In last month’s posting, we briefly discussed about the importance of motors and motor testing. We also deliberated about the first step to take—personal preparation—and how they should understand the tester equipment, motor operation, and developing proficiency in testing. We then turned our attention to the second step—preparation for equipment to be tested—and criticality of equipment from multiple perspectives; operational, safety, logistical and environmental.

This month, we discuss the next three steps to take to gathering effective electrical motor data.

Step 3 – Preparation of Test Equipment

In order to perform motor testing effectively, the test equipment should be in optimal condition. The tester should be in calibration with the most recent updates to the operating software (Note: when updating software be aware of possible compatibility issues. New software updates should clearly explain any operating systems (OS) that are and are not supported.) Software upgrades are essential. Many times they correct inaccuracies or provide important safety procedures or steps. The associated equipage should be inspected and tested, as applicable, to prevent problems when in the field conducting tests. Conduct a check of the equipment the day before testing is scheduled. A simple generic checklist will help with the readiness of the equipment:

Checklist for Effective Testing:

• All batteries operable and charged
• Deep cycle batteries as required
• Test leads are free of any nicks or cuts in the insulation
• Voltage clips or test clips are clean and free of any foreign debris or corrosion
• Voltage clips or test clips are snuggly threaded or make a tight fit on test leads
• Current probes have good batteries (if applicable)
• Inspect power cords for nicks and breaks in the insulation
• Current probes jaws are clean and free of any foreign debris at both the top and hinge point
• Current probe spring tension is good
• Test lead connection points on the test instrument are clean and free of dust and foreign debris
• All electrical and data port cables on your test instrument are properly connected.

Step 4 – Check Operational Status of the Tester

Prior to acquisition of any test data, a quick operational test of the motor test instrument should be conducted. Use of a small test motor or stator will verify that the de-energized test data acquired, is accurate or repeat- able.

To verify accurate energized data, perform a quick power quality test. Place all current probes on one phase cable and run the test. Compare bus phase voltages to acquired voltages and all of the amperage readings should be the same. Once you feel comfortable that you are collecting reliable data, begin your testing route.

Step 5 – Maximize the Amount of Circuit Under Test (and the Amount of Load on the Circuit)

If you are devoting the time to conduct testing, you should test as much of the circuit as possible.

De-energized testing is usually conducted downstream of the de-energized contactor. With de-energized testing, connections made upstream will identify circuit anomalies between the connection point and the motor. Once identified, circuit isolation can be conducted and the source localized.

Energized testing should be performed from the starter cabinet, connections should provide at least one local level of circuit protection above the point of connection, i.e. connect on the load side of the main breaker or load side of the fuses. Energized testing can be used to observe voltage and current FFT data to isolate spectral peak sources from upstream or downstream of the test connection point.

Step 6 – Verify or Confirm Identified Anomalies

When a potential problem is identified, it is just that; a potential problem. You should take steps to validate that it is, in fact, a problem. Sometimes erroneous data or unique characteristics of the equipment under test may give indications of a fault. You should perform all possible equipment checks and run additional correlative tests to validate your indications. Let’s say, for example, your test data indicates a possible high resistance connection.

Check your test lead connections and re-run the resistance tests. If you get a current unbalance, check equipment loading to ensure that the unbalance is not due to insufficient loading. If load is sufficient save the data and run a quick power quality test, with all the current probes on one phase, to make sure that you do not have a defective probe. If you have Fpp (Field Pole Pass Frequency Sidebands) sidebands indicating possible rotor bar anomalies, check for swirl effect, current modulation, increased current draw for a given load and reduced

in-rush current with longer start duration. These simple checks and correlative measures, can prevent erroneous data leading to bad calls, which can cast doubt on either you, the technology, or both.

Next month, the last two steps will be examined.

The past two month’s postings have revolved around what it takes to get good, effective motor testing data. The first month we examined personal preparation and preparation for equipment to be tested. Last month we discussed preparation of the test equipment, checking the operational status of the tester, maximizing the amount of circuit under test, and finally, verifying or confirming identified anomalies.

In this last installment, we examine the last two steps to gathering effective electrical motor data.

