Rotating machine insulation materials and …

1 Indian Jo urna l of Engineering & Materia ls Sciences Vol. 7, October-December 2000, pp. 370-374 Rotating machine insulation materials and techniques -An overview R C Chauhan, Manmohan Singh & Baljit Singh Sant Longowal Institute of Engineering & Technology, Longowal, Sangrur 148 106, India Received 25 February 2000; accepted 14 September 2000 An insulation system forms the heart of a Rotating e lectrical 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 sy ste ms, , thermoplastic a nd thermosetting systems.

2 Presently, th e rmosetting materials /systems a re s lowly replacing the thermoplastic materials /systems. Rotating mac hine in sulat ion 1- 14 has traveled a lo t through the various developmental stages a imed at making the machines s maller. This can only be accomplished when insulating material is capable of withstanding hig her e lectrical 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 stre ngth and environment friendly.

3 The paper provides an overview on the in sul ation materials and techniques for alternators, classified in two major categories, , turbo-alternators, and hydro-turbine alternators. The bas ic requirements for insulation are different for both of th em. Some of the turbo-alternators are used to generate at 33 kY. So, the in sulation is subjected to severe electrical stresses res ulting in the partial discharges in voids, if prese nt. It has to withstand mechanical shocks also due to s uch hig h speeds, which is the inherent feature of turbo-alt ernators. On th e other hand, hydro-turbine a lternators are low s peed alternators exhibiting electrical stresses as in the case of turbo-alternators.

4 In a machine , the in sul atio n area can be broadly divided as: (i) Slot in sul atio n, (ii) Over-hang portion insulation , (iii) T erminal insulation , and (iv) Rotor ba r insulation (or Pole insulatio n). Thermal Classification of electrical Insulating materials The e lectrical insulating materials are broadly cl assifi ed as given in T able 1. Mica as insulating material For all the high voltage mac hines, the use of mica as main dielectric component in o ne form or other is very common. It is a mineral sili cate. Its chemical composition varies with its sources.

5 Micas are Table I-Classifi cation of e lectrical in sulating materials C lassification Class -y C lass -A Class -E C lass -B C lass -F C la ss -H C lass -C materials Cotton, silk, paper without impregnation Cotton, si lk , paper, suitably impregnated, coated or well immersed in dielec tri c such as oil. Synthetic resin s and ename led wires, cellulose, tri-acetate film, etc. Mica, glass fiber, asbestos with suitable bonding, impregnating a nd coating substances, e .g., shellac, aspha lt, bitumen o r synthetic resins, etc. Built up mica, fiberglass, asbestos with alk yd epoxy, cross-linked polyester resins, etc.

6 Silicone elastomer a nd combinatio n of materia ls such as mica, glass fiber with s uitable bo nding and impregnating materials . Mica, porcelain, g lass, quartz a nd asbestos with inorganic binders suc h as glass o r cement s, silicone resins. T e mperature (0C) 90 105 120 130 155 180 Above 180 Re mark s with respect to use III e lectri cal Rotating machines. Not suitable a nd not used Not generally used In use for LT moto rs Has been in commo n use in Rotating e lectrical machines New epoxy group of materials for Rotating e lectri cal machines In use fo r traction ro tating machines For terminal b ushings.

7 Housings, etc. experimentally being used with mo tors 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 .

8 At higher temperatures, the use of Phlogophite is restricted. The characteristics of mica lie in its extremely high dielectric strength ( ), 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 co-efficient (volume constancy)

9 , 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.

10 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 cut-through 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 kV operating Voltage.