to my problem. I am strongly inclined to believe that the issue lies in the rotor bar defect, especially after analyzing the motor signature and observing sidebands around the line frequency at pole pass frequency. This has led me to be more suspicious of a rotor bar issue. To add to my confusion, I calculated a pole pass frequency of 0.438 Hz based on the actual RPM taken from the 1x frequency on the analyzer, which was 995.625. I would greatly appreciate your input on the current signature and eagerly await your recommendations. Thank you.

Thorough analysis you've done on the WEG motor, I appreciate the details you've provided. You're spot on about needing to evaluate the severity of potential rotor bar defects and using noise as an indicator. Rotating looseness can indeed be tricky to diagnose without the proper expertise. Your uploaded report will be a helpful reference for those tackling similar issues on their 3-phase AC motors. Thanks for the share!

Thanks for sharing this detailed analysis. However, I think a broader spectral analysis may help to isolate and identify other potential sources of vibration and noise, including mechanical and electrical roots. Ultimately, the application of advanced diagnostic tools, like vibration analysis, may pinpoint the problem more accurately. Equally, involving field service technicians who have hands-on experience with WEG motors could be a valuable addition to help evaluate this issue. Looking forward to more insights stemming from Spectrum Motor.pdf you attached.

Your presentation provides a comprehensive analysis of the 3-phase AC Motor; the data seems to suggest a major issue related to rotating looseness. However, I'd advise performing a further vibration analysis to be sure about the rotor bars' condition. Be sure to look into other possible causes of abnormal sound, like misalignment or bearing faults. The understanding of these mechanical failures is essential to prevent any severe damage to the motor and its components. In essence, a holistic approach always works best when diagnosing a potential problem. I'm interested to look into your Spectrum Motor.pdf for more context.

The level of detail in your analysis is impressive and I definitely see where you're coming from in terms of the potentials for rotor bar issues. I agree, it is essential to figure out whether the noise is a symptom of the defect or not. It might also be worth checking the motor's bearings as they can also create similar symptoms. Looking forward to going through the Spectrum Motor.pdf to get a deeper insight. Thanks for sharing this well-presented analysis!

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Frequently Asked Questions (FAQ)

FAQ: 1. What are the specifications of the 3-phase AC Motor being analyzed in the thread?

Answer: - The motor is by the brand WEG, with specifications at 1200kW, 1630 HP, operating at 50 Hz with 6 poles and a speed of 1000 rpm.

FAQ: 2. What is indicated by the pole pass frequency side bands and harmonics in the analysis?

Answer: - The signatures reveal pole pass frequency side bands around 1x and its harmonics in high resolution, suggesting certain patterns in the motor operation.

FAQ: 3. How is rotating looseness identified in the motor analysis?

Answer: - A normal signature at 3000 Hz indicates a pattern of rotating looseness in the motor that needs to be further investigated.

FAQ: 4. How can cracked or broken rotor bars be diagnosed in the motor?

Answer: - Further confirmation and evaluation of the severity of the defect, along with determining if noise is evidence of the fault, are crucial steps in diagnosing cracked or broken rotor bars.

FAQ: 5. Why is understanding the causes of rotating looseness essential in troubleshooting motor issues?

Answer: - Understanding the probable causes of the rotating looseness pattern is essential for effective troubleshooting and identifying the root cause of motor performance issues.

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