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.
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
Turn, turn, turn
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.