Howard W Penrose, Ph.D.
Vice President, Electrical Reliability Programs
T-Solutions, Inc.
howard@motordiagnostics.com
Developing Your Motor Repair Specification 5
In working with the EASA AR100-2001, the first section is set up as many standards are normally organized, with general information.
The scope of the standard describes the standard as covering record keeping, testing, analysis and general repair guidelines for motors and generators. It is limited in that it is not meant to replace any customer or manufacturer specific instructions or specifications. This statement is important in that, if you have developed your own specification, the repair shop (or others) can not use this standard to refute your requirements.
The standard also excludes specific requirements, certifications and specifications for listed machines, hermetic motors (ie: many chiller machines), submersible motors, traction motors or Class 1E nuclear motors.
The next part of Section 1 is very important to the repair process and to you once the motor is returned. This includes labeling, records, nameplate information, terminal leads and connectors, the terminal box and cooling system. It ends with information on the exterior finish and packaging and transportation.
Records are important to both the repair center and motor owner, especially when a question related to a potential warranty arises. The information will be extremely important to any root-cause-failure- analysis. Although the standard does not call for it, my recommendation is that a copy of all test sheets and records are included as part of the report package supplied to the owner following repair.
The cleaning section discusses the requirement of cleanliness through the repair process. A common practice in repair is to use glyptol paint on the windings. When this occurs, it has several effects: The windings cannot be ‘dipped and baked’ in following repairs; and, The insulation class is limited to Class F (155 degrees C).
The standard also calls for proper terminal lead marking and how to attach terminal connectors. However, the terminal connection requirements only state that the terminal lugs should be crimped. In many cases, the connection is more secure if the leads are tinned as part of the terminal lug assembly.
The cooling system is extremely important as missing parts will change the airflow design. For instance, removing baffles will not improve, but will restrict airflow. Changing the fan design from original will impact both motor efficiency and the operating temperature.
In the next part of this lecture, we will cover the section on motor repair.
Howard W Penrose, Ph.D.
Vice President, Electrical Reliability Programs
T-Solutions, Inc.
howard@motordiagnostics.com
Developing Your Motor Repair Specification 4
One of the important considerations prior to repair shop selection is the development of a repair specification. In particular, this specification should be one that any qualify-able repair shop should be able to follow. One way to do this is to work with your local repair shop(s) in the development of the spec. However, you may end up with a specification that only that repair shop can provide.
I had mentioned in Part 4 of this series a good repair from a non-EASA motor repair shop and a bad repair from an EASA repair shop (EASA, by the way, stands for the Electrical Apparatus Service Association). Was this meant as a slight to the EASA trade association? No. It was meant to underscore that regardless of what organization a company is associated with, you are not guaranteed a good repair. The quality of the repair comes from the culture and policies of that individual repair shop.
The EASA organization has developed a number of tools for repair shops and repair shop customers. They also keep an eye on the status of the industry for their membership – EASA is, after all, a trade association and is responsible to the members of their trade. As such, they provide engineering support, design information, guidelines, marketing and training materials, and more, for their membership. At the same time, they have realized, and acted upon, the demands of the average customer by providing educational and decision-making tools with the significant offering of a free electric motor repair specification.
The materials and reports are readily available from the EASA website: http://www.easa.com. Through the next part of this lecture series, we will be reviewing the ANSI/EASA AR100-2001 ‘Recommended Practice for the Repair of Rotating Electrical Apparatus’ standard. If you are in the process of developing your specification, I will be commenting on the application side of each part of the standard with recommendations that can be included in your specification. (The standard can be downloaded directly from http://www.easa.com under ‘Industry Info.’ You will also find a great deal of additional information.)
Should you abandon your present repair shop if they are not an EASA member? No, to that question, as well. Many of the mid to large sized repair shops that are not EASA members have their own engineering and/or expert staff.
The key to the selection of a repair shop is to ensure that some type of Quality Assurance process is in place (“If it is not in writing, it didn’t happen”). The most common QA programs are the ISO 9000 and EASA-Q programs. A world-class repair shop will have either of these programs, or equivalent. The EASA-Q program was developed by EASA, follows the spirit of the ISO 9000 program, but is directed to repair shops by adding additional requirements related to repair.
Tomorrow morning, I will begin with the first section of the EASA Standard AR100-2001.
(Also, starting January 1, 2005, I will be available for direct assistance in the development of motor repair standards and certification of motor repair shops to your repair spec.: http://www.motordiagnostics.com/tsol/index.htm).
As some of you may have noticed, I am in the process of transitioning between positions with ALL-TEST Pro and T-Solutions, Inc.
