Voltmeter and Megohm-meter testing
Howard W Penrose, Ph.D.
General Manager, ALL-TEST Pro
hpenrose@alltestpro.com
In the past, the most common method of testing a suspected motor fault in the field was to use a multi-meter and megohm meter. In this Blog, we will briefly discuss the methods involved in testing motors in this manner, including the limits of testing.
Safety is always a concern. Ensure, if you follow any of these methods, you are complying with your company, state and/or country safety regulations related to energized, or de-energized testing, as appropriate.
Testing for loose or poor starter contacts using a volt-meter:
With the circuit energized, place your voltage leads across the supply side of the starter (contactor) to the load side, ensuring that the volt-meter is set to the appropriate range. Reduce the range until an accurate voltage reading can be obtained. A result over 1 Volt AC indicates a loose connection that must be addressed.
Testing winding shorts and grounds:
When testing a suspected motor failure, there are several steps to using a multi-meter and megohm meter. (NOTE: This style of testing is limited in accuracy in most types of winding shorts and winding failures):
1) Remove the motor connection box. Observe any overheated or burned insulation odors. Note if the lead, or cable, insulation has been overheated or is brittle.
2) Remove insulating tape and disconnect motor leads.
3) Check resistance, using a multi-meter set at the highest range, connect across T1 to T2. Adjust resistance range down until a good reading is observed.
4) Repeat from T1 to T3 and T2 to T3. Test results should be within 3% from the average to greatest reading. (Va + Vb + Vc)/3 = Vave; ((Largest difference V – Vave)/Vave) x 100 = % Unbalance.
5) Tie the T-leads together and connect the megohm meter between a good ground point on the frame to another good ground point. Test and ensure results are ‘0’ megohms. Move one lead to the T-leads and take measurements for one minute. Note the megohm results.
Per the IEEE Std 43-2000, the following megohm voltages and test results should be obtained:
Random wound motors up to 600 Volts; 500 Vdc; 5 Megohms
Form wound motors up to 5000 Volts; 1000-2500 Vdc; 100 Megohms.
Problems with this method:
1) In cases where a motor is tripping occassionally offline, the value of insulation between conductors may still be high enough that low level DC resistance readings will not read across the fault point.
2) Insulation to ground readings require a direct path to ground. Faults that occur in the end turns of the motor will not allow for a path.
3) ‘Pin-hole’ shorts between conductors in VFD applications will not be detected using these methods, although the motor will not operate properly on the VFD, but will operate satisfactorily in bypass.
4) Insulation to ground faults occur in only 1 of 6 motors that fail due to winding faults.
5) The resistance limit of the multi-meter. Some meters have a range of only 0.2 Ohms of resistance. Low voltage motors’ resistance decreases as the motor size increases. Motors up to 50 horsepower have resistances above 0.2 Ohms.
Resolution:
For troubleshooting, motor circuit analysis techniques can be used to accurately determine the cause of motor problems. The technology uses low voltage AC output in order to determine changes to the impedance (complex AC resistance), and impedance components (such as phase angle), of the circuit. Motor circuit analysis devices are available for about the same cost as a good megohm tester and multi-meter, that are just as simple, and quick, to use. Tests can be performed from a motor control center or disconnect, quickly identifying winding and rotor faults.
Science and Engineering in Motor Maintenance - Philosophy
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
“Most of the fundamental ideas of science are essentially simple, and may, as a rule, be expressed in a language comprehensible to everyone.” – Albert Einstein
“Even for the physicist the description in plain language will be a criterion of the degree of understanding that has been reached.” – Werner Heisenberg
“If you cannot – in the long run – tell everyone what you have been doing, your doing has been worthless.” – Erwin Schrödinger
1.
There are two basic items you normally, by rule, do not discuss in this forum, religion and politics. I plan on breaking both rules.
Fundamentalism, in religion and politics, has the ability to become dangerous. By its very nature, it means a belief in the fundamental philosophical aspects of either. In the form of politics, this can be seen as a person who votes for a political party because of the political party and not because of the issues, and will also believe in all of the philosophies of that party without question (the ‘party line’). In the form of religion, this is, in effect, the expectation that a person will strictly believe in the rules and philosophies of their religion without question. In itself, this can be good or bad, depending on one’s view. The problem comes in when an individual or group that opposes the fundamentalist presents an argument. Some will present their argument and a discussion will ensue. However, others will have a violent reaction, in defense of their beliefs, ranging from personal attacks to, as being seen in this day and age, terrorism.
