Indian Journal of Engineering & Materials Sciences Vol. 7, October-December 2000, pp. 370-374

Rotating machine insulation materials and techniques - An overview

R C Chauhan, Manmohan Singh & Baljit Singh

San t Longowal Institute of Engi neering & Technology, Longowal, Sangrur 148 106, Indi a

Received 25 February 2000; accepted 14 September 2000

An insu lation system forms the heart of a rotating electrical machine. This paper reviews the different insulation systems practiced on the stator windings/coils of the alternators. They are broadly divided in two types of systems, i.e., thermoplastic and thermosetting systems. Presently, thermosetting materials/systems are slow ly replacing the thermoplastic materials/systems.

Rotating machine insul at ion 1-14 has traveled a lot through the various developmental stages aimed at making the machines smaller. This can only be accomplished when insulating material is capable of withstanding higher electrical and thermal stresses and has better thermal conductivity, and also that the insulating material can be processed for specific custom based applications with smaller processing time. It should also have good mechanical strength and environment friendly.

The paper provides an overview on the insulation materials and techniques for alternators, classified in two major categories, i.e., turbo-alternators, and hydro-turbine alternators. The bas ic requirements for insulation are different for both of them. Some of the turbo-alternators are used to generate at 33 kY. So, the insulation is subjected to severe electrical stresses resulting in the partial discharges in voids, if present. It has to withstand mechanical shocks also due to such

high speeds, whi ch is the inherent feature of turbo­alternators. On the other hand, hydro-turbine alternators are low speed alternators exhibiting electrical stresses as in the case of turbo-alternators .

In a machine, the insulation area can be broadly divided as: (i) Slot insulation, (ii) Over-hang portion insulation , (iii) Terminal insul ation, and (iv) Rotor bar insulation (or Pole insulation).

Thermal Classification of Electrical Insulating Materials

The electrical insulating material s are broadly class ified as given in Table 1.

Mica as insulating material

For all the high voltage machines, the use of mica as main dielectric component in one form or other is very common. It is a mineral silicate. Its chemical composition varies with its sources. Micas are

Table I-Classification of electrical insulati ng materials

Classification

Class - y

Class - A

Class - E

Class - B

Class - F

Class - H

Class - C

Material s

Cotton, silk, paper without impregnation Cotton, si lk, paper, suitably impregnated, coated or well immersed in dielectric such as oi l. Synthetic resins and enameled wires, cellulose, tri-acetate film, etc. Mica, glass fiber, asbestos with suitable bonding, impregnating and coating substances, e .g., shellac, asphalt, bitumen or sy nthetic resins, etc. Built up mica, fiberglass, asbestos with alkyd epoxy, cross­linked polyester res ins, etc. Silicone elastomer and combination of materials such as mica, glass fiber with suitable bonding and impregnating materials.

Mica, porcelain, g lass, quartz and asbestos with inorganic binders such as glass or cements, si licone resins.

Temperature (0C)

90

105

120

130

155

180

Above 180

Remarks with respect to use III

electrical rotating machines.

Not suitable and not used

Not generally used

In use for LT motors

Has been in common use in rotating electrical machines

New epoxy group of materials for rotating e lectrica l machi nes

In use for traction rotating machines

For terminal bushings. housings, etc. experimenta lly being used wi th motors

CHAUHAN el al.: ROTATING MACHINE INSULATION MATERIALS AND TECHNIQUES 371

classified in to two principal groups described as : Granitic [Muscovite (potassium mica), Paragonite (sodium mica), Zinwaldite (lithium iron mica), Lepidolite (lithium mica)], and Pyroxenic [Phlogophite (magnesium mica), Biolite (magnesium iron mica), Lepidomelane (iron mica), Roscoelite (vanadium mica)] .

Out of these varieties, the Muscovite and Phlogophite are mainly used for engineering applications and the former as electrical insulating material. At higher temperatures, the use of Phlogophite is restricted. The characteristics of mica lie in its extremely high dielectric strength (6.5-8), low electric loss, high surface and volume resistivities, excellent thermal stability up to 650-870°C, good resistance to oxidizing action of electrical discharges, excellent flexibility and elasticity in thinner gauges, transparency in thin films, ability to withstand sudden and wide variation in temperature without undergoing any appreciable damage to its physical and electrical properties, non-compressible nature, low thermal co­efficient (volume constancy), non-inflammable character, complete inertness towards water, acids, alkalis and solvents, and high resistance to high energy nuclear radiation. Chemical compositions of Muscovite and Phlogophite are given in Table 2 and their electrical properties in Table 3.

