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  Other Insulation Materials

Introduction to Other Insulation Materials

Bamboo sticks: throughout running of motor, the coils might be emerging from the slots because of centrifugal force. This is prevented through inserting bamboo sticks at the top side of the slots. The sticks are prepared to size depending on the slot size. Hard trees are employed for making these sticks.

Paper: Paper is made from wood pulp and manila fibres beaten and rolled into sheets. The dielectric strength of paper is 4 to 10 kV/mm thickness. Paper is moisture absorbent and thus is specifically suitable for impregnation. Electrical properties are fairly good. Paper is seldom employed in unimpregnated situation but can be used effectively under oil. Paper catches fire at 1250 C that is why the temperature of any paper insulated apparatus is limited to about 1000 C.

Wood: The dielectric constant of it changes in the range 2.5 to 2.7. Dry resistivity is in the order of 1010 to 1013Ω cm. It can withstand a voltage gradient of 40 kV/mm in service. These properties change over a wide range, relies on the sort of wood, seasons of cutting, grain direction, and particularly the water content. Employed for Papers, slot wedges etc.

Asbestos board: Asbestos board's dielectric strength is 3 to 4.5 kV/mm thickness. It is extremely moisture absorbent. Asbestos board's strength increases through impregnation but heat resistance and non-in flammability decreases. Asbestos board melts at 15000 C. It is neither flexible nor mechanically strong. Purified fibres with clay filler comprise better electric strength. In thickness of 0.2 to 1.0 mm the asbestos electrical insulating paper is supplied and, relies on its thickness and includes a minimum crash voltage from 0.9 to 2.4 kV. Asbestos board is manufactured through using zinc chloride solution along with paper plate. It is in yellow colour or grey colour after the coils are inserted in the slots; this is given on the top of the slot as a protection to coils and as well employed as insulated between coils. Other sorts of boards are

A. Hard board

B. Ivory board

C. Hylum sheet

Mica: Mica is a mineral containing silicate of aluminium with silicate of soda potash and magnesia. It takes place in the form of crystals, can simply be split into extremely thin sheets. It is influenced through oils. The resistivity of it at 250 C ranges from approximately 1012 to 106     Ω cm. The dielectric strength changes from 40 to 150 kV /mm. It is slightest influenced through heat but dehydrates at high temperatures. It comprise high dielectric strength and low power loss. Mica is tough, rigid, and strong. Moisture does not include any influence on it. The electrical properties of it are deteriorated in the existence of quartz and feldspar. The Mica paper is not adequately strong or self supporting. Therefore, it has to be provided backing of glass cloth or another binding material like epoxy resin.  Epoxy resin bonded mica paper is widely used in both high and low voltage machines.  For non-epoxy system a varnish impregnation is necessary to fill the air pockets and as well to work as a barrier against moisture and chemicals exist in the atmosphere. The varnish employed should include the property of forming an unbroken tight adhesive and reasonably flexible film.

It resists to a high degree the attack of gases like combination products but is attacked through warm hydrochloric acid potassium hydrate, warm alkaline carbonates, and water that containing carbon dioxide. Mica is employed like insulation separator for commentator segments, washers, gaskets for core end bolts. It as well employed as composite tapes and sheets.

Micanite: generally, mica is existing in the form of extremely thin splitting. Therefore it is bound to a supporting sheet of electrical grade paper or glass cloth along with an appropriate binding agent.  The resultant mica sheets are termed as micanite. Macanite's dielectric strength is 30 kV/mm. It is employed as insulating sheets among coils of dissimilar phases.

Bakelite: Bakelite is a form of phenol formaldehyde. The dielectric strength of it is 6 to 15 kV/mm thickness. Bakelite is hard thermosetting and dark coloured material. Employed for creating terminal boards and slot wedges.

Glass: Glass is a thermoplastic inorganic material including complex system of oxides. The volume resistivity at 2000 C is very high, 1x1016  to 1x1018  Ω cm. Quartz glass is non-hygroscopic, comprise extremely high chemical resistance, withstands temperature fluctuations, and comprise a low co-efficient of linear expansion of 5.5x10-7 cm for each 0 C. It is not subjected to thermal ageing. Glass comprise a extremely high compression strength [6000 to 21000 kg/cm2 ] but a low tensile strength  [ 100 to 300 kg/cm2] and is very brittle. The dielectric constant changes from 3.8 to16.2. At the temperature of room it can withstand a voltage gradient of approximately 8- 20 kV/mm. Toughened glass is employed for insulation in EHV lines, of voltages that more than 100 kV. Glass threads, fiber tapes, and sleeves are indispensible part of a motor insulation.

Cotton or silk: cotton is hygroscopic (absorbs moisture) and comprise low dielectric strength, thus it have to be impregnated with varnish or wax after winding. Cotton covered wire is widely employed for winding of small magnet coils, chokes and transformer coils, armature windings of small and medium sized machines, etc.

Silk is very much expensive than cotton but occupy less space and is hence employed for windings in fractional horse power machines. Silk is less hygroscopic and comprise a higher dielectric strength than cotton, but as cotton it needs impregnation. The cotton's and silk's operating temperature is 1000 C and the material might grasp fire above this temperature.

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