From my recent involvement in the Reliability and Condition Based Monitoring programs with the military and my involvement in industry standards, certification program development, the huge success of ALL-TEST Pro with the US Coast Guard and training, I have been given the opportunity to assist the industry in developing motor diagnostics programs and electrical reliability programs. As a result, I will be leaving my position with ALL-TEST Pro effective December 31, 2004 and will be starting as Vice President of Electrical Reliability Programs for T-Solutions, Inc. on January 1, 2005. If you are wondering, I will be a senior consultant to ALL-TEST Pro for the indefinite future.
Besides, it now gives all the other stars of ALL-TEST Pro the opportunity to make names for themselves.
Will this reduce or remove my involvement from the industry? Will I reduce, or eliminate, my work with ReliabilityWeb.com and other areas that I have been involved in? Absolutely not! In fact, quite the opposite: In addition to the work that I have already been performing, without the work of overseeing motor diagnostics products, I will be assisting companies in the development of their motor diagnostics, maintenance and motor management programs. It also gives me leeway in the development of a dream: The Electrical Motor Diagnostics Institute.
T-Solutions, Inc. is a military contractor for the development of RCM programs for the US Navy, US Coast Guard and others. I will be directly responsible for the development of the Electrical Reliability Group and the introduction of T-Solutions, Inc.’s services into industry world-wide.
For more information, while the website is being updated, go to: http://www.motordiagnostics.com/tsol/index.htm.
If you have an interest in these services, contact me.
Sincerely,
Howard W Penrose, Ph.D.
Vice President, Electrical Reliability Group
howard@motordiagnostics.com
Howard W Penrose, Ph.D.
Vice President, Electrical Reliability Programs
T-Solutions, Inc.
howard@motordiagnostics.com
Developing Your Motor Repair Specification 3
1. Initial Winding Tests
Upon receipt by an electric motor shop, certain tests should be performed, as a minimum. The following steps are common to quality motor repair and also an explanation of the different methods used in repair.
The first test is an insulation to ground test, which measures leakage to ground. For motors rated under 600 Vac, 500 VDC is the acceptable limit, with a reading of 5 Megohms as the absolute lowest reading. However, a reading below several hundred Megohms should indicate some type of problem. A reading of zero indicates a direct short to ground. The applied voltage and limits for medium voltage and DC motors will be covered in the test limits blog.
In many cases, a motor repair shop will test the phase to phase resistance of the electric motor with a milli-ohmmeter, or wheatstone bridge, then attempt to operate the electric motor before disassembly (assuming the motor passes these incoming tests). This is done to indicate what types of defects are within the motor. For electrical testing, the phase current is taken at full voltage, no load, and both noted for later use and compared to ensure that one phase is not drawing more current than the others.
Modern repair shops have started to perform motor circuit analysis tests on their motors prior to disassembly. This allows for the immediate detection of winding and rotor defects prior to disassembly and test. For smaller motors, this may mean that a repair versus replace decision can be made before the motor is disassembled, saving the repair shop and motor owner hours of lost time and repair costs.
If the motor passes these tests, it is disassembled and cleaned using solvent, hot soap and water, steam, or some other accepted method. If the stator has been cleaned with soap and water, it must be dried, before further testing, in an oven set for a temperature of around 196oF (90oC). If damage occurs to the insulation as a result of cleaning, or if the insulation appears to have minor defects, it may be dipped and baked in a Class F, or better, insulation varnish.
Once cleaned, the windings should have a second MCA test prior to an AC or DC hi-potential test performed at a voltage figured in Formula 1. The AC hi-pot is a pass/fail test, as if it arcs to ground, the insulation will be damaged beyond repair. The DC hi-pot is more forgiving, especially if the leakage can be monitored. Any sudden increase indicates that the insulation has failed. If it is below the calculated voltage value, when it fails, then the winding should be rewound.
Formula 1: Test Voltage
Vac = 0.65 * (2Em + 1000V)
Vdc = 0.65 * (2Em + 1000V) * 1.7
Where
Em = the motor nameplate voltage
If the motor completes this test successfully, it should be surge comparison tested. The voltage value limit for this test is the same as that determined in Formula 1. In this test, however, wire insulation is being compared. This test is meant to detect shorts within the windings themselves. It is normally done by setting the surge tester to a value of Zero volts and bringing it up, slowly, to the calculated value. The tester sends a high frequency surge to the windings and the results are read on an oscilloscope comparing at least two of the windings at a time. Once properly set, any deviation in the scope waveforms indicate a defect. This test is considered a pass/fail test should a defect be detected, it will normally finish off the weakness. It must also be considered that the surge test will not detect broken turns, loose connections and may miss obvious shorts deeper than the first few turns of the windings. Therefore it should be coupled with the MCA test for greater accuracy.
