Magnetic RMS readings

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Roy Smithson
Roy Smithson ✭✭✭
edited July 2023 in Ask a Question

I was looking at the Magnetic RSM readings I am getting from different motors. I noticed that some of the motors the magnetic readings are separate, and some are together. Could someone explain these readings and what faults they will identify.

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  • Christian Smith
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    Hi Roy. I'll need to pull in someone from the data science or R&D teams if you want a more scientific answer but I'll tell you what I've observed in my experience and how we are using the magnetic data.

    The magnetic field RMS amplitude level is greatly impacted by the location of the endpoint along the motor. Some points of the motor housing transmit greater levels of the magnetic field. So with the top screenshot we can surmise the EPs are both in locations where they have similar sensitivity to the magnetic field whereas in the bottom screenshot one of the EPs might be further from the stator or the motor electrical junction box, or across a flange, or something else. The magnetic field RMS value from one EP to the next or one motor to the next is less important than seeing how they change against their own trend and in relation to each other.

    There are dozens of data "features" we calculate using the magnetic data. The magnetic field rms is an indication of the overall strength of the field. It is most useful to observe when a motor is on or off. It occasionally can indicate changes in load or a wiring issue (like single-phasing), but its primary use for us analysts, is on/off determination and it can tip us off to fallen EPs or changes to the EP mounting which weren't reported, such as if the customer remounted them in a slightly different spot.

    The time sample of the magnetic data is translated into a spectrum using the Fast Fourier Transform just as we do with vibration. By doing so, we are able to determine the line frequency (at the EP level this is the LF peak frequency trend) being supplied to the motor which is extremely helpful with speed identification on VFD driven motors. Since we know the line frequency and motor rpm, we can calculate the rotor slip (Slip trend) and trend it over time. Sometimes we will see an increase in slip correlated with other changes in vibration and/or temperature which indicate load problems, such as build up of ice on an outdoor fan or buildup of material on a pump impeller, bogging it down.

    Additional features are calculated from the magnetic spectrum to attempt to identify electrical problems. One example which has shown promise is calculating the total sideband energy around the line frequency peaks. But most of the other magnetic feature trends are still in the research phase. Another outside the box example of when magnetic data is useful is if a belt breaks/falls off a fan. The magnetic field rms will confirm the motor is still running but the fan vibration will reduce to "off" levels. Similar, for a coupling failure. We hope to get in front of both of these issues, but I've seen it come to this!

    I hope this helps. Feel free to ask any follow up questions and I'll do my best to answer or pull in the right people.

  • Roy Smithson
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    Christian, thank you for the explaining this. I was looking for another way to identify if an end point may have fallen off. I have a motor in a heated area on the roof that is enclosed. I tried to set a temperature threshold and an RPM threshold but because the area is heated, and the area is vibrating from other close equipment I would still get readings if the endpoints fell off. This may what I need to try to identify if the endpoints are still in place. Thank you again for sharing you Knowlege.

  • Christian Smith
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    Yes, it can be helpful in that regard. One of the first things I check to verify a fallen endpoint is the acceleration and envelope trends. If those flat line on one endpoint but remain at normal running levels on the other EPs on the machine, it's extremely likely the endpoint has fallen. Velocity levels can rise or fall depending where the endpoint landed but to get acceleration/envelope transmission, you need a solid connection to the machine, so when it has fallen the value drops extremely low.

  • Christian Smith
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    While we are on the subject of endpoint issues, a great way to tell if an endpoint is loose is to look at the time waveform. If all of the peaks are exaggerated heavily in one direction (positive or negative) it's very likely the endpoint is loose. This phenomenon will cause the acceleration kurtosis trend to change and often cause a cliff detection on that trend. Whenever I review a cliff detection for kurtosis, the first thing I check is the time waveform and confirm it isn't just a loose endpoint.

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