February 10th, 2017, Published in Articles: EE Publishers, Articles: Vector

by HS Hawkins, ICMEESA

 

From the ICMEESA archive

This article discusses the selection of correct types of insulating and impregnating varnishes.

Electrical apparatus consisting of coils of insulated wire is usually finished off with an insulating varnish. The varnish is used for the following purposes:

• To seal all fibrous or hygroscopic materials in the winding against the absorption of moisture.
• To bond the whole winding, wires and insulation mechanically, into a solid cohesive mass, so that it is made more resistant to shock, vibration and mechanical stress.
• To protect the winding against the destructive effects of oil, acid and other chemicals, petrol, damp, heat and mould growth and to provide anti-tracking properties.
• To improve the electrical properties of the fibrous or other insulators which should remain unimpaired by any of the various destructive influences over a period at least as long as the designed life of the component under the normal cycles of heat and cold resulting from it being switched on and off.

Insulating varnishes may be broadly classified under two main headings: surface coatings and impregnants.

Surface coatings

These are applied to the surfaces of the component parts of complete assemblies to impart resistance to the effects of moisture, oil, acid, alkali and fungus. The degree of protection conferred upon the component will depend upon the intrinsic moisture resistance of the varnish, the continuity of the coating and its thickness. A smooth finish will help to prevent the inclusion of conducting deposits and will provide an anti-tracking surface. Air-drying varnish used for surface coatings will only dry properly if applied as a comparatively thin film. A staving varnish with good through hardening characteristics may be applied as a thicker coating, but the capacity of the component to withstand the baking temperature must be considered. For instance, the use of a thermoplastic material in the construction of a winding may render the use of a staving varnish impracticable, though such a varnish may in fact be superior in performance to an air-drying varnish.

The construction of the component itself may affect the efficiency of the protective coating. The sharp edges of metal stampings may project through the coating, while the protruding fibres of cotton or silk can also provide a path through which moisture may penetrate. It is advisable to avoid sharp or rough edges on metal parts. The removal of projecting textile fibres by singeing is also recommended when maximum protection is required. When selecting a surface coating the following points arise:

How is the coating to be dried?

Staving, force-drying, or air-drying may be employed, but the choice will depend upon the temperature the component can withstand. A staving finish may be applied as a comparatively thick coat and will harden fairly quickly. Production conditions may require a short baking time. An air-drying finish must be applied in several thin films if good drying is to be obtained. This is equally true if the coating is to be force-dried. The application of a thick coat of an air-drying varnish will usually lead to the formation of a sticky layer beneath a hard skin, rivelling or other flaws may appear on the surface and greening of the copper subtrate may result from contact with the wet undersurface of the varnish.

What drying time can be tolerated?

Some of the air-drying varnishes, particularly those of greater flexibility, require a drying time of ten to twelve hours. Spirit varnishes (alcohol soluble) will dry in 30 to 45 minutes. Baking varnishes are suitable for a wide range of staving schedules according to the type selected.

Table 1: Classes of insulating material.

Class	Temperature
Y (formerly 0)	90°C
A	105°C
E	120°C
B	130°C
F	155°C
H	180°C
C	Above 180°C

What temperature must the component withstand?

Most baking finishes may be used at comparatively high temperatures. Air-drying spirit varnishes are not suitable because the coating softens when heated and tends to become more brittle on cooling.

What degree of flexibility or hardness is required?

The slower air-drying finishes are generally the most flexible though some of the staving materials are good. Spirit varnishes are less flexible than other types.

What colour is preferred?

Surface coatings are available as clear varnishes ranging in colour from pale gold to deep brown or as pigmented enamels.

Is there a need for high resistance to moisture, oil or other contaminating liquids?

Baking varnishes show excellent resistance to moisture, acids and alkalies, and many air-drying materials are good in this respect. Spirit varnishes are slightly less resistant to moisture but have good resistance to oil. Bitumen-based varnishes are fairly moisture resistant and acid-proof but have poor resistance to hydrocarbon oils.

What is the physical condition of the surface to be coated?

Air-drying varnishes have to be applied thinly and so will not give a heavy enough build-up to cover irregular surfaces. Baking varnishes on the other hand will usually produce a good finish on fairly irregular surfaces. Special filling compounds are available.