There are no reasons why non-destructive tests, above and beyond these, may not be performed. A world -class quality repair shop will do whatever is necessary to ensure that no surprises occur during the motor repair process.
2. Mechanical Tests
All of the mechanical fits on the motor must be tested using calibrated outside and inside micrometers. The critical areas which effect efficiency include the bearing journals and housings. If the fits are too loose, or tight, both the efficiency will be reduced and the bearing life will be reduced.
There are several ways to return bearing fits, which include:
• Peening - The practice of punching or marring mechanical fits to create a tighter fit. This practice is not recommended for repair as it is "uncontrolled."
• Metalizing - Consists of a one or two part spray process which requires metal to be removed first. This process is susceptible to separation from the material to which it is attached in instances of non-symmetrical pressure, or when the surfaces have not been properly prepared. This practice should not be used for "world class" energy efficient motor repair.
• Welding - Similar to matalizing, however, it creates a stronger metal to metal bond, when properly applied. If a repair requires adding metal, this is the preferred method.
• Sleeving - The process of returning fits by machining and sleeving a motor shaft or housing. This is the recommended method of motor repair, as it is more controlled.
• Refabrication - While expensive, this method is the best for machining severely worn motor parts, shafts in particular.
It is also highly recommended that motor bearings are replaced during each repair. They should also be replaced with the original class of bearing. Internal bearing fits and friction can have a large effect on motor efficiency. Fan replacement should also be considered, when the original fan has been damaged. The replacement fan should be original, as well. If a fan is replaced by a larger fan, or one with more fins, the motor efficiency will be reduced. If a fan is replaced by a smaller fan, or one with fewer fins, cooling will be reduced, reducing the life of the motor.
3. Coil Removal Practices
At this point, and for the purpose of this blog, it is assumed that the motor has failed at least one of the tests outlined above. The stator will have to be "stripped," meaning that the copper windings will have to be removed, before re-insulating and rewinding the motor. It is "best practice" to perform a core test before and after the stator is stripped. The Wattage per pound losses should be recorded, and should be found not to increase.
In all the motor stripping practices, one end of the coil winding is removed. The length of the end-turns must be measured first and any connection and/or other information collected and recorded. Then one of the following methods is used for removing the remaining wire:
• Direct Flame - A flame from a torch, or other source, is directed onto the core and winding. In some cases, the stator is physically placed in a bonfire! The temperature is uncontrolled and severe damage to the core will occur. The winding is reduced to ash, and the windings remove.
• Chemical Stripping - The core is lowered into a chlorinated solvent bath and kept submerged until the varnish is dissolved enough for coil removal. Chemical stripping is ineffective in many cases, such as overloaded stators. The chlorinated solvent presents potential health, environmental, and disposal problems. In some cases, the solvent is not completely removed when the stator is rewound, and the solvent works against the new motor insulation.
• Burnout - The stator is placed into a burnout oven that is set for a recommended temperature of 650oF (345oC). It is kept at this temperature until all of the varnish and insulating materials are turned to ash (8 hours, or more). If the temperature exceeds this, damage to the stator core and frame may result, reducing motor efficiency and mechanical reliability. Gasses, and other by-products, are exhausted through a "smoke stack" into the atmosphere.
• Mechanical Stripping (Dreisilker / Thumm Method) - Using a heat source, such as gas jets, a distance away from the core, the back iron and insulation is warmed until the windings become soft and pliable (approximately 10oC above the insulation class of the varnish insulation). The coils and insulation are removed using a slow, steady hydraulic pull. Temperatures remain low, stripping times extremely fast (ie: 2.5 hours for a 350hp motor), and there are no airborne by-products nor disposal problems. Attempts at duplicating this process using pneumatic pulling methods have resulted in core laminations being pulled apart. Therefore, pneumatic machines, of this type should be avoided.
• Mechanical Stripping (Water Blasting) - A high pressure stream of water is used to blast the coils out of the stator slots. This is a fast method of coil removal. Personal injury, due to high water pressure, and mechanical damage, can be avoided by experienced personnel and safety devices.
• Mechanical Stripping (Hot Vapor Process Chemical Stripping) - A stator is submerged in a bath of non-chlorinated petroleum-based solvent at a temperature of 370oF (190oC) for a short period of time. It is then removed and the coils removed with high-pressure air. The solvent has an oily smell which must be masked, and is difficult to dispose of. Personal injury and mechanical damage can be avoided by experienced personnel and safety devices.
