This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Insulating Materials”.
1. What is the property of insulating materials?
a) Prevents the unwanted flow of current
b) Allows the unwanted flow of current
c) Increases the unwanted flow of current
d) Decreases the unwanted flow of current
Explanation: Conductors, allow the flow of current through the material. Insulators are the opposite of conductors. The material doesn’t allow the flow of current through them.
2. In Transmission and Distribution sector, where should the insulators be placed?
a) Between towers and poles
b) Between towers and ground
c) Between towers and conductors
d) Between conductors and ground
Explanation: The insulators are used to block the flow of unwanted current. In power system, already the tower and the conductors are grounded. Thus the insulators are connected between towers and conductors.
3. What is the main cause for the failure of overhead line insulators?
Explanation: In overhead lines, there occurs a flow of abnormal over voltages. This abnormal over voltages, causes flashover. This flashover causes damage to overhead line insulators.
4. What happens when some serious phenomenon occurs in the insulators?
a) Puncher is produced in the insulator body
b) Insulator body bulges
c) Insulator body bursts
d) Insulator body tears apart
Explanation: The serious phenomenon is the abnormal over voltage, produced in the insulators. Due to that, flashover occurs in the insulators. This causes puncher of the insulator body.
5. Insulation Resistance should be high in insulators.
Explanation: Insulation Resistance is very important in the performance of insulating materials. If the insulation resistance becomes low, high flow of current occurs and can damage the material.
6. How should the properties of strength and dielectric strength in insulating materials?
a) High strength, low Dielectric strength
b) Low strength, low Dielectric strength
c) High strength, high Dielectric strength
d) Low strength, high Dielectric strength
Explanation: The insulator should have high strength in order to prevent the insulating materials. The insulator should have high dielectric strength, in order to hold the electric field without breaking down.
7. What is property of porosity and temperature change in insulating materials?
a) Less, less affected
b) Less, highly affected
c) High, highly affected
d) High, less affected
Explanation: The insulating materials should have less porosity as it should not lose the internal properties due to holes. The material should also be less affected by temperature change in order to preserve its properties.
8. What is the dielectric strength of porcelain insulators?
a) 60 kV/cm
b) 140 kV/cm
c) 50 kV/cm
d) 40 kV/cm
Explanation: Porcelain has a dielectric strength of 60kV/cm. 140 kV/cm denotes the dielectric strength of glass insulator.
9. What is the dielectric strength, coefficient of thermal expansion of glass with respect to porcelain insulators?
a) High, high
b) High, low
c) Low, low
d) Low, high
Explanation: Glass has a higher dielectric strength (140 kV/cm) when compared to porcelain (60 kV/cm) and glass has a lower coefficient of thermal expansion, when compared to porcelain.
10. Glass has lower tensile strength compared to porcelain insulators.
Explanation: Glass insulators have all properties better than that of porcelain. Glass has high dielectric strength, low coefficient of thermal expansion and then High tensile strength than that of porcelain.
11. What is the other name of Polymer Insulator?
a) Moisture insulator
b) Core insulator
c) Composite insulator
d) Mixed insulator
Explanation: It is also known as composite insulator. It is known as composite insulator because it consists of both core and the weather sheds in them.
12. How many classifications of overhead line insulators are there?
Explanation: There are basically3 types of overhead line insulators. They are Pin type, Suspension type and Stray Insulator type.
13. How many types of electrical insulators are present on the basis of voltage application?
Explanation: There are two types of insulators based on voltage application. They are Stay Insulators and Shackle Insulators.
14. How many discs are used in suspension insulators for 220kV?
Explanation: 3 discs are used when voltage is 33kV. 4 discs are used when voltage is 66kV. 8 discs are used when voltage application is 132kV.
15. What is the other name of shackle insulator?
Explanation: String is the other name of strain insulators, whereas, hanging is the other name of suspension insulators. Post insulator is otherwise Pin insulators.
Sanfoundry Global Education & Learning Series – Design of Electrical Machines.
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