Impregnating varnishes
The main objects of impregnating an electrical winding are as follows:

• To impart protection against the ingress of moisture by saturating all porous materials used in the construction of the winding and by filling the interstices between conductors.
• To impart good electrical properties to the assembly by the impregnation of those component materials which have low dielectric strength.
• To bond components firmly together.
• To enhance the resistance of the component to ageing by heat.
• To improve the heat dissipation of the components and to prevent ‘hot spots’.
• To protect the component against the effects of lubricating oil, acid, alkali, mould growth and insects.
• To some extent to impart a pleasing finish to the component, although this is often a secondary consideration.
• To permit dry testing to be conducted when necessary.

Interleaving paper used in a component should be porous to allow the varnish to penetrate. The paper will then become saturated with varnish which will confer improved electrical properties to it.

Methods of application

The method of application is generally determined by the type of varnish being employed and the type of component to be treated. There are four main methods of applying varnishes for insulating purposes: spraying for surface insulation; brushing for surface insulation; dipping (hot or cold); and vacuum impregnation.
The viscosity of varnishes used with the various methods of application differ and for consistency in production are usually controlled by means of a flow-cup viscosimeter.

Viscosity limits are in the following broad ranges

• Spraying: 22 – 60 seconds BS flow-cup type B4 at 25°C.
• Brushing: 100 – 200 seconds BS flow-cup type B4 at 25°C.
• Dipping: 40 – 120 seconds BS flow-cup type B4 at 25°C.
• Impregnating: 60 seconds upwards BS flow-cup type B4 at 25°C.

(It is important to note that an impregnating varnish with a comparatively high viscosity will penetrate quite readily when used by a hot dip process, since the viscosity of varnish in close proximity to the article being dipped will be reduced by heating).

Specific gravity is also controlled, which provides some control over the solids content and avoids building up a disproportionate amount of light volatile solvent in the varnish tank.

Space does not permit further enlargement on the application of insulating varnish by either spray or brush which are common methods.

Hot dip

This is the immersion of a preheated coil in cold varnish. This method may be used for the impregnation of all but the most complex windings. Complex windings require vacuum and pressure impregnation if complete penetration by the varnish is to be ensured. The coil should be preheated at approximately 110°C for a period long enough to remove all moisture. The hot coil should then be lowered slowly into the varnish and allowed to remain for a predetermined time governed by experience with the particular type of coil. It is important that the coil be lowered slowly into the varnish since rapid immersion will entrap air which will retard penetration of the varnish.

It is then withdrawn from the varnish, allowed to drain and finally baked in a well-ventilated oven at the temperature recommended for the varnish. A low temperature initial bake with good ventilation is required primarily to drive off the volatiles. The temperature is then raised to the required polymerising temperature. If the coil is put straight into the oven at the higher temperature as hardening of the varnish can occur.

Cold dip

This is where both the coil and the varnish are at room temperature. This method may be used for the impregnation of relatively open windings containing little moisture-absorbent material. Because there is 110°C local warming of the varnish to improve penetration and no preliminary drying out of entrapped moisture, this method is applicable only to windings of simple design. Baking is the same as the hot dip method.

Vacuum and pressure impregnation

This is the most efficient method of ensuring good penetration and is therefore employed on complex interleaved windings. The process commences with a preheat as in the hot dip method, after which the coil is dried under vacuum in an autoclave. After drying, the varnish is admitted into the autoclave until the coil is covered to a depth of 5 to 7 cm. Penetration is then assisted by putting the autoclave under pressure. In due course the varnish is blown back to the storage tank and the coil is drained and baked in the normal way. Frequently a suction is applied to help extract the volatiles.

Table 2: The properties and uses of modern wire enamels.

