MRO Magazine


By Chuck Yung   

Machinery and Equipment Maintenance Energy

Whether you're new to the job or a veteran, your choice of vendors is critical to successful management of plant operations -- especially when

Whether you’re new to the job or a veteran, your choice of vendors is critical to successful management of plant operations — especially when

it comes to repair and replacement of electric motors and drives. But this can be tricky given all the variables to consider, including price, delivery, service, expertise, reliability and quality.

No one wants to pay too much, but deciding on price alone can backfire. The cost of downtime, idle workers, lost production, damaged product, missed deadlines and dissatisfied customers can quickly offset whatever you might have saved on a ‘cheaper’ repair.

So what’s the best way to choose? It helps to begin by listing the types of equipment you have, your basic requirements, and any desired value-added extras that can save you time, money and aggravation. Then evaluate each prospective service centre to see how it measures up. You may find that one service centre can provide everything you need. In other cases, it might make sense to use two or more vendors, based on your requirements and their expertise.


Narrow the choice: Service centres come in all sizes, with varying capabilities and expertise. To narrow down the field, first qualify candidates based on their experience in the type of work you expect of them. If all of your motors are 460 volts and under 500 horsepower, it probably doesn’t matter that a service centre can rewind form-coil motors or rebuild DC machines.

Take a tour: If you’ve never visited the service centres that do your work or that you are considering, it’s time you did. Touring the facilities of several repairers and inspecting work-in-process provides an opportunity to compare capabilities, expertise and workmanship.

The best-equipped facility isn’t always the most capable or the best fit for your needs. It takes skilled people to troubleshoot and repair motors, drives and controls, so look for a well-trained, stable workforce. Do the technicians have the expertise and application knowledge to determine the root cause of failure? If not, they may just keep replacing that bearing that seems to fail every six months and never solve the real problem.

What to look for: A walk-through inspection allows you to observe obvious things like crane capacity, general cleanliness (especially in critical areas like the rewind department), and overall organization. Specialized equipment can be helpful, but repairs often can be accomplished in more than one way. Other equipment is essential, and it should be in good working order.

Essential equipment

Test panel: The test panel should handle a useful range of voltages. Some test panels employ a variable-voltage supply, rated 0-600 volts, with additional steps for higher voltages as necessary. Others incorporate multi-tap transformers. If your plant has special voltages (e. g. 208/575/950/7200/13,200), verify that the service centre has those voltage capabilities.

Test bed: The test bed (Fig. 1) should be able to support motors for rigid mounting during test runs and vibration analysis. Ideally, the baseplate should be grouted to a foundation with a total mass at least 15 times that of the largest motor to be tested. If you have a large number of vertical motors, ask how the service centre will support them during testing. Using a T-slotted base helps simulate actual operating conditions. The higher the motor speed (e. g. 3,600 rpm and above), the more important this is.

Vibration analysis equipment: This may be stand-mounted or portable. Portable equipment is important for final quality analysis of repaired rotating equipment, as well as for field analysis of problem machines. A stand-mounted vibration analyzer is often an integral part of a balancing stand.

Balancing stand: Each rotating element should be precision-balanced during the repair process. The higher the speed of the motor, the more important this is. Expect most service centres to have a balancing stand; subcontracting this step of every motor repair job will likely result in delays. Most commercial balancing stands have a 100% overload capacity. Balancing stands should be sized to handle most work without using that overload capacity. To protect shafts from scoring, balancing-stand rollers must be kept clean and should be covered when not in use.

Electrical test equipment: Standard test equipment should include a surge tester of appropriate size and an AC or DC high-potential tester. The latter should have a voltage rating suitable for testing new windings, as specified by NEMA standard MG1-2003. This is generally twice rated voltage plus 1,000 for the AC test and 1.7 times that value for the DC test. A megohmmeter is important as a first-line test but is not adequate of itself. (NEMA is the National Electrical Manufacturers Association. MG 1-2003 is Motors & Generators 1-2003, a standard for electric motor construction and testing.)

Core loss tester: The stator core should be tested to verify that it was not damaged by winding failure or removal, and to check the condition of repaired or restacked cores. Commercial core testers are easy to use and can document the results with a printout. A core test also can be accomplished using a power supply and multiple loops of insulated cable. (Refer to EASA’s Tech Note 17 for more information.)

Controlled-temperature burnout oven: The burnout oven (Fig. 3) should be fitted with a temperature chart recorder to monitor the stator core temperature during the burnout process. A flame-suppression system is important to prevent excess temperatures, while an afterburner reduces volatile emissions. The service centre should also follow proper procedures for loading the oven. (For more information, download Guidelines for Maintaining Motor Efficiency During Rebuilding, from

Winding area:The service centre should have an adequate inventory of magnet wire and insulation materials for the types of rewinding performed. Winding machines should be capable of making concentric or lap coils. Semi-automated winding machines with computerization are an indication that a service centre is technologically up to date.

