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    Motors 101 - How Do They Work and What Does EMT Tell Me?
    Motor Talk
    The Snell Group

    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.

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    Chart - Voltage & Current vs Time
    Motor Talk

    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.

    Chart 1

    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.

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    Personal protective equipment for electric motor testing.
    Motor Talk

    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!

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    Sample of a pump motor.
    Motor Talk
    The Snell Group

    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.

    If you liked this post, you might enjoy our Knowledge Briefs Newsletter. Receive new posts delivered right to your inbox every week!
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    Motor Current Signature - Mechanical Fault Analysis: Belt Frequencies
    Motor Talk
    The Snell Group

    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 above equation provides belt frequencies in RPM. To convert to belt frequency to Hertz divide RPM by 60.

    Pulley Diameter

    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.

    MCSA

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  • 06/08/18--05:00: Sounds May Lead to Problems
  • Sounds May Lead to Problems
    Motor Talk
    The Snell Group

    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 05/08/07

    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.

    Data 01/17/07

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    Barre, VT, November 10, 2016: The Snell Group is pleased to announce new courses in South America for 2017. These courses are presented en Espanol and are part of The Snell Group’s continuing presence in South and Central America.

    “At The Snell Group, we’re always excited to bring our knowledge to new areas and our Spanish language courses have allowed us to share that knowledge with more thermography professionals in South and Central America than ever before,” said Jim Fritz, President and CEO of The Snell Group.

    The courses offered are:


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    Barre, VT, November 28, 2016: The Snell Group is pleased to announce that they will be offering a series of eight free live webcasts for the 2016-17 holiday season.

    The Snell Group’s webcasts combine the expertise of the industry’s most experienced independent knowledge provider with the convenience of web-based learning. These sessions are perfect for thermographers and electric motor technicians who are new to the field or are looking to supplement their training. Anyone with a computer and access to the internet can attend a webcast from The Snell Group.


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    Congratulations Ray Petrisek, the winner of the $249.00 SeeK™ Compact Thermal Imager for iPhone® and Android™ in our recent Giveaway! Thanks to everyone who participated and helped make it a success and stay tuned for our next free giveaway over the next few months!

    In case you missed it, you can check out our Giveaway/Sweepstakes page here: http://www.thesnellgroup.com/giveaway


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    Barre, VT, February 14, 2017: The Snell Group is pleased to announce that they will be hosting a half-day workshop for SMRP at the 2017 Symposium. The event will be held at the Hilton Atlanta Airport in Atlanta, GA from June 7-8, 2017, 8am-5pm.


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    Barre, VT February 16th, 2017: The Snell Group is pleased to announce a new member to their staff, Kelsie Bailey.

    Kelsie has joined The Snell Group as an Administrative Assistant. She will be assisting with pre- and post-course needs, logistics, customer support, handling the high volume of shipping of class materials worldwide, and working with the sales department.


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    Barre, VT, March 17, 2017: The Snell Group is pleased to announce a new member of their staff and team, Dave Sirmans.


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    Barre, VT, May 11, 2017: The Snell Group to Present at the SMRP Symposium June 7-8

    The Snell Group is pleased to announce that one of our instructors, Dave Sirmans, will be presenting a half-day workshop at the SMRP Symposium. The Symposium will be held at the Hilton Atlanta Airport in Atlanta, GA from June 7-8, 2017, 8am-5pm.


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    Barre, VT, June 27, 2017: The Snell Group is pleased to announce a new member of their staff and team, Lori Millington.


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    Barre, VT, July 26, 2017: The Snell Group is pleased to announce a new member of their staff and team, Catrine Turillon.

    We are excited to have added another great individual to The Snell Group team. With an extensive background in design and photography, Catrine has joined us here at The Snell Group as a marketing coordinator. Her previous experience in website design, print collateral, and corporate branding will be an asset in developing and expanding the marketing materials for The Snell Group.


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    The Snell Group South Africa IR Training a Success!

    Johannesburg, South Africa - September 18, 2017 - The Snell Group Level 1 Thermographic Applications and L2 Advanced Infrared Thermography courses, sponsored by Comtest, took place in Johannesburg, South Africa this August. They were a great success!

    “We are thrilled about the enthusiasm of the attendees,” stated Instructor and Consultant Elmer DeForest. “Some delegates that attended the Level 1 returned the following week for the Level 2.”


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    Barre, VT, September 19, 2017: The Snell Group announced that they will be presenting at the 2017 EASA, The Electro Mechanical Authority, Southeastern Chapter Fall Conference in North Charleston, South Carolina. Taking place September 28-30, this is the 78th Annual Fall Conference and we are proud to be a part of it! Attendees will choose between two technical program tracks.


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    Barre, VT, September 29, 2017: The Snell Group announced that they will be presenting at the 2017 SMRP, Society for Maintenance & Reliability Professionals Annual Conference in Kansas City, Missouri. From October 16-19, this annual gathering of over 1,000 maintenance, reliability and physical asset management professionals gives attendees the opportunity to help improve their skills and knowledge.


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    Barre, VT, October 6, 2017: The Snell Group is proud to be presenting at the 2017 IMVAC, International Machine Vibration Analysis Conference, being held on November 6-8 in Orlando, FL. This conference is designed specifically for vibration analysts and condition monitoring professionals.


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    Barre, VT, November 20, 2017: The Snell Group is pleased to announce a new addition to their staff and team, Alain McMurtrie.

    Alain will be joining us as an Inside Salesperson/Customer Relations Manager. His long history of working in sales and business development will be an asset to selling our services and expanding our global presence. We are excited to add another experienced individual to The Snell Group Team!

    For more information on The Snell Group and their service offerings, please call 1.800.636.9820.


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