Is this problem unique? No. Even Galileo was seized by the Inquisition and forced to recant his theory that the earth revolved around the sun, as it had serious implications for the beliefs of the church at the time. Galileo persevered in the end. And, is it limited to politics and religion? No. It can even happen within science, itself. For instance, Einstein literally fought against the concept of Quantum Mechanics, an accepted modern branch of physics, because it did not agree with his view of a ‘unified theory’ of physics. (note: unified theory is the theory that all of physics can be brought to one basic law or formulae that can explain all physical phenomenon; Quantum Mechanics – the physics of atomic structure and the very small, the laws and theories conflict with some of the laws and theories of macro-physics and astro-physics). Authorities and experts are not always correct.
2.
Within engineering and science, there are two ‘classes,’ the scientist and the technician. The scientist pursues knowledge, development of new theories, and is, in effect, a pioneer. These are the Newtons, Einsteins, Galileos, and numerous, sometimes nameless, others. These engineers and scientists are the ‘change agents,’ the ones who introduce ‘disruptive’ thoughts, concepts and inventions into our lives. The technician is the engineer or scientist who works within the rules developed by others. This person (or group) will rely upon standards, tables and set formulae to perform their task or job. Both are necessary in order to perform science and engineering.
3.
In an interesting case, related to electric motors, that illustrates the battles of beliefs, and their impacts, due to disruptive technologies:
Nikola Tesla, the inventor of AC induction motors, transmission and generation started down the path of alternating current motors after being told by an engineering professor that it was impractical. When he first came to the United States, he worked for Thomas Edison as a technician repairing direct current motors and generating systems. He expressed his views on alternating current to Edison, who reacted negatively and they parted bitter enemies. Tesla went to Westinghouse who funded Tesla’s research as part of a deal to turn over the 1888 patents on alternating current technology. Tesla and Westinghouse worked together bidding on AC powerplants throughout the USA with Edison first fighting the bids with DC, then changing to AC. Edison went on to fight against AC by electrocuting animals on stage in demonstration of the ‘dangers of alternating current.’ When this had limited effect, he obtained a license on the AC patent and invented the electric chair for executions. He advertised it, publicly, as the first execution where the ‘victim’ was to be ‘Westinghoused.’ From historical accounts, it did not go very well, with several attempts being required before the prisoner expired.
4.
Over the past 25 years new technologies have been brought into the maintenance world. Some are accepted quickly, some are not, with fundamental beliefs being challenged in a number of cases.
Multimeters and MegOhm meters were some of the basic tools available to the average electrician, or maintenance technician, to check the condition of an electric motor for condition. In many cases, the electrician would have to make a judgment call, based upon experience, as to whether the motor was good or bad. It has been the norm to make decisions on the side of caution and many motors that are electrically good are replaced. In the 1980’s, motor circuit analysis was introduced. Based upon existing impedance-based tests, coupled with insulation to ground testing, accurate analysis could be achieved quickly using a hand-held device on any size motor or coil by comparing one phase or coil to the next. Acceptance of this technology has increased as users have experienced positive results in application.
With this, what has changed? The fundamental method of testing insulation to ground has been preserved. Instead of testing with DC resistance testing for winding shorts, an AC current at a low level is supplied. Phases are compared, just as before, except that now changes to the insulation system, and not just direct shorts, are determined.
In cases of electrical predictive maintenance, older instruments were bulky, heavy and impressed high voltages across the windings. This stressed the insulation system and, if the insulation system survived, it would be considered acceptable. If a fault, defect or contamination existed, immediate winding failure was probable as a result of the output of the instrument. The concept was similar to over-pressurizing a compressed air system in order to detect air leaks. Yes, some can be found, but others may be created in the bargain. In the 1990’s, motor circuit analysis technologies were introduced that expanded upon the initial 1980’s motor circuit analysis. The new system allowed for predictive maintenance and long term trending of insulation system condition by viewing resistance, impedance, inductance, phase angle and insulation resistance. Time estimations, without stressing the winding system, is possible with a hand-held instrument on equipment of virtually any size.
Again, the fundamental principles of insulation resistance and impedance-based testing are preserved.
5.
