Industrial Motors

Motor rewind service centers keep your motors running smoothly

If you've never visited a motor rewind service center, it's time you did

By Chuck Yung


Whether you're new to the job or a veteran, vendor choice is critical to successful plant management -- especially for repairing electric motors and drives. Selection can be tricky, given the variables to consider, including price, delivery, service, expertise, reliability and quality. No one wants to pay too much, but buying on price alone can backfire. The cost of downtime, idled 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? Begin by listing the types of equipment you have, your basic requirements, and any desired value-added extras that can save time, money and aggravation. Then, evaluate prospective service centers to see how they measure up. If one service center can't provide everything you need, it might make sense to use two or more, based on your requirements and their expertise.

Narrow the choice
Service centers come in many sizes, with varying capabilities and expertise. To narrow down the field, qualify candidates based on their experience in the type of work you expect of them. If most of your motors are 460V and less than 500 hp, it probably doesn't matter that a service center can rewind form-coil motors or rebuild DC machines.

Take a tour
Touring the facilities of several repairers to inspect 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. Look for a well-trained, stable workforce. Probe to determine if 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 without ever solving the real problem.

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

Essential equipment
The test panel should handle a useful range of voltages. Some panels feature a variable-voltage supply, rated 0V to 600V, with additional steps for higher voltages. Others incorporate multi-tap transformers. If your plant uses special voltages (e.g. 208, 575, 950, 7,200, or 13,200), verify that the service center has those voltage capabilities.

The test bed should be able to support motors rigidly during test runs and vibration analysis. Ideally, the baseplate should be grouted to a foundation having a total mass of at least 15 times that of the largest motor it can support. Using a T-slotted base helps simulate actual operating conditions, a factor that becomes important for motor speeds of 3,600 rpm and above (see Figure 1). If your plant uses vertical motors, ask how the service center will support them during testing.

Massive stability


Figure 1. A test bed having a T-slotted base helps simulate operating conditions. The base should have a mass at least 15 times the mass of the largest motor it can handle.


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

A balancing stand allows each rotating element to be precision balanced, a measure that becomes more important at higher motor speeds. Expect most service centers to have a balancing stand because subcontracting balancing likely results in delay. Although most commercial balancing stands have a 100% overload capacity, they should be sized to handle work without dipping into that reserve. To protect shafts from scoring, balancing stand rollers must be kept clean and should be covered when not in use.

Standard electrical 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 the rated voltage plus 1,000 for the AC test and 1.7 times rated voltage for the DC test. A megohmmeter is important as a first line test, but isn't adequate by itself.

A core loss tester is an important tool that verifies the stator core wasn't damaged by winding failure or removal, and for checking the condition of repaired or restacked cores. Commercial core testers are easy to use and they document results with a printout. A core test also can be performed using a power supply and multiple loops of insulated cable. See EASA Tech Note 17 for more information.

The controlled-temperature burnout oven should be fitted with a 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 center also should follow proper procedures for loading the oven. For more information, download "Guidelines for Maintaining Motor Efficiency During Rebuilding," from www.easa.com [no hyphens]. Figure 2 shows a controlled-temperature burnout oven.

Cooking the resin

Figure 2. A controlled temperature burnout oven should be equipped to monitor temperature as a function of time. The thermal profile should stay within the upper and lower control limits to achieve the best quality rebuild.


The winding area at the service center should have an adequate inventory of magnet wire and insulation materials. Winding machines should be capable of making concentric or lap coils. Computerized, semi-automated winding machines are an indication that the service center's technology is up to date.

The bearing inventory is another factor. The service center either should stock or have ready access to the bearing sizes and types found in your motors. Many service centers rely on bearing suppliers to carry this inventory for them. This helps keep their inventory costs down, and generally ensures faster stock turnover. A service center with a reliable bearings source doesn't need to stock them.

Other specifics
If your motors are rated higher than 1,000V, look for proof of form coil motor winding capability. Form coil production equipment, however, isn't essential. In fact, most service centers now order custom replacement coils from form coil manufacturers, who often can manufacture and deliver a set more quickly than most service centers could produce them in-house. Service centers 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.

Repairing babbitt bearings requires expertise, although rebabbitting of sleeve bearings is a niche market that bearing specialists serve. Whether bearing repairs are done in-house or subcontracted to a specialist, quality control procedures, such as 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 large enough to handle the range of motors repaired (see Figure 3). A service center with a vacuum pressure impregnation (VPI) vessel 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. If a service center doesn't do enough of this work to justify maintaining its own VPI system, the work is subbed out. Whether a service center uses a dip tank or a VPI vessel, it should test resin integrity. Viscosity measurements may be done in-house, at least quarterly. An outside party, usually the resin supplier, should perform regular detailed testing of the resin's chemical composition.

Into the drink

Figure 3. Immersing the winding in a dip tank filled with varnish coats the winding. After baking, the solidified varnish stabilizes the winding geometry.


Machine shop capability may be in-house or locally available. In either case, verify that the shop's 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 your typical repair (see Figure 4). 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 center 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 to minimize turnover of key employees. With traditional on-the-job training, it may take an employee more than 10 years to become fully proficient in repairing electric motors. Ask about formal and in-house training programs. Find out if employees receive training from other sources, such as trade and professional associations; motor, drive or pump manufacturers; community colleges; vocational and trade schools; or industry consultants and specialists. Whether formal or informal, training should be documented.

Turn, turn, turn

Figure 4. The concentricity achieved on a lathe is the beginning of a well-balanced rotor.


Documentation also is the foundation for any quality program. Besides ensuring that proper procedures are followed, documentation 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 center may do excellent work without it, but a paper trail is necessary in larger operations, in which one technician doesn't take the job from start to finish.

Bring your repair specification with you to ensure that the service center agrees with the details. If you don't have a spec, 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, because the standard is updated at least every five years. If you have a written specification, review it regularly to keep up with improved processes.

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

If you require pages of reports that document each step of the repair, someone at the service center must prepare them and you should expect to pay the service center's hourly rate to get them. The same applies to load testing and other nonstandard tests. By the way, if you require special tests when purchasing a new motor, you'll pay extra for that, too.

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 the cost of down time 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.

Note: MG 1-2003 is a standard for electric motor construction and testing.

Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis. Call him at (314) 993-2220 or fax to (314) 993-1269.

Figures courtesy of EASA.