LinkedIn Series on Electrical Signature Analysis

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This December we have started a linkedIn series on Electrical and Motor Current Signature Analysis Applications. The series will be announced and linked in the newsletter and on the website.  For notices, please sign up for the newsletter which is bi-weekly.

The first parts of the series:


Tech Tip: Ground Current

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Based upon several projects we have had to do a deep dive into expected ground currents in shielded, unshielded, and current loop conditions. While we have not found anything solid in standards, we have found several ‘rules of thumb.’ Note that we are discussing ground and not system neutrals.

Primarily what you should see in the ground circuit is no more than kVA/1000 of the associated ground. For instance, if I have a 480V system that carries a 500 Amp load, the leakage should be no more than ((500 * 480)/1000)/1000 Amps or 0.24 Amps (240 mA).

Causes for having higher current include using unshielded cables with unbalanced voltage, on VFDs or during startup of soft start systems. Other causes would be: leakage from contamination; ground fault; dirty (noisy) ground systems; EMI (Electro-Magnetic Induction); and a number of other conditions. This is one of the reasons you will see a requirement for shielded cabling and spacing between cables in cable trays. Finally, it is also important that you run ground cables away from individual phases in conduit and run all three phases in the same conduit so that they cancel out.

You also need to ensure that all grounds are properly bonded to the motor frame. High resistances can also create these conditions, especially as you need to run a reference ground back to the drive/soft start and one at the motor to ground. In the case of shielded cable, you must ensure that you ground one side of the shielding, as grounding both may cause a ground loop.

Conditions such as ground loops or EMI, in addition to problems with electronics, controls, mechanical and bearing failures (fluting), cause danger to personnel due to the risk of electrocution. A quick check of current on the ground conductors will provide feedback as to problem conditions. Any value of current over 1 Amp should be investigated.

Electrical Signature Analysis for Final Testing at Repair Shops

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We were quite surprised to see a tech tip produced by an organization that stated that you required 50% load or more in order to perform motor current signature analysis (or, in our case, electrical signature analysis) and that you cannot use it for final testing in repair shops. Apparently, we have been doing the ‘impossible’ with all of the ESA/MCSA instruments for the past 20 years, or so.

Figure 1: 30hp no load with broken rotor bars – note long start time uncoupled

The 50% load statement from a few manufacturers in the 1990s, and a few that use ‘low resolution’ spectra now, has to do with the slip of an induction motor. In effect, if you have a four-pole, 1800 RPM induction motor, the motor actually runs close to the nameplate value under full load. Most newer motors will have a nameplate reading about 1785 RPM full load. At no load, uncoupled, a good 4-pole machine will run closer to 1799 RPM (low inertia motor) or, more often, at about 1795 RPM. In low resolution spectra, the peaks associated with motor defects would blend into the line frequency (power harmonic) peaks. However, most have resolutions within 0.001Hz+ at the lower frequency range (ie: <300 Hz) and about 0.01-0.1 Hz at higher frequencies (<5kHz).

Figure 2: 5hp motor with no load unbalance on test bench

When looking for issues such as broken rotor bars on an incoming motor to a repair shop or to verify the condition of the rotor and airgap following a repair, the fact that the motor is not connected to a load or coupling is better as it will eliminate outside influences on your tests. For rotor bar conditions we are looking for Pole Pass Frequency (PPF) sidebands around the line frequency, and harmonics if there are a large number of broken rotor bars with good bars in between, and the number of rotor bars times the RPM +/- 60Hz, 120Hz, 180Hz for static eccentricity (rotor off center) and sidebands of running speed around those for dynamic eccentricity (orbiting rotor). The PPF is calculated as twice the slip frequency, or 2*((1RPM/!800 RPM)*60Hz) = 0.0667 Hz, well within the 0.001Hz+ resolution available in most modern ESA devices available.

Figure 3: 1hp motor with unbalance and bad rotor bars

The peaks surrounding line frequency, and elsewhere in the spectra, are related to the peak voltage or current resulting in the ability to detect issues regardless of load. The peak values are counted down in dB from the peak current providing a value that does not vary significantly (ie: 2-3dB on average) regardless of load. In fact, the -dB (dB down) values developed by Oak Ridge National Labs, specifically when they developed ESA for the nuclear power industry, do not specify load as it was unnecessary.

In addition, one of the problems that does occur with a motor with broken rotor bars is a loss of torque and an increase in slip (multiple bars), making it even simpler to distinguish rotor bar issues. When we have run into instances where the findings are in question, we sometimes reduce the voltage to the motor in order to increase the slip to demonstrate that the PPF follows.

We perform this for quality assurance in repair shops from very small, low voltage motors to very large machines, 13.8kV loaded and unloaded, coupled and uncoupled, with great success, detecting defects before they become issues.  In most cases, however, we find the motors are in very good condition both with mechanical (vibration) and electrical (ESA) testing when following a good repair specification and processes.

