Tape provides both insulation and a substrate to which the varnish bonds.
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Since their introduction, the first wave of energy-efficient motors has had time to run to failure. Not surprisingly, they fail for the same reasons that standard motors fail: heat, overloads, voltage spikes, mechanical problems.
Now, the old question arises. Does the 57% repair/replace threshold still apply?
Tradition says yes, but technology says no. Hidden beneath the hype that followed the new generation of motors was a quiet evolution in the motor repair industry. This, too, was energy focused. Old repair practices and procedures were evaluated and a few were found to degrade a motor’s efficiency.
Of the traditional repair practices that degraded motor efficiency, coil burnout was the biggest offender. Insulating varnish must be treated like paint to remove it, and it was not uncommon to see a motor turned on its end over a bucket of flaming kerosene, the insulation surrounding the coil slowly carbonizing to ash.
It was effective, but this technique put the motor iron in thermal stress, with the hot end approaching 800°F or 900°F and the cooler end at less than half that. When one end was carbonized, the unit was flipped to cook the other end. The extreme temperature gradient caused core losses in the iron and often left the slots distorted. For a motor of 50 hp or greater, the decline in efficiency was about 0.9% to 1.0%. Repeated rewinds made the situation even worse, dropping efficiency as much as 5%.
“Bucket burnout” or other direct-flame methods are no longer considered effective ways to remove damaged coils. Today burnout is done in an oven where temperature zones are controlled to within ±10°F. With temperature peaks limited to 700°F (370°C) and evenly distributed, core iron emerges both mechanically and electrically unchanged. Recently, even safer chemical techniques have shown promise to remove varnish with no danger of thermal damage.
Better balance, stalwart stators
Procedures for restoring the coils have changed, too. Replacements now are wound with overlapping layers of tape insulation that, by itself, has a high dielectric strength (Figure 1). In addition to insulation, this tape also provides a porous surface into which new insulating varnish is applied via vacuum/pressure impregnation (VPI). The porous tape gives “tooth” for adhesion of the new varnish.
The materials used in rewinds have evolved to improve motor performance. VPI is not new, but because it became the standard for infusing rewound motors with varnish, new resins with extreme dielectric strength and mechanical rigidity have been developed. Today’s rewound stator shows little tendency to shift with the same current surges that would have cracked older insulation systems.
The motor repair industry also owes grudging thanks to ISO because it forced a wholesale reevaluation of practices and procedures, particularly on the metrology side of the business. Today, the process of taking measurements and balancing rotating parts is more precise. Even on large rotors and armatures, imbalances are held to less than 0.1 mil or better.
Figure 2
