WINDING INSULATING MATERIALS

 INTRODUCTION 

The Electrical insulating materials are defined as materials that offer a very large resistance to flow of current, and for that reason, they are used to keep the current in its proper path along the conductor. This is evident when we touch an electric machine when it is under operation. We don’t receive any electric shocks, because of the insulation. Breakdown of insulation results in short-circuiting of the coils, causing electric currents to flow in unintended paths. This may also cause, electric shocks to humans operating the machinery and also damage the machines. 

Requirements of good insulating materials involve physical properties, reliability, cost, availability, adaptability to machining operations, etc. Electrical insulation and dielectric materials include various forms of materials that surround and protect electrical conductors and prevent unwanted current flow, leakage. Electrical specifications include electrical resistivity, dielectric strength, and dielectric constant. 

1.2 ELECTRICAL PROPERTIES 

Electrical Resistivity: It is the electrical resistance (ohm-cm) to the flow of current through it. Its value should be very high. Resistivity is the inverse of conductivity. 

Dielectric Strength: Dielectric strength is the maximum voltage gradient that the material can withstand before electrical breakdown occurs. This value specified as ‘kV/mm’ should be very high even for very thin films. 

1.3 CLASSIFICATION OF INSULATING MATERIALS : 

The insulating materials are classified in the following two ways : 

1. Classification according to substances and materials. 

2. Classification according to temperature. 

Classification according to substances and materials : 

(i) Solid Insulating Materials [Inorganic and organic] Mica, wood, slate, glass, porcelain, rubber, cotton, silk, rayon, terylene, paper and cellulose materials etc. 

(ii) Liquid Insulating Materials [Oils and Varnishes] Refined hydrocarbon minerals oils, Linseed oil, spirit, and synthetic varnishes, etc. 

(iii) Gaseous Insulating Materials Dry air, carbon dioxide, argon, nitrogen, etc. 

Classification according to temperature: The insulating materials are classified mainly based on the thermal limit. The performance of the insulation depends on its operating temperature. The higher the temperature, the higher will be the rate of its chemical degrading, and hence the lower will be its useful life as shown in fig.1.1. If a reasonably long life of insulation is expected, its operating temperature must be maintained low. Therefore, it is necessary to determine the limits of temperature for the insulation, which will ensure safe operation over its expected life.

Thus the insulating materials are grouped into different classes Y, A, B, and C with temperature limits of 900 C, 1050 C and 1300C for the first three classes and no specific limit fixed for class C. Class Y and A cover the various organic materials without and with impregnation respectively, while classes B and C cover inorganic materials, respectively with and without a binder. With the existence of newer insulating materials, namely, the plastics and silicones, during the middle of this century, a need was felt to reorganize the classification of the insulating materials. This classification is shown in fig.1.2. This led IEC (International Electrotechnical Commission) to come up with the new categories: 

Class Y: 900 C: Paper, cotton, silk, natural rubber, polyvinyl chloride, etc. without impregnation. (formerly O) 

Class A: 1050 C: Same as class Y but impregnated, plus nylon. 

Class E: 1200 C: Polyethylene terephthalate (terylene fiber, melinex film), cellulose triacetate, polyvinyl acetate enamel. 

Class B: 1300 C: Mica, fiberglass (alkali-free alumino borosilicate), bituminized asbestos, bakelite, polyester enamel. 

Class F: 1550 C: As class B but with alkyd and epoxy-based resins, polyurethane. Class H: 1800 C: As class B with a silicone resin binder, silicone rubber, aromatic polyamide (Nomex paper and fiber), polyamide film (enamel, varnish, and film) and estermide enamel. 

Class C: Above 1800 C: As class B but with suitable non-organic binders; (Teflon, Mica, Micanite, Glass, Ceramics, Polytetrafluoroethylene).

CHARACTERISTICS OF A GOOD INSULATING MATERIAL: A good insulating material should possess the following characteristics. 

I. Very high insulation resistance. 

II. High dielectric strength. 

III. Low thermal expansion. 

IV. Non-inflammable when exposed to arcing. 

V. Resistant to oils or liquids, gas fumes, acids, and alkalies. 

VI. Should have no deteriorating effect on the material, in contact with it. 

VII. Good thermal conductivity. Fig.1.2 

VIII. High mechanical strength 

IX. High thermal strength. 

X. Should be resistant to thermal and chemical deterioration. 

XI. Should be resistant to moisture absorption. 

THERE ARE FOUR PRINCIPAL AREAS WHERE INSULATION MUST BE APPLIED. 

They are 

a) between conductor /coils and earth (phase-to-earth), 

b) between conductor /coils of different phases (phase-to-phase), 

c) between turns in a coil (inter-turn) and 

d) between the coils of the same phase (inter-coil). 

INSULATING MATERIALS - FORMS: Insulating materials are available in different shapes and sizes. Insulating materials are available as Tapes, rolls, sleeves, paper, and cloth. 

Insulation Tapes and sleeves: Insulation tapes are used to cover the windings(coils) on the overhang side. Shellac or varnish are applied over this covering to prevent it from absorbing moisture and improve insulation strength. Tapes are sold as rolls in required lengths. Different types of Insulation tapes available are Cotton tape, PVC tape, Silk tape, Polyester tape, Asbestos tape, Glass Fiber tape, Empire cloth tape, Mica tape. Insulation sleeves are used to cover the joints made at the coil ends and coil leads. It gives physical protection to joints and also provides insulation. They come in rigid and flexible types. They are available for standard wire sizes. 

Insulation paper: A variety of insulating papers are available specifically designed for insulating electrical circuits. In motors, it is used to insulate the slots, in between coils. Following are the most often used insulating materials: Leatheriod paper, Press pan paper, Manila or hemp paper, Triflexil paper, Asbestos paper, Micanite paper 

Insulation cloth: It is inserted between the coils after they are placed in slots. Sometimes it is also used as a slot liner. Empire cloth, Asbestos cloth, Glass cloth, Mica cloth, Micanite- cloth are some of the types.