Specifications

6
manufactured by using zinc chloride solution with paper plate. It is in grey colour or yellow colour
After the coils are inserted in the slots, this is provided on the top of the slot as a protection to
coils and also used as insulated between coils. Other types of boards are
1.Hard board 2.Ivory board 3.Hylum sheet
Mica : It is a mineral consisting of silicate of aluminium with silicate of soda potash and magnesia.
It occurs in the form of crystals, can easily be split into very thin sheets. It is affected by oils. The
resistivity of mica at 25
0
C ranges from about 10
12
to 10
6
Ω Ω
Ω Ω
Ω cm. The dielectric strength varies
from 40 to 150 kV /mm. It is least affected by heat but dehydrates at high temperatures. It has
high dielectric strength and low power loss. It is rigid, tough and strong. Moisture does not have
any affect on it. Its electrical properties are deteriorated in the presence of quartz and feldspar.
The Mica paper is not sufficiently strong or self supporting. Hence, it has to be given backing of
glass cloth or other binding material such as epoxy resin. Epoxy resin bonded mica paper is
extensively used in both low and high voltage machines. For non-epoxy system a varnish
impregnation is essential to fill the air pockets and also to act as a barrier against moisture and
chemicals present in the atmosphere. The varnish used should have the property of forming an
unbroken tight adhesive and reasonably flexible film.
Mica resists to a high degree the attack of gases such as combination products but is
attacked by warm hydrochloric acid potassium hydrate, warm alkaline carbonates, and water
containing carbon dioxide. Mica is used as insulation seperator for commutator segments,
washers, gaskets for core end bolts. It also used as composite tapes and sheets.
Micanite : Normally, mica is available in the form of very thin splitting. Hence it is bound to a
supporting sheet of electrical grade paper or glass cloth with a suitable binding agent. The
resulting mica sheets are known as micanite. Its dielectric strength is 30 kV/mm. It is used as
insulating sheets between coils of different phases.
Backelite : It is a type of phenol formaldehyde. Its dielectric strength is 6 to 15 kV/mm thickness.
It is hard thermosetting and dark coloured material. Used for making terminal boards, and slot
wedges.
Glass : It is a thermoplastic inorganic material comprising complex system of oxides. The volume
resistivity at 200
0
C is extremely high , 1x10
16
to 1x10
18
ΩΩ
ΩΩ
Ω cm. Quartz glass is non-hygroscopic,
has very high chemical resistance, withstands temperature fluctuations, and has a low co-efficient
of linear expansion of 5.5x10
-7
cm per
0
C. It is not subjected to thermal ageing. Glass has a very
high compression strength [6000 to 21000 kg/cm
2
] but a low tensile strength [ 100 to 300 kg/cm
2
]
and is extremely brittle. The dielectric constant varies from 3.8 to16.2. At room temperature it can
withstand a voltage gradient of about 8- 20 kV/mm. Toughened glass is used for insulation in EHV
lines, of voltages more than 100 kV. Glass fiber tapes, threads and sleeves are indispensible part
of a motor insulation.
Cotton or silk : Cotton is hygroscopic (absorbes moisture) and has low di electric strength, so
it must be impregnated with varnish or wax after winding. Cotton covered wire is extensively
used for winding of small magnet coils, armature windings of small and medium sized machines,