Once the windings have been removed, the stator may have to be cleaned. This may be done by steam cleaning and baking, bead or cob blasting, or low pressure air. In some cases, additional copper, that may have fused to the core at the time of motor failure, will have to be removed. This is done with a small air grinder or jeweler's files.
The stator should then receive a loop test which is performed to check for "hot-spots" within the stator core caused by shorted laminations. If these are found, they may be removed by separating the effected laminations and insulating them, then pressing them back together. Other methods include a dip and bake before rewinding, or VPI'ing the stator core. In some cases, the core losses or hot-spots may be excessive causing the stator core to have to be re-stacked or the motor replaced.
4. Stator Winding
Common rewind practice dictates that the paper insulation inserted into the stator slots be of Class F insulating material, or better. The most common is Class H. The reason for this is to allow the motor insulation to survive any hot-spots which may have been missed during the loop or core loss tests. This also has the effect of potentially increasing the insulation life of the motor beyond the original design, and allowing some "forgiveness" if the original cause of insulation failure has not been corrected when the motor has been returned to service.
It is "best practice" to rewind the motor with the same wire size and type of coil winding method (lap or concentric). In some cases this is not possible. If the wire size must change, it must maintain the same cross-sectional area. A general rule of thumb is, for every three wire sizes smaller, two wires will be the same. For instance, if one number 15 wire is required, two number 18 wires may have to suffice. If the wire size is made smaller, the I2R losses will increase, decreasing motor efficiency and reliability, if it is made much larger, there is the chance of over-filling the stator slots, or increasing the motor's inrush current. It is best to create a sample coil to ensure that the coil ends are the correct length and the coils will fit in the stator slots.
There are several coil winding methods:
• Hand - Winding - Performed with a "tower-type" winding machine and mechanical counter. The winding technician must try to maintain correct tension and layering of the coils, or the coils will be difficult to lay in the stator slots. In the "worst-case," there will be wires crossing which will increase the turn to turn potential in the wire, creating an area which may short under certain operating conditions. Improper tensioning of the coils may cause more wire per phase, changing the impedance balance of the motor windings.
• Automatic Coil Winding Machines - Maintain constant tension and proper count of the coils. Still require a technician to observe operation, but still succeeds in reducing labor time.
• Computerized Coil Winding Machines - The technician is free to perform other tasks while the machine winds the stator coils. Proper tension and turn count is maintained.
The coils are then inserted by hand or machine. It is important to include phase insulation and "in-betweens," in order to avoid phase to phase or coil to coil shorts when the motor is returned to operation.
Once the coils have been inserted, the coil ends are insulated and connected. The stator connection must be the same as the original, and the coil ends crimped, silver-soldered, or braized. The lead wire must be of the correct size and type for the motor current and application. After this phase, the coil ends are tied down for mechanical strength. The ties should pass between each coil slot and tied. Care should be taken not to pull up the phase insulation.
5. Post Winding Tests
An insulation to ground test should be performed on the rewound stator of 500VDC, for motors rated under 600 Vac. The windings should show a resistance of better than 1000 Megohms (based upon experience).
A Hi-Potential test should be performed at a value calculated in Formula 2. Passing results and methods are outlined in the Initial Winding Tests. The Surge Comparison Test should be the same as in the Initial Winding Tests, except at the Formula 2 value. It should be noted that the surge test will act as a pass/fail and will not detect loose connections, broken conductors nor defects deeper than the first few turns of the coils conductors. Therefore, an MCA test is recommended along with the surge test for greater results.
Formula 2: Test Voltage
Vac = 2Em + 1000V
Vdc = (2Em + 1000) * 1.7
Where
Em = the motor nameplate voltage
Additional tests include an Impedance test(MCA) and a Spin test. The impedance test is a comparison between all three phases. The difference should not be more than +/- 3%. The Spin test consists of placing 10% of the nameplate three-phase voltage across three of the stator lead wires. A current reading is taken and compared. Then a ball bearing or test rotor is inserted into the stator core. If the windings are correct, the bearing should rotate within the stator core, or the test rotor will operate in the same direction as it is brought around the inside of the stator core.
All test results should be recorded for future reference.
6. Varnish Insulation
The final step in the rewind process is to varnish the stator. The purpose of varnish is to increase the mechanical and electrical strength of the stator windings. As with the slot insulation, it is common practice to use Class F or H varnish on the stators. There are several basic methods for insulating rewound stators:
• Dip and Bake - The stator is pre-heated then dipped into a tank full of insulating varnish. This is normally done a minimum of two times to ensure a full coat of varnish. Care must be taken as voids may be left within the stator coils which may collect moisture, or other contaminants. Additionally, all of the surfaces, including machined areas, are covered with varnish, which must be removed (and constitutes wasted varnish material). While the slots are receiving a reasonable amount of varnish, to allow for heat conduction, a blanket of varnish collects on the outer surfaces of the motor, reducing its ability to cool itself.
