Question

Insulating material, cotton or oiled cambric tapes, are used for which part of machines?
1. Rectangular conductors such as used in large D.C. machines
2. Taping armature and field coils of traction motor stator coils of high-voltage alternators
3. Insulation of commutators
4. For insulating terminals of high-voltage machines

Answer 1

Correct Answer - Option 1 : Rectangular conductors such as used in large D.C. machines

Insulation materials used in different parts of Electrical Machine:

• Cellulosic backing paper, mica splittings, shellac varnish, and industrial methylated spirit. insulation of coils up to high operating voltages such as alternator armature winding, DC series motor (traction motor), tapping, etc.
• Mica separation insulation is commonly used as commutator insulation.
• Insulating material, cotton or oiled cambric tapes, are used for the rectangular conductors or strip conductors in DC machines.

The types of materials used for insulations are and their applications:

1. Insulating Oil:

• Insulating oil has an important role as electrical insulation as well as a coolant to dissipate heat losses.
• Transformer oil is basically obtained by fractional distillation and subsequent treatment of crude petroleum.
• So Transformer oil act as liquid dielectric and coolant and it is placed in a tank in which core of the transformer is placed. 

                                

2. Insulating paper:

• Paper is a fabric made from vegetable fibers which are felted to form a web or sheet.
• The fibrous raw materials are obtained from plants including cotton, hemp, manila, straw, and coniferous trees.
• It attains a very high value of electric strength when emerged in oil under a vacuum.
• ”Craft insulating paper of medium air permeability” is used in layer winding insulation, condenser core of oil-impregnated bushing.
• Craft insulating paper of high air permeability” is used in covering over the rectangular copper conductor and continuously transposed copper conductor. 
• “Crepe Kraft paper” is used in covering over flexible copper cable insulation of winding lead.
• “Press paper” is used as backing paper for axial cooling duct.
• Cotton or oiled cambric are used as taping the coils of air-cooled transformers.

 

3. Pressboard:

• Pressboard is a widely used insulating material for making a variety of components used in the electrical, mechanical, and thermal design of transformers.
• Pressboard is also made from vegetable fibers, whose cells contain much cellulose.
• The most difficult practical insulation in power transformers occurs at the end of the windings and the lead-outs from the windings.
• Pressboard molded components can be made to any required shape.
• Angle rings and caps are the widely used moldings.
• There are many kinds of pressboards used in high voltage transformers but “soft pressboard –laminated” is used in 11 kV transformers as a block washer, terminal gear cleat, and support and spacer, etc.
• “Pressboard molding from wet sheet or wet wood pulp” is used in angle ring, cap, sector, snouts, square tube, lead-out, for insulating ends of winding, insulation between windings, and numerous other applications.

 

4. Wood:

• Wood-based laminates are manufactured from selected veneers obtained from various timbers.
• The veneers are dried and partially or fully impregnated with natural phenomena.
• In the areas which required higher mechanical and lower electric strength, densities laminated wood is used for making a variety of insulation components like coil clamping ring, cleat, support, core, and yoke, etc.

 

5. Insulated copper conductor for winding:

• Different types of insulated copper conductor windings are used in power transformers for example paper covered rectangular copper conductor, twin paper covered rectangular copper conductor bunched together, paper-covered continuously transposed copper conductor, twin transposed copper conductor bunched together, twin rectangular copper bunched together, and provided with a common paper strip between the two conductors and epoxy coated continuously transposed conductor.
• These are used to the winding space factor and mechanical strength of windings.” paper covered rectangular copper conductor” is used for making different kinds of windings.
• “Paper covered standard copper cable” is used for making lead and terminal.
• “Crepe paper-covered flexible copper cable” is used for making lead and terminal required to be bent to a small radius.
• “PVC insulated copper cable-single and multi-core” is used to control wiring in marshaling box, nitrogen, sealing system.

 

7. Insulating tape:

• Insulating tape is used for various taping purposes.
• For example cotton tape, cotton newar tape, glass woven tape, woven tape, and phenol laminated paper base sheet.
• These tapes are used in taping, banding, core bolt insulation, places where required high strength and in the banding of transformer cores.