Now, the application of motor circuit testing, along with other ‘predictive’ technologies such as electrical signature analysis, vibration, infrared, ultrasonics and others falls upon the ability of the reliability engineer (or maintenance professional) to apply them. The principles of reliability engineering require change agents within a company and disruptive concepts to how business is performed. This type of change requires patience, communication with managers and production, and, the ability to show results in order to avoid negative reactions to new maintenance and reliability philosophies.
Originally Published at Ask an Expert
Question: Can you please tell me the probable causes for 2 X line frequency in motors. We had a problem of high vibration in one of our motors. Main contributor to vibration is 2 x line freq.. When we opened motor we found all things okay. Can you tell me reasons for 2 x line frequency.
Thanx,
Jeevan Jadhav
Answer:
Mr. Jadhav:
The number one reason for any 2FL (twice line frequency) signature is electrical. Now, the type of electrical problem will be the question:
1) Stator eccentricity, shorted laminations, loose iron and a loose stator core will cause a high 2FL frequency, normally without sidebands.
This will also occur with unbalanced voltage incoming voltage.
2) Some 3600 RPM, and to a limited extent, some 1800 RPM, low voltage concentric-wound motors will show 2FL just because of the placement of the coils in relation to the stator core and rotor. This signature is more pronounced when the motor is connected for Delta.
3)Pole pass frequency (or twice slip frequency) sidebands around running speed and the 2FL peak indicate eccentric rotor conditions (usually dynamic eccentricity).
4) Broken rotor bars will show as multiples of running speed with pole pass frequency sidebands and may also show as rotor bar pass frequency
(RBPF) harmonics. 2FL sidebands around RBPF harmonics indicate looseness in the rotor bars.
5) Loose connections will appear as sidebands around a 2FL peak as 1/3 FL peaks with harmonic sidebands.
The good news is that motor current signature analysis is designed to quickly detect these issues using the motor current instead of mechanical vibration (the two technologies complement each other tremendously).
Please visit our site: www.alltestpro.com for more information on motor current signature analysis (MCSA).
Howard W. Penrose, Ph.D.
General Manager, ALL-TEST Pro
A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-3180
Web: www.alltestpro.com
Originally Published at Ask an Expert
Question: Black Band Test
Can you give me any information on a test called a black band test used for adjusting DC motor or generator interpole strength?
Answer:
The Black Band test is commonly performed by contracted professionals who have specialized equipment to perform this test. The specialized equipment allows them to adjust the buck and boost of the field strength to identify the optimal balance and minimize arcing and sparking at the commutator. The equipment is temporarily connected into the field right at the motor. Adjusting the buck and boost in this nature needs to be performed with care to prevent a flashover of the commutator. If you are looking for more details please ask and I can research it further or reference another contact.
Best Regards,
Noah Bethel
PdMA Corp
www.pdma.com
Originally Published at Ask an Expert
Question: Electric Motor Storage
When storing motors, how often should the shaft be turned and how many turns. I'm asking because we installed a 500 HP motor and saw bearing defects immediately on startup. I think this is from sitting over a year without being turned. What are your thoughts?
Answer:
I worked at ALCOA in Davenport, IA for 30 years as an electrician, the last 15 years in electrical reliability. We had an spare motor inventory of approx. 600 ac and 250 dc motors. We experienced the same situation you described in your question, with a 1000 hp, 480 volt vertical motor. This motor been stored for about 10 years when the decision was made to install it. We sent the motor to our repair facility for general checkout. After disassembly the repair facility discovered the bearing were totally destroyed . We preformed some root cause analysis and came to the conclusion the bearing failure was caused by building vibration and the shaft never being rotated. This situation prompted us to start a motor shaft rotation program. We made round laminated paper discs, about three inches in diameter. The discs were divided into 12 equal sections with each section labeled with a month of the year. For example, if the program was started in September this month would be at 90 degrees. In October the shaft would be rotated 360 degrees and October would be left at 90 degrees. We developed a schedule with which we would rotate the shafts every month. Hope this information is useful.
