An objective look at rewind technologies

June 4, 2009
The choice of insulation system should be driven by motor or generator application, dimensions and specific user requirements.

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.


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.

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.

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).

Characteristics of B-stage fully cured coil rewinds.
Criteria Properties and benefits
Repair process Coils are installed and tested coil-by-coil
Repair location In-shop or on-site (Hard coils are portable)
Single-coil repair Usually possible
Repair test protocol IEEE 1043 Voltage Endurance, IEEE 429 Water Immersion, partial discharge, doble and corona probe
Dielectric strength Excellent
Power factor/PD Excellent
Shock and vibration resistance Excellent
End-winding rigidity Excellent
Rigidity of core wedges and fillers Mechanically secured
Temperature stability At least as good as original, often better
Corrosion protection At least as good as original, often better
Voltage limit No limit (based on density of insulation build)
Size limit No size limit

The resin-rich B-stage mica tapes, for example, contain epoxy resin that produces a bonded insulation wall in the slot. Automated coil-forming technology and advanced QA methods ensure precise coil duplication for improved installation and operation. As one B-Stage fully cured coil proponent told me, “In the end, you know all the coils are good.”

Vacuum pressure impregnation

This is an engineered insulation system for severe-duty applications. Over the years, vacuum pressure impregnation has proved to be a reliable, durable, cost-effective approach to rewinding medium- and high-voltage motors and generators. The strength of the VPI process resides in the high-performance resin forced into the insulating tapes and coil structure.

In preparation for the VPI process, “green insulated” coils are inserted into the stator core. They’re connected, blocked, braced and laced into position. Once submerged inside the VPI tank, the coils are subjected to a deep vacuum that removes air and moisture replacing both with a solvent-free resin..

Resin penetration is monitored by means of capacitance change or a similar method. Next, the impregnated winding undergoes thermal curing, which produces a void-free, highly durable insulation and winding structure that resists mechanical damage, as well as abrasion and corrosion. Conductors are completely encapsulated, protected from moisture, dust, corrosives and, in the case of high-voltage units (15 kV max), partial discharge and internal corona damage. Flexing under load, which can cause minute coil shifts, is negligible because the VPI resin produces an encapsulated solid mass. VPI also eliminates the delamination that vibration or thermal cycling can cause. Finally, because the coil and winding are now part of one solid system, there are no air voids and the increased thermal conductivity to the core iron improves cooling (Table 2).

Characteristics of VPI rewinds.
Criteria Properties and benefits
Repair process Stators/rotors are rewound in mass, tested during and after VPI resin is cured
Repair location In-shop only (VPI tanks are not portable)
Single-coil repair Not possible - all coils encapsulated in resin mass
Repair test protocol IEEE 1043 Voltage Endurance, IEEE 429 Water Immersion, partial discharge, doble and corona probe
Dielectric strength Excellent
Power factor/PD Excellent
Shock and vibration resistance Excellent — Bonded as a solid mass
End-winding rigidity Excellent — Bonded as a solid mass
Rigidity of core wedges and fillers Excellent — Bonded into position
Temperature stability Highly stable
Corrosion protection Excellent for sulfuric acid, hydrochloric acid, sodium hydroxide, sodium chloride, black liquor (paper mill caustic) and detergents (Tide)
Voltage limit Up to 15 kV
Size limit Largest VPI tank (about 14 ft. diameter and 17 ft. tall)

High-quality VPI rewinds use premium epoxy resins and high-performance tapes, blocking materials and supports, as well as precise techniques for their application. Together, the combination of craftsmanship and materials yields superior dielectric and mechanical performance.

Figure 4. Coils in VPI rewinds are pre-taped with porous wraps to absorb resin.

Form coils play a key role in VPI rewind performance. Technicians wrap the coils with porous mica insulating tapes that feature overlapping mica platelets to retain the epoxy resin, which is cured into one solid mass (see Figure 4).

Connections are insulated with the same mica tape. The end windings are secured using impregnable blocking and bracing materials, plus woven lashing cords and insulated metal or glass-fiber coil head support rings. The wound core assembly is impregnated with epoxy resin using the vacuum/pressure process (Figure 5) and then oven-cured (Figure 6). At this point, the winding and insulation system achieve their final electrical, mechanical and chemical-resistant properties.

Completed VPI rewinds exhibit high dielectric breakdown resistance. Accelerated aging tests routinely predict a 40-year service life for the insulation system. Premium VPI rewinds are in service across the United States, performing reliably and efficiently in harsh, severe-duty environments. As one longtime VPI customer noted, “I’ve never seen a VPI’d motor fail because of insulation breakdown.”

Figure 5. A stator to be processed in a VPI tank will be completely impregnated and encapsulated with 100%-solids epoxy resin.
Figure 6. This stator is being prepared for final curing in the bake oven.

Picking your path

As noted earlier, the best rewind technology for your motor or generator repair is more obvious in some situations than others. Either way, you’re looking for the same improvements in reliability and performance, which reinforces my belief that repair facilities should offer both systems with the direction driven by the application, the dimensions and your requirements.

In the end, B-stage fully cured coil and VPI rewinds are both viable technologies but, like any tool, they’re only as effective as the knowledge and experience of the repair specialist you select. Accurate diagnostics and documented recommendations, faithfully executed, are the real differentiators.

Regardless of which rewind technology your repair specialist recommends, you should establish agreed-upon service standards and repair specifications. That’s how you ensure the integrity of the rewind process and achieve improved reliability and performance.

Tom Reid is senior vice-president of engineering and quality at Integrated Power Services in Greenville, SC. Contact him at [email protected] and (864) 451-5608