#129: The Basics of Rotating Machine Testing

Welcome to Energy Talks, a regular podcast series featuring expert discussions on power system testing data management cyber security and important trends in the power industry. My name is Scott Williams from The Podcast Team at Omicron And I will be your host. Hello everyone! In this episode we are diving into the heart of Power Generation at utilities & industrial plants where the reliable operation of motors and generators plays critical role.

More specifically, we are going to discuss the basics of rotating machine testing... to help keep these machines running reliably. Joining me is Alexander Herrera an Omicron Applications Engineer who specializes in Rotating Machine Testing. Alexander has spent a lot of time recently in front of the camera for new Omicron video series about the basics of rotating machine testing, where he demonstrates step-by-step how to perform recommended diagnostic tests and also how to interpret data to reliably detect developing defects or prioritize condition based maintenance.

In addition, Alex will explain why following safety protocols during testing is essential. He will also highlight the importance of comparative measurements and offer useful tips for testing rotating machines. So without further delay hello Alex. welcome to energy talks.

Thank you Scott. it's a pleasure to be here Alex. many of the machines powering our infrastructure are now approaching the end-of their lifespans. What are the specific risks of navigating reliability while budgets and technical teams are shrinking?

Well, The main risk is that we're asking many machines to operate beyond their originally expected service life. But with less margin, Less time for maintenance And often fewer experienced people available to assist their condition. For large motors and generators Aging does not usually happen overnight. The insulation system, the stator core, the rotor and end windings.

And mechanical support structures are all exposed to thermal stress electrical stress vibration load cycling contamination and moisture. over many years these stresses accumulate! The challenge is that many of this degradation processes aren't visible. during normal operation Machine can appear healthy from the outside, but internally the insulation may already be weakening.

When budgets and technical resources are limited The biggest danger is making maintenance decisions with incomplete information. That could lead to two problems Either we overhaul machines unnecessarily which is expensive or we miss early warning signs and suffer an unexpected failure. That's why condition-based diagnostics are so important, they help us focus the available budget on expertise in machines that truly need attention. Interesting!

Electrical insulation is an important component to look at in terms of machine reliability. Why is the Stator winding considered the most valuable component? And why is specialized electrical testing needed, to give operators more visibility into the condition their machines? Stator wading is one of the most vulnerable components because it's exposed several stresses at same time.

electrically It has to withstand the operating voltage and transient overvoltages. Thermally, it heats up and cools down depending on the load. Mechanically, it is exposed to electromagnetic forces by version. Environmentally...

it might be affected by humidity dust oil chemicals or other contamination. All these factors act in the insulation system. Over time. they can produce cracks delamination, voids, surface striking, looseness or partial discharge activity.

The difficulty is that insulation degradation often hidden. You can not fully evaluate the health of its data winding only by looking at it from outside or monitoring temperature and vibration. Specialized electrical testing gives us a deeper view. Tests such as insulation resistance, polarization index, capacitance, tan delta with some voltage and partial discharge measurements, each look at the insulation but from a different angle.

Together they help us understand whether the winding is clean dry mechanically stable electrically strong free-from active local defects. in your new YouTube video series you start with measuring insulation resistance. Why is this considered the first line of defense for deciding if a machine is even safe to energize it? further testing?

Well, insulation resistance and polarization index are normally the first lines of defence because they help us detect major insulation problems before applying more demanding test voltages. The insulation resistance test gives an indication on how well the insulation is resisting DC leakage current. If the measure resistance is very low, it may suggest moisture contamination surface leakage or serious insulation deterioration. The polarization index adds a time component.

It shows whether the insulation resistance improves during test which typical for healthy and dry installation system Or if remains low unstable which can be a warning sign. I like to describe insulation resistance and polarization index as a basic safety and condition screening. They do not tell the complete story of the installation, but they help answer very important first question is this machine in reasonable conditions? To continue with further diagnostic testing?

if it's snow then applying higher voltages without fewer investigation could be risky. Alex, the next measurements you performed in part two of your video series about The Basics Of Rotating Machine Testing are the Capacitance and TAN Delta Measurements. In these measurements You compare resistive and capacitive currents. How do These measurements help a testing engineer assign to health grade To A machine's insulation system?

In a healthy installation system the current is mainly capacitive. that means the insulation behaves mostly like a dielectric between the conductor and ground, but real insulation is never perfect. There's always small resistive components which represent electric losses. TAN-delta or dissipation factor or power factor measures the relationship between these losses on the capacitive current.

when TAN delta is low unstable It usually indicates that the bulk insulation is in good condition. when tan delta is high or it increases significantly with voltage. It can indicate aging, moisture contamination, delamination voids and other insulation defects. That's why this test is useful for assigning a health grade to the insulation.