Type	Thermal classification	Properties	Use
Acrylic	105°C	Resistant to refrigerants and many solvents. Non solderable. Windability not good unless overcoated e.g. with nylon.	General and can be used in sealed units (except for softer solderabIe grade-melting about 455°C).
Ceramic	220°C – 650°C	Resists heat, vapour, nuclear radiation, high vacuum, thermal shock. Windability and moisture resistance can be improved by overcoating, e.g. with PTFE, silicone, polyimide.	Use developing rapidly. Space and nuclear application. Used with inorganic insulation and encapsulants.
Epoxy	130°C	Resists transformer oil, moisture and most chemicals. Self-bonding if overcoated with epoxy or PV formal.	Broad use in low temperature field. Self bonding version converts by heat or solvents to give self-supporting coils.
Nylon	105°C	Excellent windability and resistance to rubbing. SolderabIe. Withstands strong solvents but sensitive to moisture.	General low temperature use where moisture-sensitivity not a drawback. Used as overcoat e.g. over polyester polyurethane and PY formal.
Oleoresinous	105°C	Easy to wind at high speeds.	Preferred in many non-severe application, e.g. on paper formed coils.
Polyamide/folyimide	220°C	Tough, smooth, abrasion resistant. High dielectric strength which is maintained under humid conditions and on ageing. Resists heat and pressure deformation and radiation (quoted 3 x 109 rads of gamma radiation). Chemically inert including resistance to chlorine-containing insulating liquids, impervious to and gives high scrape resistance in refrigerants. Resists strong curing agents and is compatible with most impregnants and encapsulants.	Suitable for any application except high temperature calling for ceramics and high radiation fields (e.g. within primary shield of a reactor pile). Ideal for sealed units in contact with refrigerants.
Polyester	130 – 155°C (Straight) 155 – 180°C (overcoated)	Straight polyester is not resistant to moisture (hydrolyses), heat shock, abrasion, chlorine-containing insulating liquids. These defects are corrected by topcoating, e.g. nylon top­coat gives windability. Not solderabIe.	Use is growing but is held back because of: Lack of complete polyimide systems to take advantage of the high thermal rating. Cast. General in medium to high temperature equipment when suitably over­coated.
PTFE	180°C	Chemically inert but low scrape resistance, e.g. compared with PY formal.	Specialised end uses where high thermal classification and chemical inertness outweigh disadvantages.
Polyurethane	105 – 130°C (Nylon overcoated)	Good winding properties and resistance. Low thermal classification unless overcoated. SolderabIe (decomposes).	Low temperature use with wide applications when overcoated. Self-bonding when overcoated with polyvinyl butyral.
PV formal	105°C	Excellent windability and compatible with most impregnants and encapsulants. Latter property improved by overcoating with Nylon. Not resistant to refrigerants and a modification with polyurethane is used in sealed units.	Still the largest volume wire enamel. Suitable for most low temperature electrical apparatus. Self-bonding when overcoated with polyvinyl butyral.

Baking ovens

An oven (for preheating or baking) can be either:

• Electrically heated by elements which are totally enclosed or operate at black heat and which are screened and protected in such a way that varnish cannot drip on the hot elements.
• Steam heated which is satisfactory provided that the temperature of the steam is sufficient to give at least 150°C without difficulty.
• Gas heated, but the overs employed should be of the double or triple case type in which the products of combustion do not enter the oven space.

All types of ovens should be well ventilated and equipped with a means of internal circulation to prevent the formation of pockets of stagnant solvent-laden air and to ensure that the solvent is removed from the windings. The oven should be thermostatically controlled. Conveyor type ovens should have an initial zone at low temperature followed by a zone at the full baking temperature.

The time required to bake out an impregnated coil depends not only on the characteristics of the impregnating materials but also on other factors such as the thermal capacity of the metal contained in or around the winding and the depth of the winding itself. The type of insulator covering the wire and the gauge of the wire, together with the nature and extent of any interleaving or auxiliary insulation used in the winding, can also have a bearing on the time.

Insulation for high temperature working
The ability of equipment to dissipate heat or the insulation to withstand the temperatures reached are often the principal factors in determining the design and size of the equipment for a particular duty. With effective heat dissipation the output for a given size of equipment can be increased considerably if the working temperature can be allowed to rise. It is for this reason that insulations have been classified in terms of their maximum possible operating temperature. The classes of insulating material showed in Table 1 (including varnishes) are  recognised internationally by the electrical industry for specific uses.

Materials which are completely inorganic, such as mica, glass, fibre, asbestos and porcelain are extremely resistant to deterioration at high working temperatures and will generally be rated as class C materials. When they are used under practical conditions it is often necessary to employ organic adhesives or varnishes with them, resulting in a composite insulation of perhaps only class H, F or B.

At the other end of the scale, organic insulation such as cotton, silk and paper, when not impregnated or coated, or immersed in an insulating liquid such as oil, have poor resistance to high working temperatures and are therefore assigned class Y, less suitable for continuous operation at more than 90°C. If such insulation is varnish impregnated, it is usually rated as class A, E, or B, depending on the type of varnish.

Some organic materials have been shown to be capable of continuous operation at higher temperatures. Some epoxy resins are therefore suitable for class F duty and some silicone resins are rated class H.

Wire enamels
These belong to the same family as insulating varnishes and should be considered with the subject matter of this article. Table 2 summarises the properties and uses of modern wire enamels.

Conclusion

The author lays no claim to knowledge of electrical technology. His experience is that of a varnish technician and this article is written with the bias of a varnish man addressing an electrical engineer. The author wishes to thank the board of directors of Dulux for permission to write this article.

Contact Mariana Jacobs, ICMEESA, Tel 011 615-4304, icmeesa@icmeesa.org.za

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