Step 7 – When Possible, Correlate with Other Technologies

When possible, you should correlate acquired data with other technologies. This will help confirm the existence of a problem and help quantify the severity. Sometimes reliability technicians tend to try and be a “one-man band.” But working together as a “RELIABILITY GROUP” will yield immeasurable results. Reliability technologies are like a set of wrenches or sockets, they all have specific purposes, but overlap.

When they are used together you can work on most anything. The same with the reliability technologies; when used together you can diagnose most any problem. Proper use of vibration, ultrasound (shown in image), oil analysis, infrared and electric motor testing can provide a maintenance environment that will have minimal undiagnosed failures, resulting in maximum productivity and/or reliability.

Step 8 – Generate Effective Reports (Communication)

Believe it or not, reporting is probably the most important aspect of an effective motor testing program. The report is your deliverable. It can be the basis on how you are judged as a motor test technician. If you are a service oriented company, you already know or should know this. “In house” programs tend to sometimes neglect or minimize reporting which works to the overall detriment of the program. Budget expenditures are based on perceived value of the desired item. If your motor testing program does not appear effective, you may not be receiving the funding level you may require.

Not only do you have to generate effective reports on identified anomalies, you need to generate updated reports on the overall success of the program. Work with management to establish performance metrics or KPIs. Bar graphs showing the number of identified and corrected anomalies should be posted in high visibility venues. Display monthly and yearly discrepancy counts, hopefully it shows a marked decrease. Display the budget reductions for motor rewind and replacement or the number of rejected motors not put into service that may have failed prematurely. Specific examples will help you illustrate your point. Use the data you gather not only to identify and repair problems, but also to demonstrate the effectiveness of the program.

Communication, in addition to effective reports, is key to program success. You should have a network of communication established between yourself to middle and upper management, maintenance and production departments, as well as the other PdM technology personnel.

Communications with planning, safety and logistics are also important departments for procedural and material support. Integrate your EMT results into the site EAM (Enterprise Asset Management) reporting to further support these types of communication. Other areas of important communications are with the motor shops, motor manufacturers, the tester manufacturer and your EMT knowledge provider.

An effective motor testing program should be part of an effective reliability program. Use of the above 8 steps will provide your program with a significant and highly effective tool as part of a “World Class” maintenance program.

Today’s motor testers challenge the term “field portability,” in particular the de-energized test instruments.  The power supplies necessary to provide the high insulation test voltages are the main reason.  Expedient testing can be hampered by not carrying the necessary tools with you to handle the majority of the circuits and tests to be performed. 

So, let’s build the ultimate “Motor Testing Tool Kit.” The most important component of this kit is a means of carrying the test instrument(s), the necessary gear, and tools to support testing.  Utility carts are fine, but-limited in mobility. Moving them over rough surfaces and going up a set of stairs can be cumbersome at best. 

What I found, quite by accident, was a tool cart.  Years ago I was doing motor testing at a new customer site, a paper mill.  Their insurance company mandate was that there would be nothing in electrical equipment rooms except equipment.  Back in those days, I would carry an accessory bag and my tester.  When in an equipment room I would improvise and set my tester on a box, trash can, bucket….whatever I could find as a “desk-like” substitute to hold my tester.  Well, on this job I spent all of my time kneeling on a concrete floor, running my motor tests.  Needless to say, my first stop after returning home was a place to find a cart.  All I wanted was a two shelf cart on castors, preferably plastic, and narrow enough to lay under my Tonneau cover in my pickup bed for transport.  Simple enough, right?!

 I went everywhere and no one had what I needed.  On a whim I went into Lowes and asked a sales assistant where the utility carts where.  He guided me to an isle and I didn’t see what I wanted but what I did see revolutionized my testing methodology from that time on. It was a two-wheel tool cart made of heavy plastic with a hinged lid and a retractable handle to pull it with. It wasn’t what I wanted, but it was exactly what I needed! I immediately went about making modifications to enhance the cart’s usefulness.  It was the size of a large cooler and easily held all of my gear. However, it didn’t have dividers and when I would take my tester out; everything would fall to the bottom. I found some old arc shoots from a 4160V starter and box cut them so that I would have 4 compartments inside the cart. I used Velcro to hold them in place. I then cut up an old tool bag and riveted it to the dividers in sections.  The individual pouches gave me numerous storage slots. I added an extension cord reel carriage, bolted to the non-handle end and aligned with the cart hand grip, and I was almost there. I put Velcro on the top of the cart, and bottom of my tester, and Voila – a Motor Testing Tool Cart.