This gives us global view of electric conditions. Capacitance is also valuable. Changes in capacitance can indicate changes in the insulation geometry, moisture absorption or delamination. So together, capacitance and tan delta provide a very important picture of overall condition for the main insulation system.

The next measurement you performed on part three of this video series Is the voltage withstand measurement Also known as overvoltage or high pot test. This is a more aggressive assessment. Why do you specifically recommend performing this measurement with AC voltage? to verify that the insulation can handle its rated voltage within a safe margin?

The AC withstand test, it's not a test I will just casually or repeatedly without a clear reason. It is more aggressive than diagnostic trend measurements but when applied correctly according to standards OEM recommendations and the condition of machine It gives very valuable confirmation that the insulation still has enough electric strength and safety margin for operation. So another test you perform in part four of your video series is often referred to as the ultimate predictive tool.

This is a partial discharge also known as PD measurement, what can any PD measurements see regarding microscopic voids or resin bubbles and stator winding insulation that they other three tests might miss? Well partial discharge testing. it's powerful because it detects local defects inside installation system before become major failures. A small void, crack, delamination or resin bubble inside the insulation can create a local concentration of electric field.

When the electric field in that small cavity becomes high enough... a tiny electrical discharge occurs! That is what we call partial discharge. The important point is that this defect may be very small and very local.

Because of that, it may not significantly affect global measurements such as insulation resistance or tan delta in the early stages. PD measurement can detect this electrical activity. It does not simply get us a wrong number. You also provide information about discharge pattern, severity and sometimes possible defect type.

for me PD is one of the most viable predictive tools because it helps identify active insulation deterioration while there's still time to plant maintenance instead of reacting to a failure. Alex, before touching machine to perform electrical testing It is vital to follow specific safety protocol. Could you please walk us through the five safety rules for electrical testing and tell us why earthing in short circuiting are so critical when performing diagnostic measurements on machines?

Absolutely! Safety must always come before testing. The five safety roles are first, disconnect completely. second, secure against reconnection.

third verify the absence of operating voltage. fourth carry out earthing and short circutin. and fifth, provide protection against adjacent life parts. The fourth rule is especially important during diagnostic measurements because large machine windings behave like capacitors.

they can store electrical energy during testing specially after DC tests such as insulation resistance or polarisation index measurements. there can also be induced voltage from nearby energized equipment connected cables electromagnetic coupling in the substation or plant environment. By earthing and shore circuiting, we make sure that the winding is discharged on health at Earth's potential. This protects people working with a machine to prevent dangerous voltage reappearance.

In high-voltage testing, grounding isn't just formality. It is one of most important barriers between control tests. Alex, can you describe the importance of benchmarking test results and why you consider a single-test result just at data point but a trend? A diagnosis.

If you do not have the original factory acceptance or FAT tests data how do you begin to establish reliable baseline for benchmarking? Factory's acceptance test data is always valuable because it gives us an original fingerprint on our machine. But in practice especially with older machines, that is often missing. In this case the best approach to create a reliable baseline starting from next well performed diagnostic campaign means test setup must be well documented.

The engineer should record the test voltage temperature humidity connection method grounding configuration instrument used and condition of machine at time. From there, future measurements should be performed as consistently possible. The absolute value is important but the trend is often even more important! We can also compare results with similar machines previous service experience industry guidelines and OEM recommendations...

The key message is this if you do not have a historical baseline start building one now. A single result gives information, but repeated and comparable results give insight. Now we have covered the four measurements on stator windings. these again are insulation resistance and polarization index capacitance in tan delta voltage withstand and partial discharge.

In addition to this there a few additional recommended tests One can perform on a machine's stator core and rotor. Why should one also keep an eye the condition of these machines components? Because our rotating machine is not only a stator winding, it has complete electromagnetic and mechanical system. The Stator Core is responsible for guiding magnetic flux.

If insulation between core raminations are damaged, circulating currents can develop. These currents can reduce local overheating and in severe cases, major core damage. The rotor is also critical! Rotor winding problems turn to turn.

shorts, mechanical looseness, thermal imbalance or deformation can affect the magnetic field increase vibration produce efficiency. so even if these state winding tests look acceptable ,the machine may still have serious problems in the core or rotor. A complete condition assessment should include all critical components. That is the only way to understand a real risk of the machine as an asset.

One of these tests you show on part five in your video series, it's the StatorCore Low Energy or L-SID test. What are warning signs that you can detect with this test? The interlaminar insulation is failing and how does this test prevent damaging core melt? built from many thin steel laminations.

Each lamination is insulated form the next one, thus interlaminar insulation is essential because it limits eddy currents. if that insulation is damaged local circulating current can appear between laminatons. this current produces localized heating. The gardening signs are areas with abnormal magnetic losses localized heat in or increased fault current detected during the test.