At no load, current signature analysis with the correct instrumentation is far superior to vibration for rotor bar and rotor related fault detection.

The Cost of Maintenance 2 – Overview

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We are going to take a number of the issues developed in the last Blog one step at a time. This week, we are going to focus on the reason why technology has been ineffective in many motor management programs.

Maintenance Needs

The present trend in maintenance involves reduced staffing, reactive maintenance and far too much unplanned downtime. In addition, new staff is often not exposed to experienced staff before they retire, transfer or pass away. The initial purchase of technology is often done to fill the holes left behind.

With reduced staffing occurring due to perceived profitability improvements, the shift away from planned maintenance is inevitable. The best priority for maintenance is, in order:

  1. Condition-Based Maintenance/PdM (CBM – corrections based upon actual condition)
  2. Preventive Maintenance (Lubrication, servicing, other periodic maintenance)
  3. Reactive/Corrective maintenance practices

However, it is often seen that managers, planners and maintenance personnel will often respond to reactive maintenance as a priority. I have actually observed one of the worst conditions that could be considered:

A production line was to be brought offline for eight hours to perform planned maintenance. The maintenance planner released personnel to provide planned maintenance only after they would complete reactive maintenance (that did not impact production) and pulled other personnel off of condition-based monitoring duties. In effect, the priority was: Reactive; Preventive; then, CBM. The result is a purely reactive environment, which is ineffective.

Through a focus on reactive maintenance, due to the need for personnel (pulling personnel off CBM), planned maintenance quickly moves towards breakdown maintenance. Breakdown maintenance does not involve any CBM, planned maintenance, or otherwise, and instead is an ineffective and expensive, let alone frustrating, method of performing maintenance.

MotorDoc Now Performing Shaft Current Studies

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For Immediate Release: May 17, 2017

Based upon requests from our clients MotorDoc LLC has increased our capabilities to include independent shaft current studies for motors on soft starts, variable frequency drives and motors & generators where fluting issues are suspect.

As we have previously discussed, we are seeing an increase in shaft current issues within industry.  We have added the Aegis Shaft Current inspection system to our arsenal of electric machine testing technologies.

This capability is available for troubleshooting, as part of PdM work and machine forensics.

Contact us at 800-919-0156 ext 0 or (630 310-4568 outside of the USA)


The Cost of Maintenance 1

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So, what is up with our industry right now? What is the future?

We are in a maintenance limbo: Technology is allowing us far greater capabilities than we have had in the past; Maintenance professionals are retiring from industry, with few following into our profession; Technical education through universities and training companies is mediocre, at best; Indecision by managers to act upon PdM/CBM recommendations; Over 93% of motor management programs fail; Over 57% of CMMS applications fail; and, Well over 90% of maintenance programs fail.

In order to understand, let’s break it down just a bit:

First, we have a love affair with new technology. Gadgets are cool to have and use. However, for the most part, they have been ineffective. Either they are not fully applied, or other maintenance actions call attention away from the use of the technology. In effect, technology has been applied as a ‘crutch’ for a rapidly ineffective maintenance and reliability industry. Coupled with a distrust of technology, the ability for technology to see developing problems far in advance of human detection (through the senses – hey, it’s still running!) and the occasional mis-call of expert systems, decision-makers will often not act upon reported reliability findings. The result is frustration by the technologist (reliability/maintenance tech’s) and lost opportunities that are often written off as the ‘cost of doing business.’ [Note: I have seen numerous instances where maintenance is placed in the expense column of a business spreadsheet and another column identifying projected production losses due to unscheduled downtime – not listed as an expense. Contradictory? I think so! If you have such a spreadsheet, look at the projected unscheduled downtime that is your reliability/maintenance opportunity.]

There seems to be a growing need, within the maintenance and reliability community, for new blood to replace the aging workforce. The problem is that many businesses are either not allowing time for mentoring or are using the opportunity to allow attrition to reduce the costs associated with maintenance manpower. Few people enter into our workforce because it is perceived as ‘grunt work,’ and not high tech. Why should they enter the maintenance and reliability world when they can get a job working with computers in a nice, clean, office?

Education has deviated from maintenance and, in the electrical world, power. Very few universities offer motor design courses (North America), distribution courses, etc. instead changing their curriculum from Electrical Engineering to EECS (Electrical Engineering and Computer Science). The reason is quite simple: Universities are businesses and they need a minimum number of people in coursework and they need to represent that they are cutting edge in order to obtain research grants. There is not a lot of money in maintenance and reliability research and development.

The high rate of maintenance and reliability failure can be attributed to a great number of reasons. In my experience, those amount to several key issues: It is determined by management that the activity is too expensive after it is initiated (false start); The activity is initiated with very poor planning; Programs are implemented in parallel; and/or The Entropy Factor. While the first few are self explanatory, the Entropy Factor is, by far, one of the most dangerous.