Now, what to put in there? 

(Besides the test instrument/s batteries, chargers, test leads, amp probes and PPE):

• Multimeter and test leads
• Spot Radiometer
• Digital Level and/or Inclinometer – for shaft position indication
• Machinist’s “V” Block magnetic base –for mounting inclinometer or level on motor shaft
• Electrical tape -- for phase labeling
• Wire labels – for lead removal identification
• Soft Wire Brush – for cleaning leads and terminals
• Strobe Tachometer
• Lockout Tagout materials
• Three to one jumper – for Wye Delta starters
• Miscellaneous jumpers – as needed
• Fuse pullers
• Screwdrivers – Phillips and Standard, popular sizes or an “all in one.”
• Sockets, Wrenches and Ratchets – for lead removal and re-installation as necessary
• Diagonal, needle-nose pliers, channel locks
• Solder and Soldering iron
• Flashlight
• Rags
• Clipboard, paper, pens, pencils, discrepancy labels
• Mili-Gauss meter – for flux measurement, particularly on Field Poles)
• Reference Materials: NEMA Pocket Engineering Handbook, Ugly’s Electrical Reference, Torque Specification Chart, etc. I also carry a pocket Vibration reference manual.
• Tie Wraps – various sizes
• Extra batteries (for flashlight, spot radiometer, amp probes – as applicable
• Folding Chair (sits nicely diagonally across the top of the box

What’s in your Motor Testing Tool Box?

This topic is so vast that I can't squeeze it into one post, so this is part one of a two-part blog post that I'd like to write on the subject. If your attention span is anything like mine, I'll try to keep this short! You're welcome.

One of the toughest things when discussing the real-life benefits of Electric Motor Testing (EMT) is overcoming the gap in most folks’ understanding of motors. Motors are everywhere. Take a look around wherever you’re reading this blog. I would wager that there are half a dozen electric motors of some variety within your field of vision. There’s at least one in your computer, running the cooling fan. Electric motors are a big part of our everyday lives, yet many people involved in maintenance and reliability aren’t really sure how they work, much less how the technology behind EMT works to discover motor failures in their infancy.

So for now, let’s start with how electric motors work. I’m going to simplify it here and discuss one of the most common AC motor designs, the squirrel cage motor. Its name comes from how the rotor (that’s the part in the middle that turns the shaft) looks if you could see only the rotor bars. A simple squirrel cage motor consists of a rotor and a stator. The stator is made of thin steel laminations, around which wires are wound in a particular pattern. These groups of wires are called “windings”, you know, since they’re wound around the stator. The windings consist of numerous layers (referred to in motor talk as “turns”) of wire called magnet wire. The winding turns are insulated from one another by a very thin layer of insulation, essentially like a veneer.& This insulation is so thin that many mistakenly believe the wires to be bare. It’s important to note though, the wires are in fact not bare, but are insulated from one another. If there was a conductive path through the individual wires, the motor wouldn’t operate properly. The breakdown of this insulation is one of the most common modes of failure in an electric motor, giving us what motor folks call “turn-to-turn shorts”.

The windings are wound in a particular arrangement to produce a rotating electromagnetic field when electric current is applied to the motor. When AC is applied to the motor stator, the electromagnetic field generated by the current passing through the windings induces electrical current in the rotor portion of the motor. The lines of magnetic flux (a topic for a much deeper discussion later) that emanate from the stator windings cut across the rotor bars, which is what induces the rotor current. As current is induced in the rotor, it flows through the rotor in a path that is created by the connection of the individual rotor bars and “end rings” on each end of the rotor. Many people don’t understand that the electric current on the rotor is induced by the stator, not supplied by motor wiring. Many have asked me how a motor turns while not damaging wires, believing the rotor to be wired also, which it isn’t. The current in the rotor is electromagnetically induced by the electromagnetic field produced in the stator.

Ok, stick with me here!