• Trickle Varnishing - The stator is placed on a turntable and connected to three-phase power. This both serves as a heating source for the windings and an additional powered test (the coils should heat evenly). The stator is heated horizontally and monitored with an infra-red sensor. Once the windings have reached a pre-determined temperature, the turntable is tilted to 35 to 45 degrees and varnish is trickled on to the windings through several tubes. The varnish is drawn through the slots by gravity and capillary action creating a solid slot fill. The varnish also collects on the end turns. In considerably less time than two dips and bakes, the stator windings will have the equivalent of three dips and bakes (1 to 2.5 hours as opposed to 16 to 20 hours). There is no excessive varnish, decreasing cleaning time and varnish waste.
• Vacuum Pressure Impregnation (VPI) - Due to expense, this process is not recommended for low voltage stators, but is a must for medium voltage, form wound cores. It consists of a voidless slot fill (as the trickle varnish method), but wastes varnish ( as does the dip and bake). The stator is warmed in an oven, then placed in a VPI tank. A vacuum is drawn within the tank, then varnish is flushed in from a holding tank. A pressure is then applied to the tank forcing varnish into all existing voids. The stator must then be placed in a baking oven to cure the varnish.
7. Rotor Tests
The rotor should be tested upon disassembly, using MCA, or during the repair evaluation phase using growler, die or single-phase testing. The rotor must be balanced with all rotating components mounted on the shaft and at least a half-key in any open keyways.
8. Final Tests
Once the stator has been varnished and cleaned, noting that abrasives on the stator laminations may cause shorting between laminations, the motor is assembled. (In "world class" repair centers, the stator is retested before assembly.) An insulation to ground test is performed once the motor has been assembled, and should measure at least 1000 M-ohms. The electric motor is then tested at no load and all rated voltages for 30 minutes. The current and voltage is measured and recorded, if the motor had been tested during the disassembly phase of the repair, the final results are compared to the first. Also, the temperature of the stator is checked, and should remain cool to the touch, when operated at no load (also assuming the motor is not an "air-over" motor).
The measured current readings are compared, and, if found to be in excess of 5% of each other, the phases are rotated. For example: Phase A is rotated to the Phase B location, B to C, and C to A. If the unbalance remains the same and is found to follow the line leads, then the power supply is unbalanced, if the unbalanced current remains on the motor leads, then the rewind repair is suspect and the motor should be disassembled to have the stator retested and repaired.
Motor current should also not exceed the nameplate rating during a no-load test. The "rule of thumb" for two, four, and six pole motors is that the no-load current will be in the area of 25 to 50 percent of nameplate.
It is also recommended that either a vibration analysis or Electrical Signature Analysis (ESA) is performed under part load in order to detect any operating defects prior to shipment.
Once all the running tests are complete and acceptable, the motor is electrically suitable for operation. In a few cases, the customer may require additional tests.
9. Conclusion
As shown, there is more to an electric motor repair than a good looking paint job. The type and quality of work required for returning a "world class," "good as new" electric motor following a rewind repair is extensive. It is apparent that a motor repair customer must work closely with a motor repair center to ensure that the equipment, which is sent out for rewind repair, is handled in a manner which does not reduce efficiency nor reliability.
An end-user should have pre-qualified an electric motor repair shop to ensure that their equipment will be repaired to their expectations. This prequalification should include a review of capabilities, equipment, a recognized quality control program (ISO 9000 or EASA-Q recommended), and a method for handling warranties or concerns. The end-user should ensure that all billing, terms and conditions, and reporting is understood by both parties in advance. It is also recommended that the end-user has a method for contacting the motor repair center at any time.
To achieve this, a specification for motor repair, to include pre-qualification requirements, developed through a neutral entity for fairness, must be developed to ensure that the end user is receiving the best energy efficient and cost effective repair or repair versus replace decision possible.
Contact Howard W Penrose, Ph.D. for more information on developing your motor repair specification.
Howard W Penrose, Ph.D.
Vice President Electrical Reliability Programs
T-Solutions, Inc.
howard@motordiagnostics.com
Developing Your Motor Repair Specification 2
Today we will discuss the general process of a motor repair. We will then follow with details of each stage, with alternatives, over the coming days.
Stage 1: Receipt of the Motor
When a motor is received by a repair facility, it is normally placed in a ‘receiving area.’ Nameplate data and customer information is documented and a cursory list of included and missing items is made.