Mica was used earlier in the form of Large Mica Splitting either by hand or by machine on a backing of paper or canvas. Sometimes, it was also applied as a wrapper but more commonly in the form of tape in multiple layers. Then, mica paper was developed and two types of mica papers emerged: One manufactured by the process of mechanical delaminating termed as integrated mica, and second with chemical and thermal delaminating. Around 1970, a new type of mica paper emerged under the trade name MICANITE-II, which had much larger number of flakes of mica (about five times as large as normal mica paper), higher mica content and higher cut­through resistance. It had lower breakdown voltage and hence disappeared from the market.

Different adopted insulation systems are shown in Fig. 1. Thermoplastic systems are becoming obsolete and being replaced by thermosetting systems.

Thermoplastic insulation systems

Shellac micafolium wrapping system-It has been extensively used for coils up to 6.6 kV operating Voltage. The main constituent materials are cellulosic backing paper, mica splittings, shellac varnish and industrial methylated spirit.

Bitumen mica folium system--It has been used for insulation of coils up to 11 kV operating voltages. The constituents are same as in shellac except that bitumen varnish is used.

Bitumen mica tape system-It was very popular among the manufacturers prior to the introduction of thermosetting system. Bitumen mica silk tape and glass tapes were used in half lap layers on the coils and impregnated in a vacuum pressure vessel with solvent less bitumen compound and pressed under heat.

Thermoplastic materials (bitumen and shellac) have some limitations for high temperature operations of machines as they become plastic under application of heat. Machines after long usage develop winding faults due to cracking of insulation caused by the movement of tapes and the phenomenon is termed as "Tape Migration" and were reported particularly near the slot ends. When the machine is heated up during its operation, the main insulation wall relaxes and takes the shape of the slot, but the small space between core and insulation wall is inevitable. As a result, the main insulation wall is swelled and voids are created inside the insulation. Electrical discharges take place in these voids and destroy the insulation .

Thermosetting insulation systems

Previously, polyesters were used here. Later on, epoxies were introduced. Different epoxy resin based systems, as shown in Fig. 1, are classified into two major classes, viz., resin-rich systems, and resin-poor

Table 2-Range of chemical compositions of both the micas

Items Muscovite (%) Phlogophite (%)

Si02 44.6 - 46.7 37.9 - 43 .2 AI20 3 30.0 - 38.5 12.2 - 17.0 K20 8.8 - 11.8 8.7 - 11.3 MgO 0.38 - 1.5 23.4 - 29.0 FeO 0.2 - 0.95 0.2 - 0.45 Fe20 3 0.2 - 5.8 0.2 - 1.9 CaO 0.1 - 0.5 0.1 - 0.4 Li20 0.1 - 0.8 0.01 - 0.03 Na20 0.1 - 0.6 0.3 - 0.6 Ti02 0.0 - 0.9 0.0 - 0.62 BaO 0.0 - 0.62 F 0.0-0.15 0.9-5.4 Water loss on ignition 4.1 - 5.0 1.2 - 3.0

Table 3 - Electrical properties of mica

Properties

Dielectric constant Dielectric strength (Vpm) Resistivity (Q-cm) Power factor Dissipation factor

Muscovite

6-8 1000 - 2800 101 5 _ 1016

(1- 3) X 10-4

0.0001 - 0.0008

Phlogoph ite

5- 6 1300 - 3000 1013 _ 1014

(1 - 5)x l O-3

0.003 - 0.009

372 INDIAN J.ENG. MATER. SCI., OCTOBER-DECEM BER 2000

systems. Both the systems have the ir own merits and demerits depending on the size of the machine and capital investment. Resin-rich system requires less capital inves tment and is suitable for coils up to 18 kY operating level. On the contrary, res in-poor system is capital intensive but can operate at more than 30 kY also.

Resin rich system

Resin ri ch tapes contain all the res in, which is required fo r the insulation. It must be solvent free (0.07% maximum) and fl ex ible to facilitate machine taping. The fl exibility depends on res in content, mica paper thickness (available in 50-250 gsm), degree of procure and the backing materi al. Woven glass cloth is used as baking materi al. The tape is tightly appli ed. The ai r bubbles are pressed out by hydraulic or mechanical means. Then, the winding wrapped in tape is baked at certain temperature and pressure to cure the resi n.

