Impregnating a winding with varnish or resin is the final, but critically important, step in manufacturing and repairing an electric motor winding. Our article on winding insulation only briefly mentioned impregnating varnishes and Vacuum-Pressure Impregnation (VPI) as one component of an insulation system. This article is dedicated to the VPI technology itself — what it protects the winding from, how it differs from simply dipping the winding in varnish, and how the process itself works.
Impregnation is not a decorative or optional 'just in case' step — its quality determines the winding's mechanical resistance to vibration, its resistance to moisture and contaminants, the heat transfer from the copper to the core and frame, and the dielectric strength of the insulation.
The more demanding the machine — high voltage, harsh operating conditions, large output — the less a repair shop can rely on simple dipping and baking, and the more important full Vacuum-Pressure Impregnation becomes.
After a winding is placed into the slots or the coils are wound, microscopic voids and air pockets remain between individual conductors, turns and layers of insulation. Impregnation fills these voids with liquid varnish or resin that, once cured, turns the winding into a practically solid mass.
Impregnation performs several functions at once:
An unimpregnated or poorly impregnated winding can have sound wire insulation and still lose service life far faster than a properly impregnated one, simply due to moisture, vibration or a local hot spot.
The simplest impregnation method is so-called dip-and-bake: the winding is dipped once or several times into a tank of varnish at atmospheric pressure, allowed to drain, and then cured in an oven. The method is cheap, fast, and adequate for many small, low-voltage, general-purpose motors.
The main drawback of simple dipping is that the varnish mostly fills the outer, accessible surfaces of the winding. Inside tightly wound coils, between layers of insulation, and in narrow slot gaps, air pockets often remain that the varnish, working only under atmospheric pressure, never has time to reach. These voids reduce heat transfer, become a place where moisture collects, and accelerate insulation aging.
Vacuum-Pressure Impregnation (VPI) solves precisely this problem. Before the resin is admitted, air and moisture are drawn out of every internal void of the winding under vacuum; the resin or varnish is then admitted into the already evacuated winding and additionally forced deeper into the tightest gaps by applied pressure. The result is far more complete and far deeper impregnation than any simple dipping can achieve.
| Criterion | Dip-and-bake | Vacuum-Pressure Impregnation (VPI) |
|---|---|---|
| Filling of internal voids | Partial — air pockets can remain | Practically complete — air is evacuated before the resin is admitted |
| Penetration into tightly wound coils and deep slots | Limited | Substantially deeper, thanks to vacuum draw and applied pressure |
| Typical impregnating material | Solvent-based varnishes | Most often solvent-free, 100 %-solids resins |
| Typical use case | Small and mid-size, low-voltage, general-purpose motors | High-voltage, large, critical machines and harsh operating environments |
Equipment differs between manufacturers, but the VPI technology always follows the same sequence of steps.
The winding is heated in an oven to drive off moisture and any residual solvents from the insulation. Moisture left in the winding would boil during the subsequent vacuum stage and ruin the impregnation, so this step is critical.
The stator, armature or coil assembly is loaded into a sealed impregnation tank (autoclave), which is then closed and prepared for evacuation.
A deep vacuum is drawn in the tank, pulling air and any remaining moisture out of every internal void of the winding — between turns, between layers of insulation, and inside the slot gaps.
Without breaking the vacuum, impregnating resin or varnish — most often a solvent-free, 100 %-solids thermosetting resin — is admitted into the tank. Under vacuum, the liquid is drawn directly into the voids that were just evacuated.
Once the tank is filled with resin, pressure is applied to force the resin deeper into the tightest, least accessible parts of the winding — places it would never reach under atmospheric pressure alone.
The winding is held under pressure for a set soak time so the resin distributes evenly and penetrates the full thickness of the insulation.
Excess resin is drained back into the reservoir for reuse, leaving on the winding only the layer that has actually penetrated the insulation structure.
The impregnated winding is transferred to a curing oven, where the resin fully cross-links under a controlled temperature profile, forming one continuous, solid structure.
A complete Vacuum-Pressure Impregnation cycle requires dedicated equipment, matched to one another in capacity, pressure rating and temperature range.
Impregnation quality is confirmed by a combination of visual inspection and electrical testing, never by eye alone.
As a general rule of thumb for modern thermosetting insulation systems, a polarization index noticeably below the accepted rule-of-thumb minimums usually points to incomplete impregnation, residual moisture or contamination rather than a stray measurement error.
Painting only forms a surface coating, whereas impregnation penetrates between the turns and layers of insulation, bonding the winding from the inside rather than just protecting it from the outside.
No. It is specifically the vacuum stage that removes air from the internal voids of the winding before the resin is admitted — without it, the process is reduced to ordinary dipping, with all of its limitations.
Re-impregnation is technically possible, but it does not substitute for a rewind once the insulation itself has been mechanically or thermally degraded — a repeat VPI cycle only restores the sealing and, partially, the dielectric strength of an otherwise sound winding.
No. VPI is an impregnation technology; the thermal class is determined by the entire insulation system — the wire enamel, the slot and phase insulation, and the resin itself — not by the impregnation method alone.
A control cut, or a sectioned test sample, is used to inspect penetration directly, alongside indirect electrical indicators — stable insulation resistance and a satisfactory polarization index after the soak period.
Yes, if the base insulation of the winding is undamaged and only needs its sealing and dielectric strength restored. Significant aging or mechanical damage to the insulation calls for a rewind instead.
EPR (Elektropromremont) performs Vacuum-Pressure Impregnation of armature and stator windings as part of its full rewinding and repair cycle for electric machines.
Impregnation is carried out after the new winding is laid and before final electrical testing, matched to the machine's chosen insulation system and thermal class.
The scope of work includes:
This material is for informational purposes. The values, diagnostic methods, scope of work and recommendations given here are general and do not replace the manufacturer’s technical documentation. The final decision for a specific machine is made from its own diagnostics and inspection, taking into account its type, power, design, duty, operating history and applicable standards.
We perform Vacuum-Pressure Impregnation of armature or stator windings as part of a full rewinding cycle — from drying through the final insulation tests.