TRANSFORMERS MCQs SET 02

TRANSFORMERS MCQs SET 02

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

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.
 

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

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.
 

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

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.
 

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

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.
 

5. Transformer core is generally made of ___________
a) Single block of core material
b) By stacking large number of sheets together
c) Can be made with any of the above method
d) Cannot be determined
 

Answer: b
 

Explanation: Transformer core experiences eddy current losses when transformer is in the operations. In order to reduce eddy current losses, it is advisable to use large number of sheets laminated from each other are stick together than using one single block.
 

6. Transformer core is constructed for ______________
a) Providing least effective magnetic linkage between two windings
b) Providing isolation between magnetic linkages of one coil from another
c) Providing most effective magnetic linkage between two windings
d) Cannot be determined
 

Answer: c
 

Explanation: Transformer core is so chosen that it will provide low reluctance path and will transfer maximum amount of flux from one winding to other, providing most effective magnetic linkage between two windings.
 

7. Which of the following statements is/are correct?
a) High frequency power supplies are light weight
b) Transformer size gets reduced at high frequency
c) Transformer size is more at higher frequency
d) High frequency power supplies are light weight and transformer size gets reduced at high frequency
 

Answer: d
 

Explanation: From the induced emf equation of transformer emf is given by E ∝ φf. For same emf, φf = constant φ1f1 = φ2f2 i.e. B1A1f1 = B2A2f2. For constant flux density B1 = B2. A1f1 = A2f2. For high frequency f2 > f1, A2 < A1. Thus, at high frequencies transformer size get reduced and also light weight.
 

8. Transformer operating at 25-400 Hz frequency contain core made of _____________
a) Highly permeable iron
b) Steel alloy
c) Air core
d) Highly permeable iron and Steel alloy
 

Answer: d
 

Explanation: When core is made of highly permeable iron or steel alloy (cold-rolled, grain oriented sheet steel). This transformer is generally called an iron-core transformer. Transformers operated from 25–400 Hz are invariably of iron-core construction.
 

9. In various radio devices and testing instruments we use ______________
a) Iron core transformer
b) Air core transformer
c) W/O core transformer
d) Any transformer can be used
 

Answer: a
 

Explanation: In special cases, the magnetic circuit linking the windings may be made of nonmagnetic material, in which case the transformer is referred to as an air-core transformer. The air-core transformer is of interest mainly in radio devices and in certain types of measuring and testing instruments.
 

10. Which type of flux does transformer action need?
a) Constant magnetic flux
b) Increasing magnetic flux
c) Alternating magnetic lux
d) Alternating electric flux
 

Answer: c
 

Explanation: The energy transfer in a transformer, is from one winding to another, entirely through magnetic medium it is known as transformer action. Therefore, transformer action requires an alternating or time varying magnetic flux in order to transfer power from primary side to secondary side. Since induced emf in the winding is due to flux linkage.