So, we have a rotating electromagnetic field in the motor stator that induces current in the motor rotor and the rotor in turn develops its own electromagnetic field that follows the field in the stator. The effect is much like what you see when playing with refrigerator magnets. You know how you can push one magnet around with another one? The only difference here is that the polarity is what causes that. When you try to push the positive pole of a magnet into another positive pole, the opposition in their fields pushes them apart. This is essentially what a simple electric motor does. The stator current “chases” the rotor current, causing the rotor to turn.

Chew on that for a while. Stay tuned for the next installment where we will talk about the relationship between the rotor and the stator, and how we are able to make determinations about motor health and operation by examining that relationship.

In the previous blog, Motors 101: Part I, we discussed how electric motors work, albeit a simplified version. Now that you’ve got a bit of a grip on that topic, it’s much easier to get your mind around what exactly electric motor testing (EMT) does for us. Many times when discussing the types of failures that can be discovered with EMT, I’m met with some level of disbelief. Much of this disbelief stems from a lack of familiarity of both motor design and how the testing protocols themselves work. So, let’s explore both Energized (online/dynamic) and De-energized (offline/static) EMT, and the modes of failure they can help us discover. The foundation for this understanding is your new found motor design knowledge.

In last week’s blog, we discussed the relationship between the stator windings and the rotor, in terms of the electromagnetic fields they each have present during motor operation. These two separate sets of electromagnetic fields depend on one another to operate the motor properly. When their relative proximity to one another is precisely maintained, the motor operates most efficiently. The stator fields “chase” the rotor fields and the motor turns. A number of factors however can cause the spatial relationship between motor components to be imprecise, affecting motor operation and ultimately motor health.

Motor bearing problems are chief among these. The motor bearing holds the shaft in place and is designed to make sure that the air gap between the rotor and stator remains precise while the motor is in operation. If a motor bearing begins to fail due to any number of factors, its ability to support the motor shaft suffers, and the air gap between rotor and stator becomes uneven. This unevenness, called “eccentricity”, causes the electromagnetic fields between the rotor and stator to interfere with one another. This reduces motor efficiency, which in turn causes the motor to require more current to perform the same amount of work as it would in optimal condition. More current equals more heat, and heat is the enemy of an electric motor.

Misalignment of the motor driven components can cause the same effect. Not only that, but driven component misalignment can exacerbate motor bearing problems as well. Misalignment problems are among the issues that Energized EMT can help us discover, due to the impact that this has on the electromagnetic fields of the motor. When the fields are not in precise proximity to one another the voltage, and current sine waves supplying the motor, will show distortion that corresponds to the magnetic interference caused by the eccentricity between the rotor and stator. You didn’t realize it was as simple as that did you? Well it isn’t, really. This is Motors 101 remember? Understanding the waveforms and interpreting the data provided by the Energized EMT test set takes experience.

Energized EMT and De-energized EMT provide two different sets of data because they each look at different aspects of motor health and operation. Energized EMT looks at the voltage and current into the motor and makes a determination of the motor condition as described above. De-energized EMT is a horse of a different color. This testing methodology allows us to take a look at the motor and motor circuit when the motor is out of operation. In its static condition the motor and motor circuit can yield us valuable information about the health of the motor.

The most important of these is winding impedance. Many uninitiated folk believe resistance and impedance to be synonymous, which is in fact untrue. Impedance has a resistive component but they are separate electrical properties. Resistance is the opposition to current flow while impedance is the opposition to a change in current flow. An imbalance in the amount of impedance between sets of motor windings can indicate shorts between individual turns in a set of motor windings. A great deal of electrical failures in motors begins with turn-to-turn shorts, which in turn are often caused by the motor getting too hot and the winding insulation breaking down. But that’s a discussion for another time.

Other failure modes can be detected with De-Energized EMT also, including static eccentricity, high resistance connections and rotor bar faults. Each of these is worthy of a blog of its own, so keep an eye out for forthcoming blog posts on those topics.

I was in the military for the better part of a decade, so acronyms are something I’ve grown accustomed to. But, sometimes their overuse gets downright ridiculous. I had an e-mail from a former student just a week ago reminding me of this that was chock full of letters. He wrote, “The NFPA says PPE for EMT.”Or, translated for the layperson, The National Fire Protection Association says Personal Protective Equipment for Electric Motor Testing. Whew … okay, maybe acronyms aren’t so bad, but alphabet soup aside, he needed an answer. Do you in fact need personal protective equipment for electric motor testing? Here’s what I told him.

