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.

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.

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.

Art of Electric Motor Repair 5 Developing your Specification

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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 provides a good outline of tests, procedures, etc. but does not provide pass/fail criteria.

The ANSI/EASA AR100’s big brother, the IEEE 1068-2015 “IEEE Standard for the Repair and Rewinding of AC Electric Motors in the Petroleum, Chemical and Process Industries,” which is actually applicable to all industries.


Art of Electric Motor Repair Part 4b

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The Impact of Motor Repair on Energy Efficiency

General 014

I’ve covered this topic extensively since 1994 and many have tried to tackle it as an issue since the Energy Policy Act of 1992 and as a significant topic of discussion since 1973. Legislators and policymakers have attempted to include electric motor repair efficiency as a topic with energy policy all this time and it was again tackled in the industry and government partnerships that had progressed with the Motor Challenge programs under the US Department of Energy in the early 1990s. It was lost in the noise when the Challenge programs were blended together, but was brought up during meetings I had with legislators (Congressional and Senate) when I was meeting with energy policymakers in Washington, DC, this past February, 2016, as part of the SMRP Government Affairs Committee.

The topic relates to how do you know if you have maintained or improved the energy efficiency through the motor repair process. You cannot perform a before and after efficiency on each repair as the motor is damaged, and you cannot economically perform a before and after efficiency test. In fact, you cannot rely upon the nameplate efficiency as it is often the ‘nominal efficiency’ or average efficiency and may be a calculated value. To make matters more challenging, depending on the country of origin, the efficiency calculation may be very different from the IEEE 112 Method B, which is the legislated method for evaluating efficiency.

Unfortunately, most do not know that there is an impact on the efficiency of their motor until they receive it back and it is consuming far more energy than it did before it failed. There are several papers on this in the archives, but the most telling is that if a motor is drawing a single amp more in the same conditions, there is a significant impact on the efficiency of the motor, and by direct extension, the reliability of the machine.

What we do know is that a process can be followed that will maintain efficiency based upon best practices through repair. This was proven in both the Canadian Electrical Association studies and the EASA/AEMT study. There are a few things that must be considered:

  • Wire Size – with the growing number of metric machines, and how tight conductors are packed into slots, a change in wire size to adjust for thicker insulation systems will have a direct impact on the I2R losses resulting in decreased efficiency and higher operating temperatures. With most motors manufactured over the past two decades, regardless of size, manufacturers’ insulation systems are getting thinner and slots are getting smaller while the repair industry has not adjusted significantly.
  • Winding Configuration – changing the winding from concentric to lap or otherwise will have an impact on efficiency. This is done almost routinely by many repair shops and must be performed with care and knowledge. This will have the same, or more significant, impact as with wire size. In fact, in some cases, because of the slot fits, a winding design may be changed in order to make sure that the cross-section is correct to handle the current, but the impact on the air-gap or back iron may still have a negative impact.
  • Mechanical Fits – bearing fits, in particular, but also coupling fits and rabbet fits, all have a direct impact on efficiency. Bearing fits can have a significant impact on increasing the friction and windage in a machine and the resulting heat will reduce the life of the bearing. Surfaces must be smooth and not punched, gouged or otherwise blemished as the inner and outer races of the bearing emulate the surface they are on. This can have a measureable effect on the efficiency of a motor.
  • Bearings – changing the type of bearing can have one of the largest impacts on the efficiency of a motor, in particular if a bearing is changed from shielded to sealed. As shown in previous studies, this change, alone, can reduce the efficiency of a motor by 3 percentage points, or more.
  • Stripping Process – an area of interest for over 50 years of study on efficiency and reliability. The temperature settings and times in an oven have a direct impact on the losses within the core of an electric motor, in addition to failure damage and cleaning of the stator core. The ability to verify the impact of the repair process on the core is measureable in that a before and after core loss test can be performed in order to determine if there are damaged spots (hot spots) or increases in the core losses. Another area that is impacted by this area is the mechanical fits of the components that go into the oven. In 1997, I was involved in a study in which we looked at different temperatures and the impact on airgap and soft foot. When maintaining the proper stripping temperature this becomes less of an issue. It is equally important to understand that stripping temperatures are based upon the core temperature, not the oven setting temperature, and the concept is that a temperature sensor is placed upon each stator in an oven (usually one or two). What does happen, especially in smaller shops, is groups of motors are placed in an oven and some of them ignite causing temperature differences throughout the oven, causing problems with the motor cores and even cores ‘falling apart’ through the process, or warping of cores. The IEEE 1068-2015 allows for a 20% increase in core losses for each repair activity on a motor and many companies have a limit to three rewinds before disposing of a motor due to impacts in this area.

There are other areas, of course, but these cover the main areas that have the more significant impacts.

The CEA and US DOE originally noted that there is an average of 1.1% reduction in efficiency through the repair process, based upon the studies. Due to discussions between the US DOE and EASA, programs such as MotorMaster Plus only accounted for a 0.5% decrease in repair, once and not compounded.

Starting the next part of the series we will be discussing the development of motor repair practices and specifications that will help you avoid losses in your machines.

For more information or assistance, please feel free to contact

Art of Electric Motor Repair Part 4a

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Electric Motor Repair Certification


As mentioned at the beginning of the series, this topic is an update to a blog series I had produced in 2002. At the time there were no attempts at certifying motor repair shops outside of ISO 9000 and the EASA-Q, which was the Electrical Apparatus Service Association’s work to assist repair shops in achieving a version of ISO 9000 status.