Thermoplastic Insulation System

I Micaflake Shellac Bitumen tapes with Mlcafo-thermo- lium plastic varnish bonding

r Bitumen Bitumen Micafolium Mica Tape

I I I

Mechanical Hydrostatic pressing pressing

Resin poor system

These tapes contain negligible or no res in. In th is method, the tape is applied tightly on the winding or bar. ]t can' t be taped by hand due to its poor mechani cal strength as stand-alone. Then, the bar is vacuum impregnated with a low viscosity res in and cured in a mould to obtain the desired coi l dimensions. In this method, all the air bubbles, which might be trapped in the tape, come out due to vacuum impregnation and chances of voids are far less as compared to res in rich system. Hence, it prov ides better thermal conductivity .

Comparison of both the systems, i.e., res in-rich and res in-poor sys tems is made in Table 4.

Epoxy resin The widely used res in is epoxy. Epox ides have

very good adhesion with mica, are tough, have good handling properties, have low shrinkage, low water absorption besides their good electric and thermal

Thermosetting Insulation System

Resin Rich Resin Poor System System

I Vacuum VPI Impregnation Compact pressurization System

1 Unpressed Continuous Discontinuous System System System with

Mechanical Pressing

I Mechanical Hydrostatic Pressing Pressing

Fig. 1-Diffe rent insulation systems for rotating machines

CHAUHAN et af.: ROTATING MACHINE INSULATION MATERIALS AND TECHNIQUES 373

Table 4 - Comparison of resin-rich and resin-poor systems

Resin-poor Resin-rich

High Low Capital investment Main equipment Vacuum Pressure Equipment, Taping

Machines, and Presses. Taping Machines, Presses

Application of tape With care, because of low mechanical strength

Excellent, because of high mechanical strength

Shelf - life of tapes Shelf-life of impregnating resin Curing time of insulation

Unlimited Limited Generally long

12 months at 5°C. Impregnation not necessary Shorter owing to quicker gellification of the res III

Processing Insulation thickness over 5 mm need 2 or 3 impregnation cycles

Application of tape and curing in only one cycle up to 7.8mm thickness

Mechanical strength end windings Excellent, are impregnated in the same time as the slot portion

Ordinary, hard or fl exible end winding possible

Repairing and changing of individual coils

Difficult, in general ne ighboring coils are harmed

Easy, no problems

properties . They mainly contain two materials, viz., epoxy resin (base material), and hardener (amino or non-amino). Depending upon the base material, epoxy resins are classified as: (i) Bisphenol A (epichlorohydrin); (ii) Epoxide Novolac (made by reacting epichlorohydrin with phenolic formaldehyde); (iii) Cycloaliphatic (made by epoxidation of cyclo-olefins with peracetic acid.); and Aliphatic (made by aliphatic polyols reaction with epichlorohydrin in presence of Lewis acid and subsequently dehydro chlorinated in the presence of sodium aluminate and a solvent such as dioxane).

The tape for insulating the overhang also consists of mica paper bonded to a glass fabric but the resin has the property of retaining the degree of flexibility even when cured. The resin most commonly used here is either a silicone elastomer or a flexible epoxy. Flexibility is the most important property for the overhang tape, since the forces which overhang sees, both during winding process and during operation are such that any rigid insulation is liable to crack and thus weaken the insulation. The important requirement is the compatibility of this tape with insulation on slot portion.

Environmental Considerations There is a shift from the processes and materials

involving solvent-based dip and bake varnish to solvent-less in the industry. Water based varnishes have been offered in some cases but they are not always the best choice.

Prior to 1975, most laminates were made using adhesive resins dissolved in solvents. The substrates were coated and the solvents were driven off in to the atmosphere in drying ovens. With the beginnings of health, safety and environmental regulation, coating

operations have begun to shift to high solid adhesives or solvent emiSSIOn capturing equipment. The captured solvents are recovered for re-use or incinerated to capture the heat benefit reducing emissions. Today, high solids and solvent free resins are becoming the norm.

Conclusions Previously for HV rotating machines, thermoplast ic

insulating materials were very common. Afterwards, thermosetting materials emerged as main insulating material, which not only enhanced the operating voltage levels but also mechanically and thermally more stable as compared to thermoplastic materials. Mica is the basic component for both the systems. Moreover, the trends are heading towards the solvent­less materials these days because of the "Green" movement and beginning of health, safety and environmental regulations.

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