In a word —yes. Electrical safety continues to evolve with each new revision of NFPA-70E (in case you’ve been living under a rock for the past 10 years that’s the Handbook of Electrical Safety in the Workplace) and the consensus standard that OSHA (the Occupational Safety and Health Administration) uses for the basis of their own electrical safety standard. 70E is about more than PPE, but the focus of many reliability folks rests squarely there. As with many standards it’s surrounded by speculation, mostly because people seem to afraid to just pick up the book and read it. So, some of you may be surprised to learn that even when performing de-energized EMT, you may indeed need to be adorned in arc flash rated PPE. Here’s why.

Most electrical folks are familiar with the concept of LOTO. More alphabet soup, sorry. Lock Out/Tag Out. Beginning two revisions ago or so, 70E started taking it an additional step, requiring equipment be placed in what’s called an “electrically safe work condition.” This is defined as de-energized, locked and tagged out, then measured to ensure de-energization has been achieved. Once equipment is in this condition, PPE requirements are relaxed. Here’s the kicker though, until you have measured with a meter to ensure the equipment is in fact shut down, you’re still supposed to be adorned in arc flash rated PPE applicable to the equipment in question.

So, even if you have turned off the disconnect for the motor starter, until you verify that the starter is in fact de-energized, you need to be wearing whatever the label on the starter indicates is required. Not only that, but if the line-side of the main breaker inside the starter is exposed, and stays energized when the starter itself is turned off, you’re still at risk and PPE is required if you’re inside the flash protection boundary of the starter. Depending on the starter configuration, and how far you can get away from the starter to do your testing, you may find yourself wearing arc flash rated PPE for what is considered a “de-energized” test method.

The best way to know for sure is to become as intimately familiar with the NFPA-70E as you can be. It’s only a $40 investment and it’s less than 100 pages long. That’s like eight Starbucks coffee drinks and a week’s worth of sports section. You can do it. You need to do it! Everyone wants to go home at the end of the day, eat wings, and drink beer, or whatever you do for fun. Safety makes that happen! That’s M2CW and HTH!

Just about any industrial facility, regardless of what business segment it represents, has an abundance of motors in use. When you’ve got motors, you’ve got spares. Shelf after shelf of spares, and motors are coming and going all the time. The shipping and receiving folks see a pallet with a motor strapped to it, they call maintenance/engineering and a forklift driver comes and hauls it to the designated shelf where it waits for its opportunity to get into the game. That’s the typical reality of receiving a new motor, but it shouldn’t be. The minute a new or rewound motor hits your dock, there’s an opportunity to determine if that motor should stay or go, based on its condition when you receive it.

Call it motor acceptance, Quality Assurance testing, it makes no difference. As every business, motor shops included, try to do more with less, things can fall through the cracks. Motor rewinding is an intricate process that requires an incredible level of attention to detail. Things can and do go wrong in the operation that can cause a motor to be in less than optimum condition fresh out of the process. Add to that the variable of material condition, like the porosity of the rotor or microscopic defects in stator laminations and the chance of receiving a motor that is 100% electrically and mechanically sound decreases. By following some basic testing steps however, you can separate the bad apples from the rest of the barrel and make sure your spares are ready to rock when you need them. Use any or all of the following test methods on new motors, preferably before they’re accepted from the vendor.

Polarization Index; we’ve talked about this one before. PI testing can be used to trend the condition of a motor, or to validate condition based on a standard. Insulation Capacitance can detect winding contamination, and is an excellent trending tool for motors in operation as well. Testing winding resistance for balance is very useful, because it tests the electrical conductivity through solder joints, end turn connections, essentially every connection internal to the motor. Imbalances can be the result of high resistance connections, and like many of these other testing methods is also useful for motors in service. Testing impedance of the windings can reveal turn-to-turn shorts, because they will cause an imbalance between phases. The Rotor Influence Check (RIC) is also an excellent test for a newly received motor. When the motor is received is probably the easiest time to perform a RIC test, since there’s nothing coupled to the shaft, so its position can be changed more easily. The RIC is essentially looking at impedance in different rotor positions, and can help find air gap eccentricity problems that resulted from the motor assembly process.