In about 1997-9, there were the beginnings of organizations within the USA (I am not sure about outside of the USA, I will investigate that for a future topic) on certification for motor repair shops. There are several programs that started with commercial interests in mind, and good intentions, of course, a few independent programs and, more recently, trade association programs. Following are a few of the more well-known programs:

  • SKF Certification – primarily a commercial program with a focus on bearings. Provides training for repair shop personnel and third party review of repair shop capabilities. It does focus on whether a repair shop is using SKF equipment and bearings, of course, but that is not really a down-side as the products are top-of-the-line.
  • IEMD – The Institute of Electrical Motor Diagnostics, which closed its doors in 2009, was focused on the development of certifications for motor diagnostic technologies and was also working on an independent, end-user driven, motor repair certification. Progress stopped when a majority of the 200+ members were impacted by the economic downturn of 2009/10. There are presently discussions about re-starting IEMD or rolling its work into one of several organizations.
  • Advanced Energy – Closest to independent as IEMD’s work, Advanced Energy’s program focus’ on the evaluation of a repair shops ability to maintain an electric motor’s efficiency through the repair process. At the present time, it is one of the more intense programs which includes proving that a repair shop is capable of maintaining efficiency by performing before and after efficiency tests on a repaired electric motor. It does include a site visit and review of the repair facility to EASA standards.
  • EASA Repair Certification – Released within the past 12 months, this program is EASA’s first real attempt to verify that members are meeting the best practices reviewed and recommended as a result of the EASA/AEMT Repair Study. A growing number of repair shops are joining in.

The development of certification for motor repair is still in its infancy. Primarily because the days of a company witness testing or performing their own in-process inspections of their repairs has all but ended. What this does mean, at this time, is that while a repair shop can meet or pass these certifications, they do not generally follow the practices that are required. While the certification programs that have been developed, or are being developed, have been implemented, and repair shops have passed the certification process, I have been in a few that do not actually follow what they are supposed to do in the normal course of business.

What does this mean? It means that you still have to be very attentive to whether or not you are getting what you think you should be getting from a repair shop. The important thing is that the above mentioned certifications say that the repair shop that holds that certification is capable of meeting industry standard. However, it does not mean that they would meet your repair specification nor that they normally perform to industry standard. In most cases, they do. However, it is still incumbent upon the end user to verify, especially critical machines, through in-process and final inspections and witness testing.

At the present time, in the USA at least, there are no recognized certification programs for individual motor repair technicians. With the high turnover of motor repair specialists, there are some training programs that are available through EASA, but most new technicians are trained OJT, which means they may know what they are doing, but not necessarily why, which is a bad combination.

MotorDoc LLC provides repair shop inspections as well as in process and final witness testing. Contact us at for more information.

Art of Electric Motor Repair Part 4

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Selecting the Right Motor Repair Shop for You

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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 in the past. 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. 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:
  • 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).
  • Alternate motor repair: Disassembly and test – 2 hours; Wire removal (mechanical stripping at 410 degrees F) – 2.5 hours; Coil winding and insertion – 12 hours; Trickle system with full cure – 4 hours; Re-assembly – 4 hours. Estimated linear time for proper repair: 24.5 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.

There is a great checklist built in to the IEEE 1068-2015 “IEEE Standard for the Repair and Rewinding of AC Electric Motors in the Petroleum, Chemical and Process Industries,” which is a comprehensive repair document we will cover in future blogs. Note: the title relates to the IEEE standards group that developed it. The standard, itself, is valid for all industries.

I use the checklist from the standard as a guide and the ability to ‘grade’ the repair shop. However, understanding what you are looking at is just as important as obtaining the grade. For instance, I have been in shops that have temperature controlled burnoff ovens, only to find that the water lines are not hooked up or are even turned off. In other cases, all the wire exists and the winding area is segregated, but open to outside air so that dirt, dust and other contaminants coat the wire, insulation material and working surfaces. In one case, I was touring a repair shop and had noted a blackened area on the concrete pad in the shipping area and discovered that they used it to stack wood into stators in order to cause a bonfire to remove old windings.

Yes, there are repair shops that still use a version of flamethrowers, bonfires, torches and other uncontrolled temperature means to remove coils. These repair shops must be avoided at all costs from both a reliability and efficiency/environmental standpoint.

Starting in the next lecture (part 5), we will discuss how to create a motor repair specification. In the next parts (part 4a and b) we will discuss motor repair certification and efficiency.

Art of Electric Motor Repair Part 3

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The motor has failed, now what? This can be a failed electric motor or it has been found to be in a state of failure by the condition maintenance program. What is the next step?

Download a free paper on Repair Vs Replace decisions here.

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 a few hundred dollars plus time. I have literally seen a company invest signficant 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 20 years later. You get the idea.

Even in 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 right-sized? In effect, a basic review of the system will allow you to avoid having to spend time on the same piece of equipment overa and over 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 (you never know, it might be that bathroom fan!), you will want to look a little deeper. First, you will want to determine the cause of the failure and how it effected its associated system. The information may be obtained through using troubleshooting tools, such as electrical motor diagnostic techniques (MCA and ESA), 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:

  • Verify the accuracy of your motor diagnostics program through confirmation in the repair facility;
  • Provide information to the repair facility;
  • Perform a more in-depth root-cause-failure-analysis (forensics) quickly in order to prevent the same type of failure, or to allow for earlier detection; and,
  • Help determine if there are additional monitoring requirements for this particular equipment.

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 operation conditions, or parts are swapped until the system works again. This is less effective and incredibly expensive.

However, using proper tools and planning, you can plan an outtage or address the problem with more leisure and the ability to address the root problem.

Remember the following:

  • The motor 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?


To have a custom electric machine system PdM program planned and implemented for your facility or for assistance with a long-term electric machine system problem, email us at