Don Shaw
PdMA Corp
www.pdma.com
Originally Published at Ask an Expert
Question: Measuring winding resistance
I want to measure winding resistance of an electric motor
Answer:
Measuring the resistance of an electric motor is commonly performed to identify power circuit anomalies such as: Corroded terminals Loose cables Loose bus bars Corroded fuse clips Corroded contacts Open leads Different size conductors A 1994 demonstration project on industrial power distribution systems found that connectors and conductors were the source of 46% of the faults reducing motor efficiency. Comparing winding resistance before and after a rewind can verify that no reconfiguration of the windings occurred during a rewind which could reduce the efficiency. For accurate resistance measurements a 4-wire bridge resistance measuring device is recommended. The size of the motor can make a difference as well in that more resolution may be required for larger motors with very low resistances. PdMA Corporation manufacturer the MCE which offers digital low resistance micro-ohm range accuracy. Another manufacturers of DLRO capability is AVO-Biddle.
Best Regards,
Noah P. Bethel
Product Development PdMA Corporation
PdMA Corporation develops and manufactures dynamic and static electric motor test equipment for full evaluation of your electric motors. For more details please visit our website www.pdma.com or call us at 1-813-621-6463
Originally Published at Ask an Expert
Question: Stator slots
I noticed on the PDMA nameplate that they ask for # of stator slots. Is there a test out there some where we require to know the # of stator slots on a motor?
Answer:
I have not heard of an electrical test specifically requiring the number of stator slots. However, a future use of this information will be in providing additional information for stator winding anomaly indications. Loose stator windings are supposed to produce large peaks in the high frequency current spectrum at line frequency sidebands around the slot pass frequency (#stator slots) x (shaft RPM). Vibration analysis also looks for running speed sidebands around the slot pass frequency to identify loose stator coils specifically for a synchronous motor.
Best Regards,
Noah P. Bethel
Product Development
PdMA Corporation
PdMA Corporation develops and manufactures dynamic and static electric motor test equipment for full evaluation of your electric motors. For more details please visit our website pdma.com or call us at 1-813-621-6463
Ask your own questions here
More Motor Resources
Originally Published at Ask an Expert
Question: Test panel
We are starting a small rewind/repair shop & are thinking of building our own test panel to keep cost down,I would like to know want size transformer we would need to test up to 500hp 600volts, lock rotor test @ 120volts & no load test @ 575volts.Where could we get a wiring diagram.
Answer:
Design a soft-start system using either a 'step-voltage' system (ie: 120, 240, 460, 580 V, stepped manually using a push-button system working off of the transformer taps) or a soft start. This will reduce the inrush current to something manageable. In this case, a step-up transformer of 75 to 100 kVA would most likely work fine. Assuming that you wish to perform a locked rotor test at 120 Volts, the current could see the effective full load current (~440 Amps) which would require the application of a transformer rated at (440 A * 120 V * 1.73)/1000 = 91.3kVA. Recommendation: Delta-Wye transformer rated for at least 100 kVA with taps that would allow for 120V, 240V, 460V, and 580V. Set up a series of starters that will shut one voltage off before starting another using a push button control for soft starting.
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager,
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-7784
All Test Pro
Originally Published at Ask an Expert
Question: BEST WAY TO USE A MEGGOMETER
PLEASE TELL ME THE BEST WAY TO USE A MEGGOMETER ON AC/DC MOTORS, PM MOTORS,TRANSFORMERS AND TROUBLESHOOTING CONDUCTOR INSULATIONS.