This damage can come from mechanical impact, throwing objects overheating previous repairs of vibration. The so-called ELSI test helps us detect this problem areas at a relatively low excitation level. That is major advantage because we can identify core faults without needing to fully excite the core to the rated flux. If these defects are not detected, local heating can grow and eventually cause severe cold damage.

Sometimes referred to as a core mat. that type of failure can be extremely expensive may require major core repair or replacement. so ELSIID is very useful preventive test. it helps us find localized cold effects before they become catastrophic.

For the sweep frequency response analysis or SFRA on rotors, you inject a signal from Hertz to megahertz. How does this wide-frequency range help you catch a tiny short circuit between winding churns? SFRA works by creating an electrical fingerprint of the rotor winding over a wide frequency range. Different frequency ranges are sensitive to different physical and electrical characteristics.

At lower frequencies, the response is influenced mainly by inductance and magnetic behavior. At higher frequencies capacitance winding geometry and local connections become more influential. If there's a small turn-to-turn short circuit with an electrical path in the winding changes that changes the inductance losses And sometimes the capacitance of the winding. these changes appear as deviations to frequency response.

The value of SFRA is that it's very sensitive to small changes. By comparing the response with a previous measurement, another pole or similar rotor can identify abnormalities which may not be obvious during operation. so SFRA helps us detect developing broader winding problems before they evolve into more serious electrical and mechanical failures. So cross-test correlation seems to be an important area to cover.

For example, if an engineer sees low insulation resistance or polarization index but high tan delta levels for example how does cross testing help us decide of the machine simply needs cleaning we're drying or even a more thorough intervention? Cross-testing is essential because each test sees the insulation from different perspective. Low insulation resistance or low polarization index often point toward moisture contamination, or surface leakage. High tan delta can also be caused by moisture or contamination but it may also indicate deeper aging and degradation of the bulk insulation.

so if both insulation resistance polarization index and tan delta are poor The first question is Is this a surface or environmental problem? Or is it an internal insulation problem. A practical approach to inspect clean and dry the machine if moisture or contamination is suspected, And then repeat the measurements If results improve significantly. The problem was likely related to surface condition or humidity But if tan delta remains high If it increases strongly with voltage, its capacitance has changed or if partial discharge activity is present then we may be dealing more serious insulation degradation mechanism.

That's the value of cross-testing. It reduces uncertainty and helps avoid loss overreaction and underestimation. How can an asset manager use this condition-based data to prioritize the targeted diagnostic outage instead of a costly general overhaul of machine? Condition based data allows the asset manager to move from calendar-based maintenance, To risk-based Maintenance.

Instead of saying This Machine is old so we need to overhaul it We Can Ask A Better Question What is the actual condition of this machine? And what's the risk... of continued operation. If diagnostic tests show stable results over time, The machine may not need a major intervention immediately.

but if data shows increasing partial discharge activity rising tan delta low insulation resistance abnormal current decadence or widening deviations then that machine should receive. This allows the asset manager to plan a target's outage. The outage can focus on this specific component and failure mechanism indicated by diagnostic results, that means less downtime better use of maintenance budgets at lower risk of unexpected failures. In other words condition-based data helps convert technical measurements into practical maintenance priorities.

What is one common mistake or misunderstanding you see most often during offline testing that should be altered, or avoided? One common mistake in comparing test results are not really comparable. for example an engineer may compare today's insulation resistance or tan delta result with a previous value but The temperature, humidity, test voltage, grounding configuration or connection method may have been different. In that case the difference in results might not come from this machine but it comes with a test condition.

Good diagnostics require consistency. The setup must be correct and properly isolated. The grounding is well controlled and the environmental conditions must be documented. The other important point is interpretation, a test result is not just number.

it must connected to physical condition of machine. so my advice do not only measure correctly document carefully and interpret results in context. So Alex, before we go do you have any last tips or words of advice to our listeners about the basics of rotating machine testing? Yes I do.

My main advice is be proactive and systematic. Rotating machine failures often give warning signs before they become catastrophic. The challenge is that We need the right diagnostic tools And the right interpretation to recognize those Signs early. Start with safety then use consistent test procedures, build trends over time.

Do not rely on only one test combine the results and look for this story behind the data. also remember that diagnostics do not replace engineering judgment they support it. The real value comes when we connect measurements with machine design operating history maintenance history and failure mechanisms. When we do that properly, testing becomes much more than a maintenance activity.

It becomes a powerful tool to improve reliability reduce risk and extend the life of critical assets. Very well said! Alex thank you for joining me in this episode of Energy Talks. Thank You Scott.

it was a pleasure to join us on talk about these important topics. I hope this discussion helps listeners better understand how rotating machine testing supports safe, reliable and cost-effective operation of critical electrical assets. Omicron has several years of experience in power system testing, data management and cybersecurity. And offers a matching solution for your application.

For more information visit our website at omicronenergy.com. Please join us on the next episode of Energy Talks. Goodbye everyone!