Bearing inventory: The service centre should stock or have ready access to the bearing sizes and types found in your motors (e. g. angular contact bearings). Many service centres rely on bearing suppliers to carry this inventory for them. This helps keep their inventory costs down, while generally ensuring faster stock turnover. If a service centre has a reliable source for bearings, it doesn’t really need to stock them.

Other specifics to look for

If your motors are rated above 1,000 volts, look for proof of form coil motor winding capability. Form coil production equipment, however, is not essential. In fact, most service centres now order custom replacement coils from form coil manufacturers, who can often manufacture and deliver a set of custom coils more quickly than most service centres could produce them in-house.

Service centres that manufacture their own form coils should have obvious expertise, an exceptionally clean work area, and access to a large assortment of rectangular wire sizes.

Repair of machines equipped with babbitt bearings requires expertise, although rebabbitting of sleeve bearings is a niche market served by bearing specialists. Whether bearing repairs are done in-house or subcontracted to a specialist, quality control procedures like ultrasonic testing should be in place to verify the integrity of the bond between the babbitt and the bearing shell.

Winding treatment, at a minimum, requires a dip tank (Fig. 2) large enough to handle the range of motors repaired. A serviceable vacuum pressure impregnation (VPI) vessel also is important if your motors operate in wet or corrosive environments.

A VPI vessel is only as good as the insulation system and procedures used with it. Verify that written procedures are in place to control the rewind and VPI processes. Some service centres subcontract VPI processing to another repairer if they do not do enough of this work to justify maintaining their own VPI system.

Whether a service centre uses a dip tank or a VPI vessel, it should verify the integrity of the resin by having it tested. Viscosity measurements may be done inhouse, at least quarterly. More detailed testing of the chemical composition of the resin should be done regularly by an outside party, usually the resin supplier.

Machine shop capability may be inhouse or locally available. In either case, verify that the equipment is in good condition and suitable for the types of work anticipated. Lathes, horizontal mills and vertical boring mills, for example, should be sized appropriately to handle the repairs you typically need. Equipment also should be available to handle larger work, if necessary, even if it will be subcontracted. Prepare for this scenario by asking if the service centre has a working relationship with a larger machine shop should the need arise.

Technology changes quickly, so a good training program is essential to keep technicians current and minimize turnover of key employees. With traditional on-job training (OJT) it may take an employee more than 10 years to become fully proficient in the electric repair industry.

Ask about formal training programs. Is there an in-house training program? Do employees receive training from other sources, such as trade and professional associations; motor, drive or pump manufacturers; community colleges; vocational/trade schools; or industry consultants and specialists? Whether formal or informal, training should be documented.

Documentation is also the foundation for any quality program. Besides ensuring that proper procedures are followed, it paves the way for continuous process improvement. Look for evidence that quality-control steps are in place and well documented for each job. The one-person service centre may do excellent work without it, but a paper trail is necessary for larger operations where one technician does not take the job from start to finish.

If you have a repair specification take it along and make sure the service centre agrees with the details. If you don’t have one, don’t reinvent the wheel. Reference ANSI/EASA Standard AR100-2001: Recommended Practice for the Repair of Rotating Electrical Apparatus. This will make your repair specification a living document, since the ANSI/EASA standard must be updated at least every five years. (This document can be downloaded from at no charge.)

Technology changes rapidly. If you have a written specification, review it regularly to keep up with improved processes.

The calibration of test and measurement equipment should be current and performed in accordance with standards set by the National Institute of Standards and Technology (NIST) or their equivalents. This is particularly important for micrometers, electrical test meters (including test panels), resistance measurement meters, and vibration measurement meters and analyzers. Ask to see annual calibration records traceable to NIST.

A couple of other points should be mentioned. First, you usually must pay for what you get. If you require pages of reports documenting each step of the repair, someone will have to fill them out. Expect to pay the service centre’s hourly rate for this. Ditto for load testing and other non-standard tests.

By the way, if you require special tests when purchasing a new motor, you will pay extra for that, too. Second, the strict quality control and testing you want for medium-voltage (form coil) motors may not be as critical for your smaller, NEMA frame motors, so write your specification accordingly. For example, it might cost more to repair and load test a 20-hp motor than to simply purchase a new one.

Finally, remember that the price of the repair is usually a tiny fraction of what downtime will cost if an unexpected failure occurs. Quality repairs are worth the extra money. Don’t fall into the trap of spending a dollar to save a dime.

Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO, an international trade association of more than 2,150 firms in 50 countries that sell and service electrical, electronic, and mechanical apparatus. For more information, visit


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