Stage 2: Disassembly and Inspection
The motor is inspected by a technician who may first test for insulation to ground, shaft rotation and winding continuity. The motor would then be operated, if safe to do so. The motor would then be disassembled noting electrical and mechanical condition. The stator may be cleaned and further tested for winding shorts and insulation defects. The mechanical fits may be measured for proper fit or wear.
The condition of the motor and repair estimation is normally provided to the customer who would approve repair, replace or discard.
Stage 3: Repair
Assuming that there are electrical and mechanical repairs required, these are normally performed parallel to each other.
Rewinding is performed using one of a number of coil removal, winding and replacement practices. Machining is performed using one of a number of machining practices. Other parts are cleaned and re-conditioned.
Prior to varnishing, a series of winding tests is performed to ensure quality of repair.
The winding is then varnish and prepared for re-assembly.
Stage 4: Re-Assembly
The motor is re-assembled and new parts are installed, including bearings (if roller or ball).
Stage 5: Final Testing
The assembled motor is tested for continuity and insulation to ground. If it is OK, it would normally be run, under no-load conditions, for 30 minutes and checked for phase balance and to see if the bearing housings become hot.
Stage 6: Painting and Shipping
It is normal practice to paint the motor and palletize it for shipping.
These are a simplification of the repair process. This week we will review the process a little more in-depth before discussing tolerances, etc.
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
Developing Your Motor Repair Specification 1
Why is it so important to develop and agree to a motor repair specification? Consider the following facts (Motor Diagnostics and Motor Health Study review of “Industrial Motor Repair in the United States,” BPA/US DOE, 1995):
• 81% of repair shops reported that they modify windings because of equipment limitations or shop preference. Only 4% modify windings for reliability or energy improvements;
• Although insulation, winding resistance, vibration, phase balance (surge or MCA) and core loss testing should be done routinely as part of a quality repair, only insulation testing was performed routinely;
• 33% of repair shops used written quality standards and were familiar with any type of quality assurance procedures;
• Of the repair shops that used quality assurance procedures, 40% were repair procedure specifications, 25% were test specifications, and 21% were EASA standards. Only 1.5% of surveyed repair shops used any form of quality assurance testing;
• A majority of the repair shops viewed resistance testing as a method to evaluate DC electric motor fields only;
• 49% of shops perform a no-load test prior to performing repair and rely upon the tests performed to determine the motor condition.
Only the largest repair shops had a full compliment of test equipment for detailed analysis, including before and after testing:
• 85% or the repair shops had: Meg-Ohm meters; Low resistance ohm meters; and, AC high potential testers;
• Up to 80% of the large repair shops, up to 40% of medium shops, and under 15% of the small shops had specialty equipment, including: Dynamometers; Core loss testers; Three phase Wattmeters; and, Acoustic testers. Some of the dynamometers were homemade test beds or used a shaft connected to a brake;
• All or the large repair shops, 66% of the medium shops and up to 20% of the small shops had: Vibration testers; DC High Potential testers; and, Surge comparison testers.
A number of conclusions were drawn in the repair section of the MDMH report:
• Many motor repair shops will adjust the original winding design, including reducing wire size or the configuration for convenience or ease of winding (60% or shops surveyed – 73% of the 81% of shops that make changes). Wire size changes will modify the motor’s I2R losses, winding configuration changes may modify the electric motor’s impedance balance or change the motor’s output torque. In each case, the motor will be different from the original capability and reliability of the motor and it’s design;
• Few electric motor repair shops perform before and after verification tests of the winding to determine if changes have occurred. This leaves either the motor owner to perform before and after tests, the motor owner to provide test requirement specifications, or a combination of both in which the owner performs a commissioning test upon receipt of the motor from the repair shop;
• If commissioning tests or specifications are provided by the owner, the motor repair shop should be informed prior to the receipt of the motor;
• A survey and qualification of each vendor service shop should be performed and agreements made prior to repairs. Ensure that the service shop has the required test instruments to provide equivalent tests to those performed by the motor owner.
Now here is the biggie:
Almost all industry standards provide an outline of tests to be performed, ALMOST NONE of those standards provide any electrical PASS/FAIL criteria. That is normally left to the determination of the repair technician!
For instance, the repair industry created ANSI/EASA AR100 motor repair standard (downloadable for free from www.easa.com) provides a good outline of tests, procedures, etc. but does not provide pass/fail criteria!
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
Selecting the Right Motor Repair Shop for You
If performed properly, a successful motor management program has partnered with a motor repair vendor. When this is not the case, you will be in a position to have to expect the standards and quality of the shop you send your equipment to. Unfortunately, this may also mean that you are focusing on price versus quality and, perhaps worse, you may be expecting an extremely fast turnaround.
