Effect of Humidity on a Hot-Curing Epoxy Compound
Abstract Generated abstract
This paper examines how severe humidity conditions affect the electrical properties of a hot-curing epoxy compound used for potting low-voltage direct-current machine armature windings. Models with epoxy-potted windings were compared with windings impregnated with varnish 447 after exposure to tropical humidity, seawater, and sea fog, with insulation resistance, dielectric loss tangent, capacitance, and breakdown voltage measured. The epoxy compound showed much smaller deterioration than the varnish system, including higher post-exposure breakdown voltage and more stable dielectric characteristics. The authors attribute this performance to the monolithic, highly crosslinked structure of the cast epoxy insulation and suggest that it can improve reliability in tropical and marine operating conditions.
Full Text
EFFECT OF HUMIDITY ON A HOT-CURING EPOXY COMPOUND
R. M. Kessenikh, Yu. P. Pokholkov
As is known, in polymer systems an activated form of moisture sorption (diffusion) is observed. As a result of thermal motion, the links of macromolecules form “microcavities” of different sizes in the polymer, and therefore water molecules can penetrate into them and become fixed in them owing to Van der Waals forces [1]. Naturally, the introduction of a molecule into a polymer is associated with an expenditure of energy, i.e., with activation energy. In this connection, the diffusion of moisture should depend on temperature. An increase in temperature causes an increase in moisture diffusion. When epoxy compounds are used under conditions of elevated relative humidity, under the action of seawater, etc., it is necessary to have an idea of the change in electrical properties with the duration of exposure to severe climatic conditions.
We used a hot-curing epoxy compound (ED-6 resin, hardener—phthalic anhydride) for potting the armature windings of low-voltage direct-current electrical machines. Under operating conditions, the indicated electrical machines are exposed to a tropical climate, seawater, and sea fog. Hot-curing epoxy compounds have a greater degree of crosslinking than cold-curing compounds. As a result, the chain segments between crosslinks have lower mobility, which promotes a less intensive process of activated diffusion. Thus, a hot-curing epoxy compound should possess fairly good moisture resistance [2]. The experimental results confirm this.
The effect of climatic conditions on electrical properties was studied on models of armature windings with commutators. The objects under investigation were placed in special climatic chambers (a tropical chamber, a sea-fog chamber, and a seawater chamber). To obtain comparative data, the effect of moisture was simultaneously studied on windings potted with epoxy compound and on windings impregnated with varnish 447.
Table 1 presents data from the study of the dielectric characteristics of the models after exposure to a tropical climate (humidity 98% at \(t = +55^\circ\mathrm{C}\)). It follows from this table that armatures with commutators potted with epoxy compound change their electrical properties only slightly, whereas in armatures impregnated with varnish 447, the resistance—
…the insulation resistance decreases by three orders of magnitude, the dielectric loss tangent increases by two orders of magnitude, and the capacitance increases approximately 2.5-fold. The dielectric properties of insulation impregnated with varnish 447 also deteriorate significantly when the temperature is raised from 20 to 55°C under conditions of elevated humidity.
Table 1
| Name of impregnating or potting compound | Temperature, °C | Initial state: $R$, ohm | Initial state: $\operatorname{tg}\delta$ | Initial state: $C$, pF | After 176 hours of exposure to tropical climate: $R$, ohm | After 176 hours of exposure to tropical climate: $\operatorname{tg}\delta$ | After 176 hours of exposure to tropical climate: $C$, pF |
|---|---|---|---|---|---|---|---|
| Varnish 447 | 20 | $3.14 \cdot 10^{12}$ | 0.007 | 174 | $3.4 \cdot 10^{9}$ | 0.28 | 470 |
| Varnish 447 | 50 | $8.1 \cdot 10^{11}$ | 0.013 | 210 | $7.3 \cdot 10^{8}$ | 0.55 | 620 |
| Hot-curing epoxy compound | 20 | $2.3 \cdot 10^{12}$ | 0.003 | 189 | $7 \cdot 10^{11}$ | 0.056 | 200 |
| Hot-curing epoxy compound | 50 | $1 \cdot 10^{12}$ | 0.008 | 230 | $1.3 \cdot 10^{11}$ | 0.081 | 220 |
The breakdown voltage at technical frequency of the objects studied, after being kept under the conditions indicated above, proved to be more than twice as high for the models potted with the epoxy compound (Table 2).
Table 2
| Impregnating or potting compound | Varnish 447 | Hot-curing epoxy compound |
|---|---|---|
| $U_{\mathrm{br}}$, kV (average value after 200 hours of exposure to tropical climate) | 3.45 | 7.20 |
After the action of seawater and sea fog on armature windings (Table 3), the greatest deterioration of electrical properties is observed in windings impregnated with varnish 447. The results obtained show that the hot-curing epoxy compound can ensure reliable operation of an electrical machine under severe climatic conditions. In this case the varnish film, having capillary—
Table 3
| Potting or impregnating compound | Initial connections: $R$, ohm | Initial connections: $\operatorname{tg}\delta$ | Initial connections: $C_1$, pF | After 250 hours of exposure to seawater: $R$, ohm | After 250 hours of exposure to seawater: $\operatorname{tg}\delta$ | After 250 hours of exposure to seawater: $C_1$, pF | After 600 hours of exposure to sea fog: $R_1$, ohm | After 600 hours of exposure to sea fog: $\operatorname{tg}\delta$ | After 600 hours of exposure to sea fog: $C_1$, pF |
|---|---|---|---|---|---|---|---|---|---|
| Varnish 447 | $1 \cdot 10^{13}$ | 0.05 | 110 | $8 \cdot 10^{9}$ | 0.28 | 680 | $2 \cdot 10^{11}$ | 0.25 | 235 |
| Hot-curing epoxy compound | $5 \cdot 10^{12}$ | 0.01 | 150 | $3 \cdot 10^{12}$ | 0.04 | 160 | $2.5 \cdot 10^{12}$ | 0.15 | 165 |
porous structure, promotes a sufficiently rapid penetration of moisture into the volume of the structure. The epoxy compound is a cast insulation that cures without the release of by-products. This circumstance makes it possible to obtain a monolithic structure with thermosetting properties. Therefore this type of insulation should significantly increase operational reliability. The proposed casting compound simultaneously also serves as an impregnating compound, thereby ensuring good monolithic integrity. The operating temperature of armature windings should not exceed 140°C. Thermal-vibration aging makes it possible to consider that, at the specified operating temperature (140°C), the service life of the product should be more than 10,000 hours. For the practical casting of armature windings of special electrical machines with an epoxy compound, a fundamental restructuring is required of the technological process used at some plants (varnish impregnation technology), along with a significant improvement in production culture and the simultaneous strict observance of safety practices.
REFERENCES
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M. M. Mikhailov. Moisture Permeability of Organic Dielectrics. Gosenergoizdat, 1960.
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N. Marsal and S. Slarinka. Electrical properties of epoxy resins under the action of moisture. In Elektrikal, No. 1, 1962, Heft 1.