How to Get Good Rewind on Motors

You spent big money replacing standard efficiency motors with energy-efficient models, but one just failed. Do you rewind it or replace it? A knowledgeable service center can rewind energy-efficient motors without a loss of efficiency. However, make sure you discuss specific procedures with them before sending a motor out for rewind. Ask for a repair report, and build guidelines into it to ensure

You spent big money replacing standard efficiency motors with energy-efficient models, but one just failed. Do you rewind it or replace it?

A knowledgeable service center can rewind energy-efficient motors without a loss of efficiency. However, make sure you discuss specific procedures with them before sending a motor out for rewind. Ask for a repair report, and build guidelines into it to ensure you get the best repair. Winding resistance (corrected to standard ambient) serves as a good quick-check, and core-loss test results confirm no damage resulted from the motor failure or burnout process. Most importantly, insist on documentation.

What to look for. Take a stator core apart, and you have a pile of laminations. These are thin pieces of steel, coated with insulation to reduce eddy-currents in the core. If you take a motor apart post-mortem, look for core damage: a hole, evidence of rotor drag, or laminations fused together. Then, check the windings. To remove the old windings, most shops process stators in special burnout ovens to burn off insulating varnishes and epoxies. They maintain the appropriate oven temperature to preserve the core's interlaminar insulation. The insulation reduces efficiency-sapping eddy currents. Newer insulations can withstand higher temperatures than previous formulations.

Because winding insulation materials burn at lower temperatures than the interlaminar insulation, a proper burnout process won't harm the interlaminar insulation. A recent study in the United Kingdom settled on 750 DegrF for standard laminations. It showed oxide-coated newer steel laminations could withstand temperatures in excess of 900 DegrF with no loss of efficiency. The Electrical Apparatus Service Association (EASA) recommends oven temperature should not exceed 680 DegrF for organic or 750 DegrF for inorganic interlaminar insulation.

The repair shop should perform a core-loss test before and after removing the old windings. The watts/pound readings help determine if the core is good, and safeguard against burnout problems. Commercially available core testers simplify the process and usually provide printouts documenting the core's condition. Alternatively, they can use a wattmeter, power supply, and some manual calculations to determine core loss.

Windings. The cross-sectional area of the conductors determines the amount of copper in a motor. A larger cross-sectional area reduces copper losses (heating due to resistance). We usually report this as circular mils per amp (CM/A). The CM/A for the new winding must not be lower than that of the original winding or efficiency will decrease.

Motors manufactured before 1964 generally had at least 550 CM/A. From 1964 until the advent of the energy-efficient motor, T-frame TEFC motors usually had lower current densities (e.g., 350 CM/A to 450 CM/A) to meet demands for lower cost, lighter weight motors. Today's high-efficiency models may have current densities of 600 CM/A to 1000 CM/A. Ensure your shop understands that changing current density affects efficiency. Don't let them reduce wire area (and thus efficiency) just to make a particular motor easier to rewind.

Random-wound motors may have either lap or concentric windings. Manufacturers use concentric windings in smaller motors, because they improve automated production. This keeps new motors economical for the buyer. What's the downside? Not every turn in a concentric coil is equally effective.

Viewed from the end of a stator core, the concentric winding has coils with two or more different slot spans. Each span has a different angle or "chord factor," which determines the effectiveness of the turns within that coil. Depending on the chord factor, 10 turns in a coil spanned at 1-9 will not have the same strength as 10 turns of a coil spanned at 1-10 or 1-8. If the turns in one span are half as effective as the turns in another, you need twice the turns. This doubles resistance of that coil.

The distance around the coil changes with the span, so a larger span for the same 10 turns requires a longer conductor. Because a longer conductor has greater resistance, the total winding resistance depends partly on the coil span(s).

The distance the coil ends extend beyond the core is one factor in determining the conductor length. Keeping these "coil extensions" to a minimum by careful fitting helps control the "mean turn length" This length is the average distance around each coil. The shorter the mean turn length, the lower the total winding resistance.

Rewind shops have an advantage over manufacturers, because they can use lap windings. All coils in a lap winding have the same span and mean turn length. It takes longer to insert a lap winding, but this isn't a problem for most shops. They install lap and concentric windings manually because the sheer variety of equipment they repair makes automation impractical.

Because all coils in a lap winding have the same mean turn length, careful fitting may allow you to produce a winding with a lower resistance than the original winding. A carefully fitted rewound motor can be more efficient than the original. Generally, the shop should replace the stator winding with the same wire size, winding type, turns, span, and coil extension as it originally had. Allow the shop to convert from concentric-to-lap only if mean-turn-length calculations for both windings prove you can reduce total winding resistance. Regardless of type, winders must carefully fit the coils and avoid lengthening coil extensions to ease insertion. Lengthening coil extensions increases total winding resistance, and the portion projecting from the iron is inefficient.

Bearings. Quality bearings of C-3 internal clearance are the standard, except for some specialty applications. Sealed bearings prevent contamination and require no periodic lubrication. Unfortunately, they create more friction than shielded or open bearings. For maximum efficiency, stick with the open or shielded bearing style installed by the manufacturer. For greater reliability, consider sealed bearings, despite the expected efficiency drop.

Windage. External fans lower efficiency. Windage varies among fan designs. Diameter, number and size of blades, material, and surface finish (including paint) all affect efficiency. Diameter has the most effect. Replacing a damaged fan with a dissimilar one is unwise.

Testing. Core-loss tests are critical for verifying the stator core's condition. The shop should take before and after burnout watts/pound. It should have an adequate test panel, instruments, and power supply to perform no-load tests. It should have an accurate bridge to measure extremely low winding resistance. Make sure it records and corrects the winding temperature at the time of measurement to a standard temperature (usually 25 DegrC).

Look for calibration information on the burnout oven's temperature control. Ensure the shop can monitor the burnoff so temperatures don't exceed safe levels. A recording instrument works well for this, (e.g. a paper chart recorder).

Dynamometer testing is expensive because of the setup time, test time, and power consumption. With smaller motors, the expense is difficult to justify. Very large motors require a substantial investment in switchgear and power supply. Most end-users make the load-test decision based mostly on how critical the application is. Manufacturers are developing instrumentation to test efficiency without applying full-load tests.

Getting significant efficiency improvement from motors not originally built as energy-efficient is seldom practical. The stator core and rotor losses depend on the core length and type of steel in the laminations. The frame fits the existing core, but would probably need modification to fit a new one. Adding laminations to the stator and rotor, then rewinding the stator and rebuilding the rotor is rarely feasible. The slot configuration holds a specific amount of copper, so increasing the cross-sectional area of conductors to reduce copper losses is seldom possible.

Yung is a Technical Support Specialist with the Electrical Apparatus Service Association (EASA), St. Louis.

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