The Entropy Factor exists when a program is becoming successful. The initial cost avoidance and payback appear during the growth stage of a program. The continuing, crucial, portion of the program exists after that point: Maintaining the success of the program. However, this is also the point where some enterprising manager will determine that expenses can be reduced in maintenance and reliability because they have not been having equipment failures and there are expenditures without obvious payback. The program is cut, then there is a lag-time before equipment starts failing (usually 12-24 months) at a high rate. In effect, the success of the program ensures its failure: The cause – poor management and training of managers and poor goal setting. Management and Sales (typical MBA stuff) goals are forced upon the maintenance community. This is bad practice as the concept of MAINTENANCE is to MAINTAIN the FUNCTION of EXISTING RESOURCES. It cannot, by its very nature, be a growth part of the company. The fact that maintenance and reliability of machinery is able to produce a return on investment by reducing unplanned downtime is actually a measure of historical MANAGEMENT FAILURE and POOR PLANNING. Sinking in yet?

The primary problem that we have in the Maintenance and Reliability industry is that the metrics used to evaluate the success of our programs is built around the WRONG MODEL. This is exacerbated by the myriad (thought I would use a few big words so I sound like I know what I am talking about – basically, made worse by a lot of) consultants who try to massage maintenance cost avoidance into the profit/loss models of sales and business. In effect, the MBA programs, management training programs, and all the other BS that has been passed around over the past few decades, has focused on just one part of the business, but not the underlying structure.

This is the first in a series related to this topic.

Electrical Signature Analysis Series – Part 1

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In this lecture series, we will be discussing Electrical Signature Analysis (ESA), which is a method for evaluating electrical machinery while energized. The topic will be quite broad and is to include an analysis of supply power through the driven load.

While we will rely upon some of our previous discussions to provide information and definitions for some of our new information, we will start this series by providing some definitions unique to ESA:

  • Voltage: Electrical pressure, is also termed as electromotive force. Voltage is generated.
  • Current: Defined in classical physics as electron flow. Current is demanded in order to produce work and is a result of the load.
  • Upstream/Downstream: Upstream refers to the electrical system in the direction of generation or distribution from the point of test. Downstream is towards the motor and load from the point of test.
  • FFT: Fast Fourier Transform (FFT) is a mathematical method of separating the frequencies of a ‘sine wave’ and presenting them as frequencies and amplitude.
  • Spectra: Is the graph of frequencies and amplitudes resulting from an FFT.
  • Voltage and Current FFT: Spectra of voltage and current.
  • Motor Current Signature Analysis (MCSA): A method of viewing demodulated current and current FFT’s to evaluate the condition of machinery downstream of the point being tested.
  • Voltage Signature Analysis (VSA): A method of viewing voltage FFT’s to evaluate the condition of machinery upstream of the point being tested.
  • Torsional Analysis (TA): A method of viewing the current resulting from the load and its torsional effect (pulsating loads, etc.).
  • Inrush Analysis: A method of viewing the inrush effects on voltage and current when electrical machinery is started.
  • Power Quality: The industry has defined this as reviewing voltage and current. Voltage unbalance, over/under voltage, voltage and current harmonics and current unbalance.
  • Power Analysis: This is defined as viewing power quality as well as surges, swells, transients, interruption, etc. and requires datalogging capabilities.
  • Electrical Signature Analysis (ESA): A method of evaluating the motor system, which includes supply, control, motor, coupling, load and process, utilizing MCSA, VSA, TA, Inrush Analysis and Power Analysis.

The purpose of ESA is to obtain enough information, concerning the circuit being tested, to evaluate the health of the electrical system from supply through load.

ESA has been successfully applied in these applications:

  • AC induction motors
  • Variable Frequency Drives (VFD’s)
  • Wound Rotor Motors
  • Synchronous Machines
  • DC Motors
  • Alternators and Generators
  • Machine Tool Motors and Servos, including robotics
  • Driven equipment including Belted, Direct Drive and Geared
  • Transformers
  • Traction Equipment
  • And numerous other applications

What it comes down to is the ability to evaluate the information provided by ESA. That is the purpose of this lecture series.

Electric Motor Repair Part 8

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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.  One way to do this is to work with your local repair shop(s) in the development of the specification, while also ensuring that you are following the minimum of standards such as IEEE Std 1068, so that you are not creating a specification that only that shop can follow.

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 electric motor repair specification.

The materials and reports are readily available from the EASA website: 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 as well as the IEEE Standard 1068. 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.

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.

For information on the development of a custom motor repair specification or assistance in reviewing your repair vendors, contact us at

The Art of Electric Motor Repair Part 7: The Process

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Stator of a big electric motor. repair

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 resistance (ie: Megger) 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 before proceeding with other tests. 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


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.

  1. 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 metalizing, 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

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.
  1. 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.

  1. 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.

  1. 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.

The Art of Electric Motor Repair Part 6

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2015-06-19 12.01.14

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. We are going to review the process a little more in-depth before discussing tolerances, etc.