If your facility already has a comprehensive motor testing program, you may already be checking newly received motors. If you’re not, shhhhhh don’t tell anyone, just start doing it. If you don’t have a motor program, acceptance/QA testing is a great way to start one. Either way, check those motors when they hit the dock, and have increased confidence in your spares.

Motor Current Signature Analysis (MCSA), is a highly effective method for identifying many mechanical faults. Success will be directly related to the amount and accuracy of collected drive train data. Vendor software’s vary in the amount of drive train information, which can be included in the database. They also vary in capability of automated fault frequency identification. It is highly recommended that you conduct a “drive train map,” that will include all applicable information, such as; coupling type, pulley or sheave diameters, number of belts and belt lengths, sheave pitch diameters, number of fan or impeller blades, gear box ratio’s, bearing type, number of rolling elements, contact angles etc.  

If all of the information is not available, an effective alternative is to map the RPM at each stage of the drive train. Perform the mapping when the motor is at normal load, then set up alarm band frequency parameters based on load variance. When performing analysis, you can then refer to the annotated parameters and determine what drive component is the likely cause of the suspect frequency.

Most mechanical anomalies will cause an increase in the run speed spectral peak. 

Evaluating Belt Frequencies: Belt related faults are easily displayed in a current spectrum. They will always be less than the RPM. Because of this they will readily show in the low frequency spectrums. Identifying belt problems is especially easy as there are only a few things that can be anomalous with belt driven systems and all of the problems result in similar spectral representation. Belt faults include:

1. looseness
2. excessive tension
3. misaligned sheaves
4. eccentric sheave
5. bad belt seam
6. dished belt

When belt frequencies are present, the best resolution is to check belt tension and alignment of the sheaves. After alignment is completed and belt frequencies are still present, the problem is one of three things:

1. defective belt
2. eccentric driver sheave
3. eccentric driven sheave

Belt Frequency is calculated by:

The pulley diameter is the diameter to where the belt rides on the pulley, referred to as the pitch diameter.

If the belt length is not available or access for precise measurement is not possible other calculations can be made to determine the belt length. You will need to measure the pulley or sheave diameters for the driver and driven sheaves and the center to center distance between the sheaves. Run speed must also be known. 

MCSA provides an expedient method for identifying problems with belt driven equipment. Use of MCSA is also provides the capability of identifying many other electrical and mechanical problems. For more information on MCSA and many other motor testing methods, consider attending one of Snell Groups Electric Motor Testing formal training courses.

While conducting a mechanical IR route I was looking at a large 1000hp medium voltage motor, and something just did not sound right. There was a rhythmic hum in the motor that I had not heard before. There were no problems indicated in the thermal inspection in either the bearings, the stator area, or in the driven equipment. Noting the equipment number for later reference, I completed my route and returned to the control room to ask the operator if there were any problems with the 1000 hp that might have been indicated in the control software. The operator reported that all was normal and no alarms have been indicated. I returned to the PdM office and reviewed the motor data for that piece of equipment and did not see any anomalies. Having not found any data that would explain the sounds I heard, I asked the vibration tech in charge of that area if he had anything that might explain what I had heard. The vibration data did not have any alarm points triggered. The vibration section recorded data on that motor weekly. Whereas the online testing was conducted twice a year. I then decided to take online readings, and discovered that the data indicated there was a possible rotor issue, the pattern in the low resolution pointed to rotor bars as being the issue. Not having any other data that would support removing and replacing the 1500hp motor. The motor program was just starting (about one year) and there was not any confidence in the program at the maintenance management level of the customer (they put their faith in the vibration program). I decided to remove the inspection ports (a small 2.5 inch cover used to measure the air gap during assembly) while the motor was in use (operations would not shutdown the motor) a strobe light was used to examine the end ring of the rotor on the non-drive end. There were cracks at the point were the rotor bars were welded to the end ring. (data and images following)

Once the motor was replaced and the “damaged” motor taken to a motor shop, the motor shop reported that there were over 50% of the end ring welds were broken.

Vibration data “saw” the bars as the running frequency sidebands with slight elevation in the readings that did not trigger any of the alarms.