Answer:
The standard that all of us have lived by for insulation resistance testing was re-issued in May, 2000. The IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery, had been updated because during the 1970's, several changes were made to the types of insulation used in rotating machines. The insulation resistance characteristics of these newer insulation systems are different from the older systems, and therefore required a substantial revision to the standard for measuring insulation resistance. The revised standard drastically changed a number of traditional testing programs for insulation resistance that had been in place for over 50 years, including insulation to ground tests. The purpose of insulation resistance (IR) reading is to evaluate the condition of the insulation between conductors and ground. This is done by applying a direct voltage between the conductors (windings) and the casing of the electric motor (machine) and measuring current leakage across the insulation system. The readings are applied to Ohm's law (R = V/I) and an resistance is provided. In the case of an insulation system, the current may be measured in milli- or micro-Ohms, with the lower the current reading, the higher the insulation resistance reading. These IR readings change over time because of dielectric absorption. Basically, the insulation system consists of polarized atoms that line up (polarize) with the applied DC voltage. As they polarize, the insulation resistance will increase. Insulation resistance readings have been used to troubleshoot and evaluate the condition of electric motors since the test method was first introduced, often with disastrous results. There are very clear limitations on the ability of insulation resistance tests to evaluate the condition of an electric motor for operation. For one thing, the fault has to have a direct path between the windings and casing of the machine. Air, mica or any other non-conducting material between the winding and ground will provide a high insulation resistance. Faults on the end-turns of motor windings will also not provide a clear path to ground most winding faults start out as internal winding shorts and may graduate to insulation faults, but not always. Excerpt from IEEE Std 43-2000: "Insulation resistance test data is useful in evaluating the presence of some insulation problems such as contamination, absorbed moisture or severe cracking; however, some limitations are as follows: a) IR of a winding is not directly related to its dielectric strength. Unless the defect is concentrated, it is impossible to specify the value of insulation resistance at which the insulation system of a winding will fail. b) Windings having an extremely large end arm surface area, large or slow speed machines, or machines with commutators [ie: DC] may have insulation values that are less than the recommended value. c) A single IR measurement at one particular voltage does not indicate whether foreign matter is concentrated or distributed throughout the winding. d) Direct voltage measurements, such as IR and PI tests, may not detect internal insulation voids caused by improper impregnation, thermal deterioration or thermal cycling in form wound stator coils" The recommended safe minimum values for rotating machinery are: a) For insulation systems before 1974 = 1 Meg-Ohm + 1 Meg-Ohm / kV rating of the machine. b) For insulation systems after 1974, random wound and under 1,000 Volts = 5 Meg-Ohms or greater. c) For armatures, form-wound equipment and machinery = 100 Meg-Ohms or greater
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-3180
All Test Pro
Originally Published at Ask an Expert
Question: Motor Test Voltage
I would like comments on how differing voltage levels will affect trending and/or accuracy of the data.
Answer:
Your Full Question: For offline motor testing (RTG) IEEE suggests the following test voltages for motors rated as follows: (3 phase line to line, single phase line to ground, and DC direct voltage) 500 volts for motors rated < 1000 volts 501 to 1000 volts for motors rated 1000 to 2500 volts 1000 to 2500 volts for motor rated 2501 to 5000 volts 2500 to 5000 volts for motors rated 5001 to 12000 volts 5000 to 10000 volts for motors rated > 12000 volts EASA suggests the following: 500 volts for motors rated <2400 volts 1000 volts for motrs rated > 2400 volts I know experienced motor testers who use 1000 volts on 480 motors and 5000 volts on 4160 motors on a regular basis and I have heard justification for these voltages due to 480 volts being an RMS value therefore the windings see 480 x 1.414 or ~ 678 volts. I disagree. The 480 volts is a phase to phase measurement and the test voltage applied is phase to ground. The motor sees ~ 277 volts rms phase to ground or 277 x 1.414 = ~392 volts peak. I would like comments on this logic and some ideas on how differing voltage levels will affect trending and/or accuracy of the data. Thank you. If it doesn't really matter...you can tell me that too!--------------------------------------------- Excellent question and it highlights the common misconceptions about insulation to ground testing. Even within the IEEE standards and guidelines you will find disagreement as to the values to be used when performing insulation to ground testing. Most rely upon the same data presented as far back as the early 1970's and have become ingrained into how testing is performed based upon habit. The thought that you should meet or exceed the voltage applied (ie: the peak voltage, for instance) again shows our ignorance of the actual purpose of insulation to ground testing. It is not, nor was meant to be, an insulation stress test, unlike that of a high potential test where the test is designed to provide an over-potential reading of the insulation. One of my favorite references is a book called "Preventive Maintenance of Electrical Equipment" by Charles Hubert, first published in 1955. The voltage ratings recommended are exactly the same as those shown in modern standards (500 Volts for 440-575 V equipment; 1000 - 2500 V for 2300 V equipment; and, 1000 to 5000 V for 4160 V and greater equipment). This was presented for the insulation systems of the time and the values are still used even though newer insulation systems exist. The reason is very simple: Insulation resistance (IR) (MegOhmMeter testing) is not an overpotential test! The purpose of an IR test is simply to evaluate the capacitance (leakage) of the insulation system between the windings, through the ground-wall insulation of the stator slots, to the frame. The only purpose for the higher voltage is to reduce the time to charge the winding and insulation system to the point where the potential across the winding is equal to the potential applied at the instrument and enough to allow a readable current for the instrument. The MegOhmMeter will then sense the value of the milli or micro-Amps of current across the insulation system (leakage through the dielectric) and convert the results to millions of ohms. The actual voltage value does not matter other than, if you are planning to trend insulation resistance, using the same voltage value, correcting for the WINDING temperature (not the ambient) and the surrounding humidity is necessary.