Here are a few secrets:
1. In the world of motor repair, cheaper rarely means better as the backbone of motor repair is the craftsmen. Few motor repair shops are union shops, therefore the wages are set by the employer. Lowest cost normally means lowest wages, which in turn may mean (but not always) lower quality workmanship. Also, parts may not be of the highest quality. The best motor repair shop that I worked for had one of the highest prices in the Chicago area and would, literally, turn away customers that would insist on discounts on their prices. Their feeling was that if you wanted their level of quality, you would pay their level of pricing. The workers were well paid and the work environment conditioned and exceptionally clean. It was extremely rare to see a workmanship related warranty. They are now one of the largest repair shops in the USA (and not an EASA member). I also had the unfortunate experience of working for a low cost repair shop for about two weeks in Virginia. They were the lowest cost, lowest quality that I had ever seen. I actually watched the shop manager glue a bearing into a housing because he forgot to check the housing on disassembly and they had not quoted machining. The final straw was when a part from a motor, that another technician was working on, sprung and stuck in the concrete inches from my head due to the company not purchasing required tools and the technicians not being paid enough to buy their own. One of the rewinders had even determined how much to reduce the wire size in order to get a particular high volume stator, from a large customer, to just make it through the warranty period. The burnout oven was often allowed to exceed 1,000 degrees F. (this was an EASA repair shop). They went out of business about a year after I left and I worked as General Manager for a competing motor repair shop.
2. With only a few exceptions, proper motor rewind repair practice processes do not allow for a 24 hour turnaround. When this type of turnaround is required or the repair shop, shortcuts must be made, which reduce reliability. For the following examples we will use a 50 horsepower motor:
a. Standard motor repair: Disassembly and test – 2 hours; Wire removal (burnout oven at 650 degrees F) – 7 hours; Coil winding and insertion – 5 hours; 2-dips and bakes – 20 hours; Re-assembly – 4 hours. Estimated linear time for proper repair: 40 hours (we will be describing these steps in-depth).
b. Alternate motor repair: Disassembly and test – 2 hours; Wire removal (mechanical stripping at less than 400 degrees F) – 1 hour; Coil winding and insertion – 5 hours; Trickle system with full cure – 1.5 hours; Re-assembly – 4 hours. Estimated linear time for proper repair: 11.5 hours.
When selecting a motor repair shop, you must consider a number of facets:
• Create a motor repair specification;
• Certify the repair shop to your specification;
• Include the repair vendor in your motor management program;
• Periodically visit and audit the repair shop and witness tests; and,
• Commission test all repairs.
Starting in the next lecture, we will discuss how to create a motor repair specification.
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
The Motor Has Failed, Now What?
As in the last lecture, the motor has failed, or has been determined by your reliability program to be in a state of failure. What is the next step?
If the motor is not critical, nor has any impact on production or safety, then just remove and replace it, right? No. You still have an investment, even if it is just a few hundred dollars plus time. I have literally seen a company invest significant man-hours over a bathroom fan! Even to the point where the plant manager was aware that there was difficulties with a motor in the bathroom and specialists were brought in to figure out the problem. As it turned out, the whole problem had to do with the mounting of a vibration transducer… on a bathroom fan? And, here it is, I specifically remember the incident over 10 years later. You get the idea.
Even on non-critical equipment, you need to know the basic cause of failure. Was there excessive dirt in fan blades? The motor? Did it single phase? Did a single phase motor blow a capacitor? Was the motor installed correctly? How long did it last? Is the protection sized right? In effect, a basic overview of the system will allow you to avoid having to spend time on the same piece of equipment again. It is less the importance of the cost of the motor and associated equipment and more the investment of valuable man-hours… Yours.
On critical, production or safety related equipment (and it just may be a bathroom fan in a commercial building), you will want to look a little deeper. First, you will want to determine the cause of the failure and how it affected its associated system. This information can be obtained through using troubleshooting tools, such as motor diagnostics MCA and ESA systems, infrared, vibration, ultrasonics and other tools. This allows you to make a repair versus replace decision during the removal of the failed equipment. It also allows you to: a) Verify the accuracy of your motor diagnostics program through confirmation in the repair facility; b) Provide information to the repair facility; c) Perform a more in-depth root-cause-failure-analysis quickly, in order to prevent the same type of failure, or allow for earlier detection; and, d) Help determine if there are additional monitoring requirements for this particular equipment. We will cover root-cause-failure-analysis as part of the motor repair inspection.
