Industrial Motors

How to ensure effective motor repair and rewind

Speak the same language as your service center when it comes to setting performance expectations.

By Tom Bishop, P.E., EASA

The Electrical Apparatus Service Association (EASA) has published two documents to help users and service providers ensure that motor repairs performed reflect good practices that maintain or improve a machine's energy efficiency and reliability: ANSI/EASA Standard AR100-2015: Recommended Practice for the Repair of Rotating Electrical Apparatus and the "Good Practice Guide" of the 2003 study The Effect of Repair/Rewinding on Motor Efficiency, by EASA and the Association of Electrical and Mechanical Trades (AEMT). These documents serve as tools by which service centers and end users can speak the same language when it comes to level-setting service and performance expectations on motor repair and rewind.

Also, a little more than a year ago, EASA launched its electric motor repair accreditation program, based on AR100 and the "Good Practice Guide." The program benefits both end-users and service providers by ensuring that electric motor repairs conform to the good practices identified in the aforementioned documents. "

Electric motor efficiency can be maintained during repair and rewind by following defined good practices. This article builds on my previous discussion of PM and PdM for three-phase squirrel-cage motors ("PM and PdM for electric motors") by outlining some of the expectations and good practices for repairs of these types of motors.

Initial inspection and testing

The repair process should begin with rigorous testing and inspection as specified in ANSI/EASA AR100-2015. Besides providing an accurate assessment of the machine's condition and repairs needed, this good practice often identifies latent faults or weaknesses that can be addressed during the repair process to improve the machine's reliability.

Careful inspection of the lubricant reservoir, for example, might indicate water intrusion of which the end user was not aware. Rather than simply removing the contaminant and adding fresh lubricant, the service center might recommend retrofitting the motor with a sealing arrangement (i.e., a new feature that would improve reliability).

Windings

A good practice rewind typically duplicates the original winding's magnetic parameters and losses to ensure that the motor retains its original energy efficiency. Sometimes, however, the service center can make winding changes that will increase efficiency and possibly reliability, too.

Winding copper losses are the product of current (I) squared times resistance (R), expressed as I2R. Because the EASA-accredited rewind process strives to maintain the motor's original design, the current should not change. But there are some good practices that can reduce the winding's resistance. The most common are increasing the wire area of the winding turns, reducing the average length of an end turn (see Table 1), or doing both.

The resulting lower value of R in the I2R formula will reduce power losses and therefore increase efficiency. Lower losses will also reduce heating, thus extending the thermal life of the winding's insulation system and improving the motor's reliability.

For example, increasing the winding wire area of a motor with low slot fill will reduce resistive losses and therefore decrease heating (see Figure 1). A cooler winding also will have less electrical resistance than a hot winding, so it will be more efficient. As an added benefit, a tighter slot fit will improve heat transfer from the winding through the insulating varnish to the core and cooling air. It also will permit less wire movement, thus increasing the fatigue life and therefore the mechanical life of the winding. The combined effect will be a more reliable and energy-efficient motor winding.

Bearings

Studies indicate that more than half of all motor failures begin as bearing failures. If the correct replacement bearings are installed properly, the bearing losses and reliability should be the same as when the motor was new.

Good practice repairs assure that replacement bearings maintain the same losses and reliability by duplicating the originals, including the type of enclosure, if any. Among the key parameters that are preserved are the internal fit class and the sealing arrangement. For example, the replacement for a bearing with a C3 internal fit tolerance and no shields or seals would be an open bearing with a C3 fit.

Replacement bearings also should be installed using a heating method (typically a temperature-controlled induction heater) that will not jeopardize their mechanical integrity. The lubricant chamber is then filled to the correct level to minimize friction and maximize the amount of lubricant that reaches the bearing.

Applications with higher-than-normal radial (overhung) load or where shaft currents exist are often good candidates for retrofits (e.g., converting a drive-end ball bearing to a roller bearing to increase bearing dynamic capacity and life). To conform to EASA's accreditation good practices, the service center would determine the root cause of failure before recommending modifications. If the cause were overtensioning of worn belts to prevent slipping or squealing, the right solution would be to install and properly tension new belts and/or sheaves. Converting to a roller bearing without correcting the cause (high radial load because of overtensioned belts) would likely result in a bent or broken shaft.

EASA-accredited procedures also specify that the location of the cooling fan should not be changed. It may seem like a small detail, but the purpose is to avoid reducing airflow. For the same reason, a replacement fan must be a duplicate of the original. Both are critical factors because less airflow could increase motor heating, while more airflow could increase windage losses and possibly cause turbulence that may reduce cooling.

In conclusion

Overall, aligning with the recommended practices in ANSI/EASA AR100 and the "Good Practice Guide" ensures that repair processes follow a routine but prescribed order and scope, delivering continuous improvement for the service center and more-reliable repairs for the end user.

Repairs that are in compliance with EASA's electric motor accreditation program include subtle additions to these good repair processes, including the use of calibrated instruments for tests and measurements (see Figure 2), testing stator cores both before and after winding removal, and third-party auditing that verifies service center conformance to the program.

Electric motors that conform to the requirements of the accreditation program are labeled by the service center with an EASA-approved sticker (see Figure 3). Conversely, motors that do not meet the requirements cannot be labeled. Note that the labels used by EASA member service centers are slightly different from those used by accredited nonmember service centers.