# Transformers Questions and Answers – Transformer Construction (Core)

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This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Transformer Construction (Core)”.

1. 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

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.

2. 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

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.

3. 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

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.
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4. 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

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.

5. 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

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.

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

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.

7. Different core construction is required for core type and shell type transformer.
a) True
b) False

Explanation: In the “closed-core” type transformer, the primary and secondary windings are wound outside and surround the core ring. In the “shell type” transformer, the primary and secondary windings pass inside the steel magnetic circuit (core) which forms a shell around the windings.

8. There is only one magnetic flux path in the circuit. The transformer is definitely ________________
a) Core type
b) Shell type
c) Can be any of the above
d) Depends on other parameters

Explanation: In core type transformer, winding is placed on two core limbs, while in case of shell type transformer, winding is placed on mid arm of the core. Other limbs will be used as mechanical support. Core type transformers have only one magnetic flux path.

9. Which of the following is correct statement?
a) Core type transformer has more output than shell type
b) Core type transformer has higher efficiency compare to shell type
c) Core type transformer has lower efficiency than shell type
d) Can’t predict

Explanation: In core type winding is surrounded with considerable part of core whereas in shell type core is surrounded with considerable part of winding of transformer. In core type output is less, because of losses. In shell type transformer output is high because of less loss, thus efficiency will be more in case of shell transformer.

10. Core type transformer is with ____________________
a) Large size
b) Small size
c) High voltage
d) Everywhere

Explanation: Core type is very useful when we need large size of the transformer with operation at low voltage. While shell type transformer is very useful when we need small size high voltage. Cooling is more in core type.

11. Which of the following is the correct statement?
a) Shell type has more mechanical protection
b) Cooling is more in shell type
c) In core type sandwiched wiring is used
d) In core type concentric winding is used

Explanation: Shell type has less mechanical protection to coil while Core type has better mechanical protection to coil. Core type is easy to repair and maintain. In core type transformer concentric cylindrical winding are used. In shell type transformer sandwiched winding are used.

12. What is the purpose of providing an iron core in a transformer?
a) Provide support to windings
b) Reduce hysteresis loss
c) Decrease the reluctance of the magnetic path
d) Reduce eddy current losses

Explanation: Iron core is used in a transformer to carry flux from one winding to another winding, so there should be minimum opposition to flux passing through iron core. Hence, transformer function is to decrease the reluctance of magnetic path.

13. What is the thickness of laminations used in a transformer?
a) 0.1 mm to 0.5 mm
b) 4 mm to 5 mm
c) 14 mm to 15 mm
d) 25 mm to 40 mm

Explanation: Laminations are made to reduce the eddy currents and is made of thin strips. Generally, the steel transformer lamination range for 50 Hz varies from 0.25mm to 0.5mm, if it is a 60 Hz transformer then it ranges from 0.17–0.27mm.

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