Sincerely,
Howard Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
All Test Pro
Ph: 860 399-5937
Ask your own questions here
Originally Published at Ask and Expert
Question: PAM motors
We have a couple of PAM motors on our site. I would like to get some information on these, especially a winding connection diagram and more info on how they work.
Answer:
The PAM motors are a special design and connection that provides odd speed ratios through one winding. Teco-Westinghouse keeps their PAM motor information very close to their chest. So far, in the past projects that I have been involved in, they will not release any information to repair shops, consultants or even the customer. The reason is simple, they want the repairs to go to Teco-Westinghouse authorized repair shops. However, I do have a contact there (a few years old) with a direct phone number, who may be able to assist you. Because it has been a few years, he may not even remember my name: Gerry Avery - 512-218-7249. He is the Westinghouse motor redesign specialist, and has such information available. He provided assistance when we were helping a PAM owner who had a motor repair shop remanufacture the rotor using the wrong rotor bar materials. Hope this helps. If we can be of any more assistance, please let me know.
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-7784
All Test Pro
Originally Published at Ask an Expert
Question: stator RTD's
What is the deciding factor when choosing stator RTD's? The choices being 100 Ohm Plat and 120 Ohm nickel.
Answer:
The primary differences are: Platinum RTD's tend to be more compact, have a greater temperature range and are more accurate. However, they tend to be more costly.
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro, A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-7784
All Test Pro
Zen and the Art of Motor Maintenance
Howard W Penrose, Ph.D.
General Manager, ALL-TEST Pro
A Division of BJM Corp
hpenrose@bjmcorp.com
Introduction
I have had a very successful carreer in the maintenance, trending and troubleshooting arena. Interestingly enough, very little of it has to do with my formal education, but more along the lines of my field experience and quality control training. In fact, there are times when the formal education gets in the way of real diagnostics.
In an excellent book, “Zen and the Art of Motorcycle Maintenance,” by Robert M Pirsig, the author covers a number of subjects related to items such as quality and, for the purposes of this paper, using the scientific method in troubleshooting. If you are involved in quality, reliability or maintenance, I highly recommend reading this book, several times, to see the philosophy behind quality, reliability and maintenance. The purpose, here, is to cover a little of both from a slightly different direction than is considered common.
Quality
Quality is a term that is tossed around far too much. It is often presented as if it has a truly definable meaning. Mind you, in the Webster Dictionary, quality is termed as “being of high quality,” and quality control: “an aggregate of activities (as design analysis and statistical sampling with inspection for defects) designed to ensure adequate quality especially in manufactured products.” As can be seen, the definition is kind of vague. From a philosophical standpoint, quality of a product or system can be thought of as something that ‘feels’ right.
In one extreme, if you have your brakes repaired on your car, take it from the repair shop and notice a loud ‘squeeking’ noise, you could consider the repair of low quality. On the other hand, if you take the car out of the shop and either do not notice any difference, or notice that the car handles better, then you would consider the repair to be of good quality. In one case, it feels wrong, in the other it feels right. In almost every case, the true meaning of quality is individually determined.
In the case of motor repair, the true rewind artist will have a knack for making, connecting and installing coils in such a way that the appearance of the motor winding is pleasant. No crossed wires, no scratched insulation, uniform distribution of the coils, well insulated connections with insulation that matches in length and appearance. Whereas, a winding replacement technician will put in coils and insulation, wires may cross and may even become scratched. Stray, or loose, wires in the coils will stick up, or will be ‘formed’ back into the coil. In both cases, the motor will most likely run. However, the first case will most likely run much longer, even in adverse environments. Traditional testing, in the second case, may show some defects in the beginning, but will most likely see them much later in its life (which may be seconds or years).
Most of the time, onsite inspections of your vendors coupled with proper testing with motor circuit analysis and motor current signature analysis will catch virtually all of the defects. This type of testing will also allow you to view the original condition of your equipment beforehand in order to compare to the final result. Short-cuts, changes to conductor size, poor insulation or shorted turns are all signs of less than acceptable repair practices. The test results can be used in order to provide quality control and allow you to determine if the motor feels right before and after installation.
