The function and application scope of insulating paint

Category Characteristics and Quality Control System of Insulating Varnishes for Electric Motors  As a key foundational material in the field of electric motor manufacturing, the technological evolution of insulating varnish has always been closely linked to the development of motor design. From early basic insulation suitable only for conventional operating conditions to today's specialty coatings supporting high-temperature, high-power-density motors, continuous breakthroughs in the composition innovation and manufacturing processes of insulating varnishes have constantly expanded the performance boundaries of traditional motor design. The synergistic evolution between materials and equipment has enabled motors designed for extreme operating conditions, once confined to laboratories, to gradually move towards industrial application. Precisely because of this, the technical characteristics and quality control of insulating varnishes remain a core focus of the motor industry.

Category System and Application Scenarios of Motor Insulating Varnishes

Based on their functional roles in motor manufacturing, insulating varnishes have formed five major product categories, each undertaking a unique insulation mission and collectively building the safety barrier for motor operation:

Impregnating Varnish: The "Lifeblood" of Motor Windings

As the most consumed category of insulating varnish, the core function of impregnating varnish is to penetrate and fill the gaps and micropores in winding coils, solidifying the loose windings into a rigid whole after curing. Its technical forms are mainly divided into two branches:

Solvent-based Impregnating Varnish:** Viscosity is adjusted by adding solvents such as toluene or xylene. Typical products like 1032 Melamine Alkyd Varnish usually have a solid content ≥45%. Multiple impregnation-drying cycles are required to form a dense paint film, suitable for mass production of small and medium-sized motors.

Solventless Impregnating Varnish:** Based on low molecular weight prepolymers, with volatile content during curing ≤15%. It can complete impregnation and curing in a single step, significantly improving production efficiency, and is widely used in high-voltage motor windings.

Production data from a motor factory shows that using solventless impregnating varnish can reduce winding curing time from 8 hours to 2 hours, with insulation breakdown strength increasing by 20%.

Wire Enamel: The "Protective Armor" for Magnet Wire

As the core component for electromagnetic conversion, the surface insulation of magnet wire relies entirely on the protection of wire enamel. This type of coating must withstand the harsh tests throughout the motor manufacturing process:

Mechanical Stress:** Withstands stretching (tension ≥15N) and bending (bend radius ≤2 times wire diameter) during coil winding without film cracking.

Thermal Shock: Maintains insulation integrity through repeated thermal cycling (≥50 cycles) from room temperature to 180°C.

Chemical Resistance: Withstands long-term immersion (≥1000 hours) in media like transformer oil and Freon.

The paint film formed by high-quality wire enamel has uniform thickness, controlled between 0.02–0.05mm, and passes the twist-and-peel test (180° twist angle × 3 times) without peeling.

Covering Varnish: The "Outer Shield" of the Insulation System

Applying covering varnish to the winding surface after impregnation is like adding a protective shell to the insulation system. Its technical requirements focus on environmental resistance:

Physical Protection:** Film hardness ≥2H (pencil hardness), impact strength ≥50 kg·cm, capable of resisting mechanical impacts during assembly.

Environmental Adaptation:** After 1000 hours in 95% relative humidity, insulation resistance retention rate ≥80%.

Chemical Resistance: After 168 hours immersion in 30# mechanical oil, the film shows no wrinkling or peeling.

The typical application thickness for covering varnish is 30–50Мm, applied by dipping or spraying, and requires baking at 120°C for 2 hours to achieve optimal performance.

Lamination Coating: The "Invisible Film" for Reducing Core Loss

As the core material of the motor iron core, the surface insulation coating on silicon steel sheets is crucial for suppressing eddy current losses. The technical characteristics of lamination coating are:

Ultra-thin Coating:** Dry film thickness strictly controlled at 1–3Мm. Excessive thickness reduces the stacking factor of the core (each 1Мm increase reduces stacking factor by 0.5%).

High-Temperature Curing: Requires rapid curing at 300–400°C for 10–30 seconds, compatible with the silicon steel annealing process.

Insulation Resistance:** Coating volume resistivity ≥10¹⁴ Ω·cm, ensuring interlamination insulation.

Experimental data indicates that high-quality lamination coating can reduce motor core loss by 15–20%, significantly improving energy efficiency class.

Corona Suppression Varnish: The "Voltage Stabilizer" for High-Voltage Environments**

In high-voltage motors (voltage ≥10kV), the strong electric field at the coil ends can easily cause corona discharge. Corona suppression varnish achieves uniform electric field gradient distribution by precisely controlling the film resistivity (10⁴–10⁸ Ω·cm). Its special features include:

Conductive Fillers:** Uniformly dispersed carbon black or graphite particles (particle size 50–100nm) form conductive pathways, suppressing local high field strength.

Gradient Design:A coating system with progressively varying resistivity (3–5 layers) from the coil surface to the end eliminates abrupt field strength changes.

Application Precision:** Wet film thickness controlled at 80–120Мm, with post-curing deviation ≤5Мm.An application case in a 50MW turbo generator showed that after using corona suppression varnish, the corona inception voltage at the coil ends increased from 3kV to 15kV.

Core Performance Indicators and Technical Requirements for Impregnating Varnish

As a key material in the motor insulation system, the performance indicators of impregnating varnish directly determine winding reliability. The industry has established systematic technical specifications:Process Compatibility Indicators  Viscosity Characteristics:** Dynamically adjusted according to coil gap size. For 0.2–0.5mm gaps, viscosity is controlled at 20–50s (No.4 viscometer, 25°C) to ensure complete penetration within 10 minutes.Solid Content:** Solvent-based varnish ≥45%; Solventless varnish characterized by volatile content during curing (≤15%). High-solid, low-viscosity products can reduce the number of impregnation cycles. Drying Rate:** After baking at 130°C for 30 minutes, the film is dry-to-handle (no mark by finger touch method), meeting continuous production rhythms.