An objective look at rewind technologies

The choice of insulation system should be driven by motor or generator application, dimensions and specific user requirements.

By Tom Reid

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Motor and generator repair specialists typically offer one of two technologies for medium-voltage and high-voltage rewinds — vacuum process impregnation (VPI) and B-stage fully cured coils. Both technologies are proven solutions that provide superior performance and service life. Instead of promoting one technology over the other, I contend that a well-equipped repair facility should offer both systems. The choice should be driven by the type of application, the motor or generator dimensions, and specific user requirements.

An objective look at both rewind technologies supports this point of view. With that in mind, I’ve prepared an appraisal with input from repair specialists who use both systems to provide plant professionals a more thorough understanding of the factors that influence the selection. In general, repair specialists who offer one rewind technology typically don’t spend much time extolling the virtues of the other.

From the top

The decision to rewind a motor or generator begins with an assessment of its performance and condition. The tools include testing that combines electrical measurements, such as megger, polarization index, hi-pot, surge and partial discharge levels, with a comprehensive visual assessment. Repair specialists collect these results and take into account the required reliability the customer needs before making a recommendation.

Motor and generator repair specialists typically offer one of two technologies for medium-voltage and high-voltage rewinds — vacuum process impregnation (VPI) and B-stage fully cured coils.

– Tom Reid

Let’s stipulate that both rewind technologies deliver performance and reliability that matches or exceeds original performance specifications. Regardless of which technology you select, you should expect to receive a comprehensive, written evaluation of your motor or generator’s insulation system. Visual inspections can reveal problems such as ground shorts and rotor strikes as well as cracks, shrinkage, coil abrasion and corona discharge. These inspections should be combined with a review of measured and recorded data to reveal how the existing insulation system performed. Your service provider’s recommendation also should consider operating conditions and specific process requirements.

B-stage fully cured coils are the logical choice for extremely large motors or generators. Many high-voltage motors are too large for immersion in the largest commercial VPI systems. Fully cured coils also are appropriate for hydroelectric generators. These large generators can’t be removed from the generating stations and shipped to a repair center, so the rewind must be done in the field.

Figure 1. Size can determine how a motor or generator is to be repaired. These hydroelectric generators are too large to fit inside a VPI tank or be moved to a service center and must be rewound with fully cured coil technology.
Figure 1. Size can determine how a motor or generator is to be repaired. These hydroelectric generators are too large to fit inside a VPI tank or be moved to a service center and must be rewound with fully cured coil technology.

On the other hand, both fully cured and VPI rewinds usually are recommended for motors and generators working in harsh environments, where abrasive particles such as fly ash or chemicals such as sulfuric acid can attack the motor or generator insulation materials and coils. The primary objective is to ensure the insulation system is engineered to your specific reliability and performance requirements. Let’s take a closer look at each technology, and match it with your specific requirements.

B-stage fully cured coils

This is the preferred insulation system for very large or permanent motors and generators. For older, permanently mounted motors or generators, or anything typically measuring more than 14 ft. in diameter, replacing decayed or damaged coils with new B-stage fully cured coils wound to OEM specifications is a proven solution. In most cases, dielectric and thermal performance can be improved because modern insulating materials are superior to those that OEMs used during the 20th century. Some hydroelectric generators, for example, were commissioned in the early 20th century or even the late 19th century (see Figure 1).

In a fully cured coil rewind, individual coils with insulating materials impregnated with B-stage resin are pressed with heated forms (Figure 2) to catalyze the resin, and then post-cured.

Figure 2. Technicians prepare a hard coil for hot pressing and post-curing.

Figure 3. Finished hard coils are installed into a large stator.

Figure 2. Technicians prepare a hard coil for hot pressing and post-curing.
Figure 3. Finished hard coils are installed into a large stator.

Technicians can test coils individually as each is manufactured and installed to prevent dielectric damage in the finished unit. During production, coils are tested at voltages far beyond the unit’s normal operating voltage to ensure that they can withstand abnormal voltage stresses the power system might impose. Performance improvements and increased power density, reliability and efficiency come from improvements in both materials and craftsmanship (Table 1).

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