Troubleshooting
What most people do not realize is that true troubleshooting uses the scientific method. Yes, that is right: View the system; Form a set of hypothesis for what is wrong with the system; and, then test each hypothesis until you confirm the fault. The true troubleshooting artist will understand the system they are evaluating, then will identify the potential problems. This requires experience with the system or type of system being tested.
For example, a noisey motor and gearbox. I will visualize (or sketch) the components of the system and how they work together. Using this knowledge, I will determine where the noise will most likely be coming from then determine what tests are necessary to confirm my hypothesis.
In another example, an electric motor, pump and variable frequency drive system has a problem. A pulsating noise eminates from the motor. The question will be whether or not the resonance is a motor problem, a drive problem or a pump problem. By considering the system, I can isolate the problem by setting up a series of hypothesis:
1. A pump problem will be the result of a flow issue or impellor
2. A motor problem may be the result of an eccentric rotor, rotor rub or a winding problem
3. The drive problem may be the result of the power electronics, a poor ground, poor feedback control, or may be a power supply problem.
To confirm my hypothesis, I select motor current signature analysis and motor circuit analysis techniques. The MCSA system allows us to check the pump, flow and impellor, the motor eccentricity, late stage winding problems and the condition of the drive output. The MCA system allows us to confirm the condition of the rotor and early stage winding faults.
Therefore, by visualizing the potential faults using experience, a series of hypothesis can be determined and tested.
Conclusion
Simply put, quality in evaluating systems is related to feeling that the system is right, quality control is a method of performing tests to an acceptable standard and troubleshooting involves visualization and the scientific method. Tests using motor circuit analysis and motor current signature analysis in the motor system can be used for quality control and troubleshooting in order to verify the condition of your equipment.
Both quality electric motor repair and troubleshooting methods are maintenance arts that require experience and knowledge of the system being tested. Tools such as motor circuit analysis and motor current signature analysis support the maintenance or reliability professional.
About the Author
Dr Howard W Penrose is the General Manager of ALL-TEST Pro, A Division of BJM Corp. He has over 20 years in the motor repair and reliability industry. Dr Penrose can be reached via email: hpenrose@bjmcorp.com or through the ALL-TEST Pro website: www.alltestpro.com.
Originally Published at Ask an Expert
Question: Out of Balance Motor
Being a tradesperson for many years I remember as a apprentice my foreman indicating 2 pole motors 3 phase will have sometimes a slight out of balance current characteristic compared to a 4 pole motor. At the time this was validated by a practical example,the motor resistence, voltages, were all equal and there was no shorts in the windings. 20 years on from this a apprentice has asked me why. Is this myth or can you provide some technical information for a explanation.
Answer:
The phase unbalance, in current, that you are seeing in 3600 RPM motors most likely has to do with the winding design. In high production, machine wound machines, the most common type of winding design is called 'concentric' winding and may sometimes be referred to as a 'basket' winding. The other type of winding is called 'lap' and appears almost as a vortex if you were looking through the motor. Lap windings tend to be very balanced, but time consuming to install. Concentric windings are used so that it is easy to pull them through the stator with little or no human intervention. There is a larger coil that goes across the diameter of the stator, then a slightly smaller coil, and so on. The first set of coils (one phase, usually) is positioned against the stator core, with the second set on top and the third set close to the rotor. The first set of coils is usually a little larger than the other two so that it can be positioned away from the core. The relationship between the position of the coils, the back iron of the stator core, air gap and the rotor, cause each phase to be slightly unbalanced electrically. The tolerance is set by NEMA as less than 5% impedance unbalance. As the coils have almost the same length of wire (ie: phase A might have 60 ft, phase B 55 ft, and so on), the phases will be balanced in resistance to within a few milli-Ohms. However, the position of each coil will impact the inductance of the coil, hence the impedance (consists of resistance, impedance, inductance, capacitance and frequency), which is the AC resistance of the coil. Because, simplistically, current is equal to voltage over impedance (not taking into account vectors), the current will have a resulting unbalance. This unbalance may appear to be greater in delta versus wye connected motors. This is also true in low voltage 4-pole motors, but not to the same extent because of the span (side to side distance) of the coils. To see the impact of the coils in the motor, you require the use of impedance-based measurement tools for de-energized testing.
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
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