Impregnation The technician's corner

Duty classes are divided into four main categories:

• S1 - continuous running duty (hydraulic pumps, fans, blowers)
• S2 - short-time duty (household appliances such as egg beaters, hair dryers, and similar)
• S3 - intermittent periodic duty (hoisting motors)
• S6 - continuous-operation periodic duty (woodworking machines, hydraulic pressure pumps).

Impregnation uses thermally insulating resins and/or varnishes, with formulations based on polyester, epoxy, polyurethane, silicone, and others.

The correct resin type must be selected according to the thermal class of the motor:

• Class a (105°c)
• Class e (120°c)
• Class b (130°c)
• Class f (155°c)
• Class h (180°c)
• Class n (200°c)
• Class r (220°c)
• Class s (240°c).

It follows naturally that resins with a lower thermal class have lower characteristics and performance compared to those designed for high-temperature operation, such as classes h, n, r, and s.

The main objectives of winding impregnation are:

• increased electrical insulation
• improved mechanical retention of the lamination stack-winding assembly
• saturation of the lamination stack slots (up to 100% fill).

The most widely used technologies in the e-mobility industry span drop-by-drop application, immersion, and controlled dispensing/filling, through to vacuum-assisted solutions for minimising entrapped air voids.

• Trickling (drop impregnation): resin is metered through nozzles onto specific points while the component rotates on its own axis. This is the preferred choice when high repeatability, a clean process with no post-application cleanup, and integration into automated lines are required.
• Roll dip (immersion with rotation): combines immersion and rotation to achieve robust, industrial-grade saturation. It is selected when a good compromise between impregnation quality and process simplicity is needed.
• Cold dipping / hot dipping / joule dipping: dipping is suitable when a well-established process is required. The hot variant (pre-heating) is preferred to improve resin penetration speed, while joule dipping is used when closer control of the thermal behaviour of the workpiece and higher slot saturation levels are sought.
• Vacuum potting: a filling and encapsulation process using resins, predominantly two-component epoxy-based formulations with medium filler content. The vacuum reduces air entrapment in the stator slots and improves fill quality. It is chosen when the priority is to maximise slot fill with medium-to-highly filled resins in order to enhance heat dissipation from the motor during operation.
• Dispensing / encapsulation: precise resin application for localised treatments or integrated impregnation cycles. Ideal when dose accuracy, repeatability, and material control are required, particularly on critical areas or complex components.

Destructive testing: once the impregnation and resin curing stage is complete, the stator is sectioned along the lamination stack and the slots are examined under a microscope to calculate slot fill (individual slots and the average across all slots). In some special applications, both winding heads of the stator are cut and the winding bars are extracted from the lamination stack for bending and torsion resistance testing.

Non-destructive testing: this consists of a comprehensive electrical test of the impregnated and cured stator. On automated production lines, 100% of output is electrically mapped both before and after impregnation, providing vital data on each individual part produced and on the level of electrical improvement resulting from the process.

The main tests performed on the stator are:

• Ac dielectric strength test (hi-pot)
• surge test
• rotation test
• ohmic resistance test
• inversion test
• insulation test with partial discharge measurement.

For impregnation, the most effective controls start with the resin: viscosity verification, batch traceability, and stability of operating conditions.

During the cycle, it is essential to record and maintain consistent key parameters such as resin flow rate and volume, rotation speed, temperatures, and cycle times, as these determine uniform slot saturation and prevent defects such as dry zones or resin runs.

Downstream, quality is confirmed through result-based checks, for example weighing the part before and after to assess resin uptake, and through the electrical tests described in point 3.

For powder coating, which is used to protect and insulate critical areas such as welds on hairpin stators, the main controls concern coating coverage and thickness, adhesion, correct oven curing, and the absence of micro-defects that could compromise dielectric integrity. Application-specific electrical tests on the insulating coating are added as required by project specifications.

Initial part cleanliness and resin management (contamination monitoring, replenishment, and condition checks) also have a significant impact. A properly controlled system reduces scrap and rework.

Rotors (wound or complex geometries): impregnation is carried out primarily to fix and protect the winding and structural elements against centrifugal forces, vibration, and thermal cycling, as well as to provide insulation. Tecnofirma offers dedicated solutions and technologies for rotors as well.

Permanent magnet stators: insulation typically concerns specific areas such as windings, junction zones, and protection against contamination. The presence of magnets requires attention to the maximum allowable temperature during processing and curing, chemical compatibility of materials with the magnets and adhesives, and prevention of unintended resin deposition.

Interior permanent magnet rotors (ipm): in ipm rotors the objective is not insulation alone. Mechanical stability and protection against micro-movement and delamination in service are often equally important. Typical process choices favour good fill control and thermal profiles that are compatible with the magnets and bonding adhesives.

A typical impregnation cycle consists of the following stages:

• loading the part to be impregnated
• pre-heating to the required temperature (depends on the resin type)
• active impregnation
• resin gelation in the oven
• resin curing (polymerisation).

All the stages listed above are carried out in fully automatic mode on mass-production systems, managed through treatment recipes specific to the part type in process.

The reasons it is so widely used in e-motors, particularly in automotive and e-mobility applications, are:

• Quality and repeatability: the combination of rotation and defined dispensing points enables very uniform winding saturation, which is essential for consistent quality across high production volumes.
• Less resin, less waste: being a localised application, it reduces material consumption and cleaning requirements compared to bath-based impregnation when the latter is not necessary.
• Industrial productivity: it is well suited to automated lines and optimised cycles, with precise control of flow rate and timing.
• Motor performance: a higher slot fill and thorough impregnation improve the mechanical stability of the winding and enhance thermal dissipation, both of which are critical in high-power-density e-motors.
• Fine parameter control: in more demanding applications such as hairpin stators, the ability to precisely adjust resin flow rate, rotation speed and direction, tilt angle, temperatures, and timing allows adaptation to different geometries and requirements.

• Pilot line: ideal for r&d, process validation, and small batches, as it allows different technologies to be tested and can be evolved into an automated robotic cell for flexible production.
• Ring line: the typical choice for high throughput and high repeatability. Parts are mounted on spindles and remain in continuous rotation throughout the process stages, promoting uniformity and quality at high volumes.
• U line: indicated when the objective is to manage, within a single machine, not only resin impregnation but also localised weld treatments such as gel coating, as typically required for hairpin stators. This reduces handling steps, part movements, and external integrations. Particularly suited to contexts with fixed volumes and a stable product, where having a complete, standardised, and repeatable cycle is the priority.
• Camelot line: recommended when dipping is the primary requirement and a wide range of products must be processed using cold/hot/joule cycles, with the emphasis on a proven and industrially robust process.
• Modular system: ideal for e-mobility projects that require genuine configurable flexibility and scalability. The line grows by modules, including in parallel, and allows layout and capacity to be adapted as volumes and product variants change.