In motor manufacturing, the impregnation and curing process is often considered a “behind-the-scenes” step, yet its importance is undeniable. It directly determines the insulation performance, heat dissipation, and mechanical strength of the motor windings, making it a critical factor affecting overall motor performance and lifespan. However, in practice, insufficient varnish on windings is a common issue and a major “hidden killer” of motors.
Direct Effects of Insufficient Varnish on Motor Performance
During the impregnation and curing process, insufficient varnish can degrade multiple aspects of motor performance:
· Insulation Performance Degradation
One core function of insulation varnish is to fill gaps between windings, forming a uniform and dense insulating layer. Insufficient varnish leaves air or moisture inside the windings, reducing insulation resistance and increasing local partial discharge. For high-voltage motors, this can lead to winding breakdown and even motor burnout.
· Reduced Heat Dissipation
Insulation varnish not only provides electrical insulation but also acts as a thermal conductor, transferring heat from the windings to the core and housing. If the varnish fails to fill gaps adequately, thermal pathways are blocked, increasing thermal resistance and causing excessive motor temperature. Long-term high-temperature operation accelerates insulation aging, creating a vicious cycle.
· Insufficient Mechanical Strength
The goal of the impregnation and curing process is to bond loose wires into a solid structure. With insufficient varnish, windings may loosen under electromagnetic forces, centrifugal force, or vibration, leading to insulation wear and turn-to-turn short circuits. Mechanical stability is especially crucial under frequent start-stop or fluctuating load conditions.
Causes of Insufficient Varnish
This problem often arises from materials, equipment, and process factors:
· Improper Varnish Viscosity Control
Viscosity is critical for effective impregnation. Too low viscosity results in insufficient solids content, preventing effective film formation; too high viscosity hinders penetration into deep winding gaps, especially at the slot bottom or between layers. Even with VPI (Vacuum Pressure Impregnation) systems, lack of dynamic viscosity monitoring leads to inconsistent results.
· Suboptimal Impregnation and Curing Parameters
o Inadequate pre-baking: Residual moisture or air in windings blocks varnish penetration.
o Mismatched curing temperature/time: Too low temperature leaves varnish uncured and prone to flow; too high temperature hardens the surface quickly while trapping solvents inside, forming “dry outside, wet inside.”
o Insufficient curing time: Shortened cycles for efficiency reduce mechanical strength due to incomplete curing.
· Equipment and Operational Issues
In traditional ovens, stationary windings allow varnish to flow downward under gravity, causing uneven film thickness. Parameters like vacuum level and pressure holding time in VPI equipment also directly affect impregnation results if not regularly calibrated.
Systematic Solutions to Ensure Adequate Varnish
To achieve uniform and sufficient varnish coverage, improvements should target materials, process, and equipment:
· Material Management
o Implement strict viscosity monitoring, adjusting dilution according to temperature and humidity to balance flow and penetration.
o Choose compatible insulation varnishes to avoid chemical reactions with magnet wire or slot insulation.
· Process Optimization
o Enhance pre-baking: Use stepped temperature ramps to fully remove moisture from windings.
o Adopt rotating curing: Slow rotation during curing uses centrifugal force for uniform varnish distribution, preventing local excess or shortage.
o Define precise curing curves: Set heating, holding, and cooling stages based on varnish characteristics to ensure synchronized inner and outer curing.
· Equipment Upgrades and Process Control
o Regularly check VPI vacuum systems, pressure tanks, and pipelines for leaks to maintain process stability.
o Introduce automated control systems to monitor varnish time, vacuum, and pressure in real-time, minimizing human error.
o Perform sampling inspection of varnish coverage per batch and correlate with electrical performance for closed-loop quality control.
Case Study
A high-voltage motor manufacturer experienced batch rework due to insufficient winding varnish. Investigation showed poor viscosity control and outdated curing practices were the main causes. Corrective actions included:
1. Installing online viscometers for real-time varnish monitoring and adjustment.
2. Switching from static to rotating curing, increasing uniformity by over 30%.
3. Creating an impregnation-curing process database to optimize parameters using big data.
After these improvements, average varnish coverage increased significantly, failure rates dropped by 45%, and motor lifespan improved substantially.
Conclusion
Although motor winding insulation is a traditional process, its technical depth and management requirements are substantial. Insufficient varnish reflects not only process issues but also a lack of systemic quality control. Integrating materials, equipment, process, and management ensures “saturated filling” of winding insulation, laying a solid foundation for reliable motor operation. In today’s advanced motor technology era, attention to detail ensures quality, and optimized processes achieve excellence.
Source： Jintian Copper