Using Vacuum Pressure Impregnation (VPI) to impregnate electrical windings more exhaustively is nothing new. The U.S. Navy had already employed this process some 50 years ago to impregnate their electrical winding. In the past, the varnishes utilised for impregnation were primarily solvent-containing systems with about 35% to 45% solids, and the rest were solvents.
While this caused the electrical windings to wet deeper and more exhaustively through VPA, it also usually led to the varnishes driving out all the solvents during the process and resulted in pinholes and a weak sponge-like structure that leaves plenty of void and airspace. In other words, by using solvent-containing varnishes, the enhanced penetration and wetting provided by the VPI did not lead to any significant functional improvement.
Fortunately, over the past years, the chemical industry has come out with solventless varnishes or liquid resins that can be applied without using solvents. By adding a bit of curing agent, these resins can also be transformed from liquid to solid structure, provided that the liquid resin impregnants are properly used. Read on to understand more about how the VPI works and its advantages for electric motor rewinds.
What is vacuum pressure impregnation?
A completely wrapped electric equipment rotor or stator is totally submerged in a resin during vacuum pressure impregnation (VPI). Through wet and dry vacuum and pressure cycles, this resin is absorbed throughout the insulating system.
The impregnated windings acquire a uniform, monolithic structure after being thermally treated. Various VPI systems come with different components. However, a complete and efficient VPI system usually consists of a chiller storage vessel, a pressure vessel, a vacuum booster, a vacuum pump, a transfer pump, automatic monitoring and process control instruments, and big baking facilities.
The Usual Procedure of VPI
The VPI process typically starts with the electrical winding (rotor or stator) undergoing proper cleaning and baking. Once the winding has been cooled down to a pre-set temperature in the oven, it is transported to the pressure vessel. The lid of this pressure vessel should be closed and the vessel evacuated.
The vessel will then be vacuumed until it reaches below 5 millimetres of mercury and removes nearly all the moisture and entrapped gas. The vacuum level should be maintained inside the vessel for about an hour. After the winding is submerged in the high vacuum environment for an hour, the storage vessel’s valve should be opened, and the resin will flow into the container under the vacuum condition.
At this point, the resin is already allowed to flood up the top of the electrical winding. When the resin has reached a constant level, the next thing to do is shut off the valve since the vessel continues to vacuum. Once the vacuum process is complete, the pressure cycle shall then begin.
The pressure vessel should gradually build up its internal pressure by the dried air until about 100 psi or 7 bars. This pressure should be held for an hour or longer for a bigger rotor or stator. Then, the resin should be allowed to penetrate all pinholes and void in the winding’s insulation system.
Once the pressure cycle is complete, the pressure inside the vessel should be released, and the resin should be returned to the storage vessel. The lid should be opened, and the winding should be lifted and placed in the electric oven for curing. Depending on the size of the machine, the baking process usually takes around 10 to 12 hours.
Benefits of Using VPI in Electrical Winding
The use of VPI in electrical winding has some real advantages, especially for those users who require extended machine life and enhanced reliability. This is particularly true when severe environmental conditions arise in applications, such as chemical, petrochemical, marine, cement, steel, and food industries.
Due to the better heat transfer and removal of hot spots that enable heat to disperse through the machine’s structure, VPI can also maximise tolerance to the machine’s marginal overloading. To overcome issues of end winding movement, VPI can likewise be utilised either through VPI treatment alone or with additional bracing materials before the treatment.
Lastly, since using VPI allows the core iron to be filled and sealed, it has the potential to eliminate problems that arise due to loose cores and to provide robust protection against the migration of moisture and other contamination through the core.
Overall, if carried out properly, VPI can bring about several advantages, particularly regarding electrical winding matters. To ensure that VPI is conducted the right way, it is highly advised to seek the assistance of a specialist in electric motor rewinds or electric motor overhauling. Specialists of this kind usually have the vital skills necessary to perform procedures as complex as VPI with relative ease and accuracy.