At this point, the motor should be removed. The technician must note such things as:
• The condition of the motor feet;
• The condition of the base and any grouting;
• Tightness of bolts;
• Condition of the coupling, belts and/or sheave;
• Condition of the conductors and connectors;
• Condition of the starter or drive;
• Condition of the driven equipment, including gear boxes, fans and pumps;
• Condition of surroundings, such as contamination, water, steam, etc.; and
• Other obvious conditions that would affect the new or repaired motor that will be installed.
At this point, any corrections to the base, coupling, belts, sheaves, etc. should be addressed prior to the installation of the new, repaired or spare motor. This is one of the reasons why condition based monitoring is so important. If a motor fails during a production run, the motor is removed and replaced quickly with little regard for its operating conditions, or parts are swapped until the system works again. This is less effective and incredibly expensive.
However, using CBM, you can plan an outage or address the problem with more leisure and the ability to address the root problem.
Repeat the following and quote me often:
• The motor is often acts as a fuse for the real problem.
• Utilizing a proper program, you can expect a 20-year average life from your motors.
Do you get a 20 year life out of your motors? Or are you replacing the same ones on a regular basis.
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
Three Common Reasons for Motor Repair
There are three common reasons for motor repair. These are, simply:
• Scheduled Maintenance: The ancient practice of periodically shutting down and removing equipment for a general inspection and overhaul regardless of condition. For example: Every five years a power plant shuts down, removes all critical motors and has them overhauled (this, of course, is not limited to power plants, by any means). I refer to this as an ancient method as it has been realized more and more (in general, nowadays) that this method is far too expensive and programs, such as Reliability Centered Maintenance and Condition Based Monitoring, can reduce the amount of work. Removing and installing periodically, regardless of condition, exposes the owner to infant mortality problems.
• Condition Based Maintenance: The modern art of identifying critical operations equipment, and monitoring them, to determine the point where a motor is no longer performing its function, as defined by the operation and owner. For instance, the detection of bearing failure, winding contamination or early stage winding shorts. The key to this approach is that the owner can decide how soon to remove the motor on schedule, reducing unplanned downtime, and can make an informed repair versus replace decision, bringing equipment back on line much faster.
• Motor Failure: Both the ancient and modern practice of allowing the motor to run to failure. Why both? If the equipment is critical and no monitoring is being performed (or the wrong monitoring), then this is the ancient art of allowing equipment to fail, opening operations to extremely high maintenance costs (proven over and over again!). The ‘reactive’ approach to maintenance results in patchwork, low grade repair, and more issues, that generates reduced system reliability. However, when part of a planned process, there may be equipment that has little to no effect on the mission of the company, such as production, safety, etc. In this case, it makes very little sense to invest manpower and equipment for testing.
These three reasons will come back to visit us frequently during this part of the lecture series. Each method will have an impact on your repair program and the success of your maintenance program.
Howard W Penrose, Ph.D.
howard@motordiagnostics.com
“There is no such thing as something for nothing.” Napoleon Hill
81% of electric motor repair shops modify your windings through the rewind process. 73% of the modifications are for ease of rewind and shop preference. Less than 4% of modifications are for reliability. --2003 Motor Diagnostics and Motor Health Study
Introduction:
I started my career in the electric motor repair business, first as a US Navy electric motor repair journeyman (NEC 4621) then with Dreisilker Electric Motors, Inc. (Glen Ellyn, Illinois) and Dunkums Electric (Richmond, Virginia). This experience included both performance, standards work, research and development, root cause failure analysis, warranty investigations and much more. I have literally performed both the types of repair that I have recommended and warned about.
In this part of the Penrose Lecture Series, I am not only going to provide information concerning repair standards, specifications and practices, but information on the parts that directly impact reliability and motor life that are not covered by standards, etc. This is important to understand, in your motor diagnostics program, because how your vendor takes care of your electric machine will directly impact your bottom line. We will cover both the electrical and mechanical repair processes.
What is Electric Motor Repair?
Within this part of the lecture series, we are going to define electric motor repair as the function of returning an electric motor to the ‘as new’ condition, regardless of its condition prior to the repair.
The scope of this part of the presentation will start with removal, transportation to a repair facility, the repair process, the communication process between a repair facility and motor owner, testing and repair standards, transportation to the motor owner, commissioning and installation, root cause analysis by the owner and repair facility and developing a motor repair specification that the motor owner and repair facility can live with. Also included will be the repair versus replace decision, impact of repair on reliability and other considerations. As such, we will explore each part of the repair process and the options for each, which will be graded upon my personal experience and observations.
So, sit back, enjoy and get ready to explore the mysterious world of electric motor repair!