All electrical equipment, from generators to electric motors, experiences a natural decline in efficiency over time due to wear and tear. When it comes to the former, overhauling and generator maintenance can help restore them to their original condition. However, electric motors require a more specialised solution: rewinding. When a motor’s performance drops below acceptable levels, rewinding the motor’s windings is the only effective means to restore efficiency.
But rewinding is just the first step. An additional process is required to varnish the windings to ensure long-term reliability and performance following an electric motor rewind. Applying resin or varnish to the windings offers multiple benefits. It acts as a protective shield, safeguarding against contaminants, preventing electrical short circuits, and providing mechanical rigidity to the windings. Furthermore, it enhances thermal resistance and dielectric strength, reducing the likelihood of electrical failure while ensuring optimal insulation performance.
Given the importance of this step, selecting the right varnishing technique is key. Below, we delve into the four most commonly used methods to help you choose the best fit for your application.
1. Trickle Varnishing
This method involves heating the motor winding on a rotating table using electrical resistance. Once the winding reaches the desired temperature, a small, controlled stream of varnish is applied to the winding head. The varnish seeps into the winding slots, ensuring complete coverage and eliminating the risk of partial discharges in random windings.
As the varnish saturates the windings, the current is increased, causing the varnish to cure directly on the machine. This direct curing process makes trickle varnishing faster than traditional methods like dip-and-bake, making it an excellent choice for emergency repairs or situations where quick turnaround times are essential.
2. Dip and Bake Varnishing
The dip-and-bake technique is one of the most common methods for varnishing motor windings. In this process, the windings are immersed in a tank of varnish to ensure full saturation. Once coated, they are placed in an oven to cure. To achieve optimal coverage, it is recommended that the windings undergo two dipping cycles.
However, a common oversight in many repair facilities is failing to let the windings cool completely after the first curing cycle before performing the second dip. If the windings are still hot, the varnish becomes less viscous, causing it to run off too easily and compromising the effectiveness of the second dip.
To meet quality standards and ensure thorough insulation, it is crucial to allow the windings to cool to room temperature before the second dip. By adhering to this step, you can achieve uniform varnish coverage and enhance the overall durability of the windings.
3. Vacuum Pressure Impregnation
Vacuum Pressure Impregnation (VPI) is a more advanced technique designed for maximum varnish penetration and adhesion. This method uses a specialised vacuum pressure tank to impregnate motor windings and insulation fully with resin or varnish, ensuring deep saturation.
The process begins with preheating the windings to improve resin absorption. They are then placed in a vacuum chamber, where air and moisture are removed from the windings. Once the vacuum is complete, the tank is filled with resin or varnish until the windings are completely submerged. The chamber is then pressurised to force the varnish deep into the winding slots and insulation tapes.
This process involves precisely timed vacuum and pressure cycles, with capacitance monitoring used to confirm complete varnish penetration. Although VPI is the most time-consuming of all varnishing methods, it is particularly suitable for medium-voltage electric motors. Traditional techniques often fail to saturate coils and insulation tapes thoroughly, making VPI indispensable for achieving high-quality, long-lasting results.
Conclusion
Motor winding varnishing plays a crucial role in enhancing the performance, reliability, and lifespan of electric motors. By choosing the right varnishing technique, you can ensure that your motor windings are well-protected and equipped to meet the demands of your application. Understanding these methods empowers you to make the right choice, ultimately safeguarding your investment and keeping your equipment running at peak efficiency.
