Application of Epoxy Resins in the Main Insulation of High-Voltage Motors

High-voltage motors can be categorized into generators and motors. Historically, as power generation capacity increased, generator sizes continuously expanded, placing higher technological demands on generator design. The development of large-capacity generators has also driven advances in motor technology. Although generators have existed for more than a century, their fundamental structure has not undergone significant change; improvements have primarily occurred in cooling and insulation technologies, highlighting the critical role of insulation in electrical machines.

The service life of a motor largely depends on the lifetime of its insulation system, which is why insulation is often referred to as the “heart” of the motor. Among the various insulating materials used in electrical machines, the most essential is the groundwall insulation of stator windings in large high-voltage machines, commonly known as the main insulation. The main insulation plays a decisive role in the entire insulation system.

Researchers and engineers around the world have long pursued the development and optimization of new main insulation systems to enhance insulation performance and strengthen product competitiveness. For example, Siemens introduced the VPI technology from Westinghouse in 1957 and, in the early 1960s, established its own Micalastic insulation system. Improvements made in the 1970s led to the second-generation Micalastic system, and further advancements in 1987 resulted in the third-generation system. To this day, major electrical machine manufacturers continue to refine their insulation technologies, making main-insulation development a long-term and ongoing endeavor. At the core of these advancements lies the epoxy resin curing system, where the material also functions as a highly reliable epoxy resin electrical insulator in modern insulation designs.

I. Classification of Main Insulation Manufacturing Processes and Material Components

1. Classification of Manufacturing Processes

Although domestic and international motor manufacturers have their own insulation structures, the processes can generally be divided into two major categories:

Low-resin VPI insulation

High-resin molded (or liquid-molded) insulation

Low-resin VPI processes include:

Vacuum Pressure Impregnation of individual bars, used primarily for stator bars of large generators.

Vacuum Pressure Impregnation of entire windings (global VPI), used mainly for high-voltage motors.

ABB, for example, applies global VPI even for 300 MW-class generators by installing the stator core under external pressure.

High-resin molded insulation involves wrapping the stator bars with epoxy glass-mica tapes and subsequently performing mold pressing and curing—each bar processed individually.

2. Material Components of Main Insulation

Regardless of the manufacturing method, the main insulation system is fundamentally composed of three parts:Mica (mica paper), Glass fabric and Epoxy resin

(1) Mica Powder (mica paper)

Mica belongs to the silicate family. Muscovite mica is predominantly used in main insulation due to its excellent electrical insulation, low dielectric loss tangent, superior arc resistance, corona resistance, high-temperature stability, and corrosion resistance. Since natural block mica is scarce, mica paper made from ground mica is used as the primary high-voltage-resistant material in electrical machines.

(2) Glass Fabric

Glass fabric serves as a reinforcement layer within the mica tape. It provides adequate tensile strength and good bonding with resins. In China, 2–3% paraffin sizing is commonly added during weaving, which negatively affects adhesion and must be removed before producing mica tape. To ensure high insulation quality, alkali-free electrical-grade glass fabric must be used.

(3) Bonding Resins and Impregnation Resins

Bonding resins are used in low-resin or high-resin mica tapes to bond mica paper with the reinforcement materials, enabling tape fabrication. Impregnation resins are used in VPI processes.

Among the three insulation materials, mica and glass fabric are inorganic and inherently provide excellent thermal and electrical properties. Therefore, the performance of the epoxy resin system becomes the decisive factor in determining insulation quality.

Globally, advancements in insulation systems essentially refer to improvements in bonding and impregnation resins. Enhancing main insulation performance fundamentally depends on optimizing the epoxy resin system. As such, research in this field has always centered on epoxy resin materials, a focus also shared by every leading aliphatic epoxy supplier committed to high-performance insulation solutions.

Epoxy Resin Curing Systems: The “Soul” of High-Voltage Motor Main Insulation

As the “soul” of the main insulation system, the performance of the epoxy curing system directly determines the insulation level and operating lifetime of high-voltage motors. From low-resin VPI to high-resin molded insulation, the ultimate performance always relies on the synergistic composite system formed by mica, glass fabric, and epoxy resin.

Mica and glass fabric act as the inorganic framework, providing structural stability; epoxy resin functions as the critical bonding and impregnation medium, integrating these materials into a dense and robust composite. With innovations in material tetra further enriching formulation possibilities, the tunable chemistry of epoxy resin enables insulation systems to meet the increasing demands for higher voltage levels and harsher operating environments.

For this reason, the evolution of insulation technology among global motor manufacturers is essentially a history of epoxy resin system development. As the industry moves toward higher power density, greater reliability, and extended service life, future main-insulation innovation will continue to focus on epoxy resin materials—developing new formulations and curing systems with:Higher thermal conductivity, Lower dielectric losses, Greater mechanical strength and Better environmental performance

The application of epoxy resins in main insulation is not only a proven and mature solution, but also a dynamic frontier of continuous innovation. Epoxy systems will remain the “guardian” at the heart of high-voltage electrical machines, supporting the ongoing advancement of the modern power industry.