The data that I pulled on this motor was not scheduled. The normal route would have been in another month and half. That motor may not have lasted until the scheduled route. The sound difference that the motor was making led me to investigate the cause and prevented a catastrophic failure.

Being aware of changes in the sounds that you hear even while traveling thru the plant, they may lead to a finding that validates online motor testing.

Data taken (05/08/07) compared to data taken on 01/17/07 indicates that a change has happened for the determent of the motor. Data indicates that the rotor bars have suffered damage that may damage the stator if left in service.

In the upper table is a 10 second current trace on the motor while online showing the “surging” or cyclic pattern of the motor. This surging is related to the rotor bars that are not connected to the rotor circuit (broken end ring or solder joints between the copper bars and the end ring of the rotor) creating higher current demand to perform at the same level while the broken components are at the power point in the rotation (rotor slip).

The data below, in the same trace has far less “swing” in the current trace. And is not in a cyclic pattern.

The lower table above is the FFT current the sidebands around the LF (line frequency) are indicators of rotor bar faults. @ -39.85 db down faults are present. The FFT below has minor side bands @ -64.32 db down, at these lower levels “normal running” bar patterns are expected.

My job is quite fun. I get to spend time with a new group of people with every class, talk about something I enjoy immensely, and in a lot of cases I get to help folks even after training. If you’ve taken a class from me, you’ve heard me encourage you to call if you have questions. Often, I learn by figuring out answers to questions people ask. In some cases, the calls I get lead me to write a blog about that topic of discussion…, you know…sometimes…

Let me paint a picture here. Imagine you’re doing an inspection route of MCCs. You open a bucket, and you detect an anomaly. You’re excited, because this is your job after all; detecting defects. Oh wait, there’s more. There’s not just one anomaly, there’s two! One is on the output terminal block, one is on a fuse clip. They’re kind of low grade in terms of their apparent temp and the Delta over the nearby similar components, but daggone it you found them. Heck yeah! You write up your data, your escort closes the door and you move on to the next bucket.

Fast forward a week, you’re alerted that the motor being fed by the bucket you reported was replaced. The failure mode is a late-stage bearing failure. You’re asked to follow up on the bucket and see if there are still defects. In your mind, you say “Uhhh, what? How’s a bearing defect going to give me the kind of anomalies I found? It wouldn’t.” Then to your surprise, you inspect the bucket again and can’t find the issues you saw a week ago. You measured the load on the circuit at that point, but you didn’t on the previous inspection cycle. Oops.

Scratching your head? Yeah, I get that. How are these dots connecting? This next part is speculation on my part, but I think it makes sense. The current out of the bucket at the time of the second inspection (after the motor with the bearing issue was replaced) was 44% of the FLA rating of the motor. The NFPA-70B recommends loads at 40% minimum. The reason for this is that at loads below 40%, insufficient levels of heat are being generated for defect detection. My theory is that the load on the motor was higher during the first inspection cycle. The images taken of the detected anomalies had the distinct pattern that is consistent with increased electrical resistance. If the current on the circuit was higher at the time of the first inspection, those defects would be much more easily detected than at 44% of full load. There would have had to have been some other condition in addition to the detected bearing failure causing the set of conditions that made the discovery possible. If we knew what the load readings were at the time of the first inspection, we could maybe make some informed guesses. However, it wasn’t collected. I’ve no doubt that the previously detected defects are still present, and maybe with a tighter span setting might have been detected.

Let me mention something of great importance here. Only qualified electrical persons should attempt measuring current on energized electrical equipment. Even with a qualified person, there are risks. Appropriate arc flash PPE is imperative, as is shock protection. Applying a current probe will take the hands of the qualified electrical person inside the restricted approach boundary for any electrical device above 50 volts. The NFPA-70E specifies that the qualified electrical person be insulated from energized conductors or circuit parts when working within this shock protection boundary.

We beat this horse beyond expiration in Level I; it is imperative to collect every single piece of data that might be relevant to your findings. You can’t know when you’re collecting it whether you’ll ever need it or not. There’s an old adage about it being better to have something and not need it than need it and not have it. Collect it all, use what you need, archive the rest. Once you develop the habit of collecting all the pieces of data, it doesn’t take any additional time and it might even save your bacon one day. Do it!