Transformers Questions and Answers – Basic Materials Used in Transformer Parts

This set of Transformers Questions and Answers for Campus interviews focuses on “Basic Materials Used in Transformer Parts”.

1. The majority of power transformers in use throughout the world are oil filled using a mineral oil.
a) True
b) False
View Answer

Answer: a
Explanation: In majority power transformers dielectric material used is the oil, which serves the dual purpose of providing insulation and as a cooling medium to conduct away the losses which are produced in the transformer in the form of heat.

2. Dielectric mineral oil is used in ____________
a) Small transformers
b) Medium transformers
c) Large transformers
d) In all transformers
View Answer

Answer: c
Explanation: Because of the fire hazard associated with mineral oil, it has been the practice to use designs for smaller transformers which do not contain oil. It is usual, therefore, to locate transformers with mineral oil, out of doors where a fire is more easily dealt with and consequentially the risks are fewer.

3. The purpose of the transformer core is to provide ____________
a) High reluctance path
b) Low reluctance path
c) High inductive path
d) High capacitive path
View Answer

Answer: b
Explanation: The purpose of a transformer core is to provide a low-reluctance path for the magnetic flux linking primary and secondary windings. In doing so, the core experiences iron losses due to hysteresis and eddy currents flowing within it which, in turn, show themselves as heating of the core material.

4. Transformer core is designed to reduce ______________
a) Hysteresis loss
b) Eddy current loss
c) Hysteresis loss and Eddy current loss
d) Cannot be determined
View Answer

Answer: c
Explanation: Hysteresis loss and eddy current loss are the losses which take place in core of the transformer thus they are also termed as core losses. While other losses take place in winding or in air gap which can’t be dealt with core design.

5. Transformers windings are generally made of __________
a) Steel
b) Iron
c) Copper
d) Steel iron alloy
View Answer

Answer: c
Explanation: In order to avoid losses due to loading current, winding materials must be chosen wisely. Winding conductors are thus made of copper or more precisely saying they are made of high conductivity copper by some industrial processes.
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6. Before using oil in transformers, insulation material was _________
a) Asbestos
b) Cotton
c) Low grade pressboard in air
d) Kraft paper
View Answer

Answer: d
Explanation: At the time of discovery of transformer, people were using asbestos, cotton, low grade pressboard in air for insulation purpose. Further, Kraft paper was invented which became much popular insulation material.

7. Which transformer insulation material is best compare to Kraft paper?
a) Oil
b) Asbestos
c) Low grade pressboard
d) Cotton
View Answer

Answer: a
Explanation: Newly developed oil-filled transformers have capabilities much greater than those transformers which used Kraft paper as dielectric material. Also, electrical properties of Kraft paper depend on physical and chemical properties of paper.

8. Which of the following is not the property of oil that should be fulfilled before using in transformer?
a) Low viscosity
b) High flash point
c) Low electrical strength
d) High chemical stability
View Answer

Answer: c
Explanation: There are various important parameters that oil must follow for its use in oil cooled transformer. These parameters include low viscosity, high stability, high flash point, high electrical strength, low pour point.

9. Transformer ratings are given in _____________
a) kW
b) kVAR
c) HP
d) kVA
View Answer

Answer: d
Explanation: There are two types of losses in a transformer, Copper Losses and Iron Losses or Core Losses or Insulation Losses. Copper losses (I2R) depends on current passing through transformer winding while Iron losses or Core Losses or Insulation Losses depends on Voltage. That’s why the rating of Transformer is in kVA.

10. Function of transformer is to _________________
a) Convert AC to DC
b) Convert DC to AC
c) Step down or up the DC voltages and currents
d) Step down or up the AC voltages and currents
View Answer

Answer: d
Explanation: A Transformer does not work on DC and operates only on AC, therefore it Step up of Step down the level of AC Voltage or Current, by keeping frequency of the supply unaltered on the secondary side.

11. What is the dielectric strength of a transformer oil?
a) 1 kV
b) 35 kV
c) 100 kV
d) 330 kV
View Answer

Answer: b
Explanation: For mineral oil, an accepted minimum dielectric strength is 30 kV for transformers with a high-voltage rating of 230 kV and above and 27 kV for transformers with a high-voltage rating below 230 kV. New oil should pass the condition of a minimum dielectric strength of 35 kV by ASTM methods of testing.

12. Which of the following is not a part of transformer installation?
a) Conservator
b) Breather
c) Buchholz relay
d) Exciter
View Answer

Answer: d
Explanation: Conservator, breather, Buchholz relay are the parts which are much important in transformer construction in order to maintain temperature of the transformer and to work transformer with good efficiency.

13. The insulating material that can withstand the highest temperature safely is _______________
a) Cellulose
b) Asbestos
c) Mica
d) Glass fibre
View Answer

Answer: c
Explanation: Mica is extremely stable when it is exposed to moisture and extreme temperatures to maintain superior electrical properties as an insulator. The mechanical properties of mica allow it to be cut, punched, stamped and machined to close tolerances along with maintenance of a high thermal conductivity.

14. The part of a transformer which is visible from outside _______________
a) Bushings
b) Core
c) Primary winding
d) Secondary winding
View Answer

Answer: a
Explanation: Core, primary winding, secondary winding of a transformer are generally kept in closed container filled with an oil so that, oil acts as a coolant and provides electrical neutrality also. Thus, only bushings are visible from outside.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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