Many motor manufacturers have adopted the VPI (Vehicle Impregnation) process, but many companies and repair shops also use the immersion process. The immersion method involves pre-baking the windings and then immersing them in an insulating varnish bath, allowing the varnish to penetrate the winding insulation and fill all gaps. The quality of the varnish depends on factors such as the winding temperature, the viscosity of the varnish, and the immersion time. Today, Ms. Can will provide a brief overview of the immersion process.
Explanation of immersion process
The number of impregnation cycles depends on the winding requirements and the type of impregnating varnish used. For motors operating under normal humidity conditions (relative humidity not exceeding 70%), solvent-based varnishes generally require two impregnation cycles, while solvent-free varnishes only require one. For motors operating in humid tropical conditions (80-95%), solvent-based varnishes generally require three impregnation cycles, while solvent-free varnishes require two. Motors operating in very humid conditions (above 95%) or under the influence of salt spray and chemical gases require an increased number of impregnation cycles.
The purpose of multiple impregnations is as follows: the first impregnation thoroughly fills the micropores and gaps in the insulation layer; the second impregnation firmly bonds the insulation layer to the wire, fills the micropores left by solvent evaporation during the first impregnation, and forms a smooth varnish film on the surface to prevent moisture intrusion. The third and subsequent impregnations form a reinforced protective outer layer on the insulation surface. The penetrating power of the varnish depends primarily on its viscosity; its filling capacity depends primarily on its solids content. Therefore, the viscosity of the varnish should not be too high during the first impregnation, otherwise it will be difficult to penetrate and will easily form a varnish film that traps moisture, affecting the effectiveness of the second and third impregnations. The first impregnation time should also be slightly longer to ensure thorough penetration. For subsequent impregnations, it is best to appropriately increase the viscosity and solids content of the varnish, while shortening the time. This ensures that the varnish fully fills the gaps without compromising the effect of the previous impregnations.
To shorten the impregnation cycle and reduce insulation treatment costs, the viscosity of the solvent-based varnish used in a single impregnation process for ordinary small motors should be strictly controlled within 35-38 seconds (20℃, No. 4 viscometer). Since varnish temperature has a significant impact on viscosity, 20℃ is generally used as the standard; if the temperature differs, it must be converted to 20℃.
During varnish impregnation, the temperature of the windings and core should be controlled between 60 and 70°C before immersion in the varnish. Too low a temperature will affect the varnish's penetration ability, while too high a temperature will cause the varnish to polymerize and deteriorate, leading to strong solvent evaporation and potentially forming a varnish film on the winding surface, hindering penetration. The workpiece to be impregnated should be immersed at least 100-120 mm into the varnish surface, until no more bubbles emerge, for at least the specified time. However, the immersion time should not be too long, otherwise it will damage the varnish film on the conductor.
For rotor or armature windings, they should be placed at a 45° angle during impregnation, and vertical impregnation is preferred if possible for better results. The commutator end of the DC armature winding should face upwards to prevent impregnation varnish from entering the commutator.
After each impregnation, the winding must be drip-dried, generally for about 30 to 60 minutes, until no more paint flows out. Windings that are not properly drip-dried will require extended drying time and may even cause fires or explosions due to the flammable paint.
For rotor or DC armature windings, in order to prevent the varnish from condensing into lumps inside the windings and burning out when heated during operation, causing accidents, a varnish-spinning process should be performed after each varnish dripping. Specific regulations exist regarding the conditions and requirements for varnish use.
After the varnish drips or spins dry, use cotton yarn dampened with a small amount of solvent to promptly wipe away any excess varnish from the inner and outer surfaces of the stator and rotor cores. For large motors, if equipment conditions limit the process, consider using roller impregnation or pouring varnish. Roller impregnation involves immersing a portion of the windings in insulating varnish, then rolling the core to ensure even penetration and filling of the winding ends and slot gaps. The roller impregnation speed should not be too fast, and excess varnish must be prevented from accumulating on the lower side of the workpiece, forming varnish nodules. Pouring varnish ensures the windings are fully and evenly coated with varnish. For both roller impregnation and pouring, varnishes with good moisture resistance should be selected, and their viscosity should be lower than that of the immersion method to facilitate full penetration.
Pre-baking before impregnation
The purpose of pre-baking is to remove moisture and volatiles from inside the windings and to bring them to a suitable temperature to facilitate insulation penetration and filling. The main process parameters for pre-baking are temperature and time. To shorten the dehumidification time, the pre-baking temperature needs to be slightly higher, but excessively high temperatures will negatively impact the lifespan of the insulation material. Specific temperature parameters should be selected based on the heat resistance class of the insulation.
Pre-drying time is generally determined based on actual tests and experience. It involves drying the windings at a specified temperature. When pre-drying begins, the insulation resistance initially decreases due to the negative temperature coefficient of the insulation material as the temperature rises. Subsequently, as moisture gradually evaporates, the insulation resistance gradually increases until it stabilizes. The required pre-drying time (tc) is then multiplied by 1.1 to 1.2. For small motors, this is approximately 4 to 6 hours, and for medium-sized low-voltage motors, it is approximately 5 to 8 hours.
The impact of pre-drying degree on the quality of impregnation treatment remains inconclusive. Some argue that drying must be complete and thorough, otherwise it will affect the quality of the impregnation treatment and the drying time after impregnation. Others believe that when the moisture content of the insulation is below 7%, it has little effect on the reduction of the insulation breakdown voltage.
The drying time after painting does not increase, and the moisture resistance does not decrease. This is perfectly suitable for low-voltage motors. Based on the requirement that the moisture content should not exceed 7%, the pre-drying temperature can be reduced and the time can be shortened, thus improving efficiency and saving energy. This is a problem worthy of study in the impregnation treatment of low-voltage motors.
Pre-drying precautions
(1) The windings must be clean, and the tooling used for installation must not be made of flammable materials.
(2) The pre-drying temperature should be increased gradually. Generally, the heating rate should not exceed 30℃/hour. If the heating is too fast, the temperature difference between the inner and outer layers will be large, which will cause moisture to diffuse from the outer layer to the inside, affecting the drying effect.
(3) Hot air circulation drying is used, resulting in a relatively uniform temperature inside the furnace, which is conducive to moisture evaporation. Cold air must also be blown in intermittently for ventilation. On the one hand, this replaces the hot furnace air with high moisture content, reducing the humidity inside the furnace; on the other hand, when cold air is blown in, the surface temperature of the windings is lower than that inside, which can promote the accelerated diffusion of moisture inside the insulation and improve drying efficiency. However, ventilation will lower the furnace temperature and consume heat energy, so it should not be too frequent.
(4) For windings with particularly high drying requirements, vacuum drying is recommended. The general procedure is to bake for a period of time, then evacuate the vacuum, and then bake for another period of time. This saves energy and removes moisture from the furnace. Under vacuum conditions, the boiling point of water is lowered, allowing drying to be carried out at a lower temperature to reduce thermal damage to the insulation.
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