This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Insulating Varnishes”.
1. Which of the following varnishes is oxidizing in nature?
c) Heat-reactive baking
Explanation: Varnishes are films of protective covering over particular materials. The three types of varnishes are air-drying, heat reactive baking (polymerization), and solventless. The air-drying type of varnishes is oxidizing and non-heat reactive in nature.
2. Dissolved resins in alcohol produce ______ varnishes.
Explanation: Spirit varnishes, either clear or black, are created by dissolving natural and synthetic resins in alcohol. These turn into a hard surface which acts as an excellent coating.
3. Which of the following is not a property of insulating varnishes?
a) Moisture resistance
b) Abrasion resistance
c) Electric strength
d) Oil receptive
Explanation: Insulating varnishes are used to protect a material with a film. These are mainly used in electrical units which contain magnet wire, which need to be protected. These insulating varnishes possess resistance against insulation, moisture, chemical, abrasion, and thermal resistance, as well as being oil proof.
4. Silicone varnishes have a ______ operation.
a) Class A
b) Class B
c) Class F
d) Class H
Explanation: Silicone varnishes are available as either 100% silicones or alkyd-modified silicones. The 100% silicones are operated in Class H. The Class H operations are carried out at 180oC.
5. Acrylic varnishes are generally used in _______ gases.
Explanation: Acrylic varnishes are dispersions of acrylonitrile polymers. These are used with refrigerant gases like R-12 and R-22. When they come in contact with moisture, the dielectric strength is lowered.
6. What kind of varnishes are used for most temperatures or classes?
Explanation: Polyester varnishes are high-temperature insulators. They are used in three classes at 105oC, 130oC, and 155oC. Silicone varnishes are used at 180oC.
7. At what temperature is Class F operation carried out?
Explanation: Insulating varnishes are used for high-temperature insulation. The Class F operation is carried out at 155oC. Class A, Class B, and Class H are operated at 105oC, 130oC, and 180oC in that order.
8. Which of the following is not a coating method for varnishes?
a) Dip impregnation
b) Vacuum impregnation
c) Thick coat
d) Flow coat
Explanation: Insulating varnishes must be coated for protection against adverse effects. Dip impregnation is used for electrical gear, vacuum impregnation is used for solventless varnishes, and flow coat is used in bulky transformers.
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