# Transformers Questions and Answers – Real Transformer and Equivalent Circuit

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This set of Transformers Questions and Answers for Aptitude test focuses on “Real Transformer and Equivalent Circuit”.

1. When does transformer breath in?
d) cannot be determined

Explanation: Transformer in the low loading condition, also called extreme condition (lower temperature), oil inside contracts and then air is taken inside thus breath in to main via the balloon like structure through silica gel breather.

2. A transformer transforms ________________
a) voltage
b) current
c) power
d) frequency

Explanation: Since, in a transformer voltage and current is changed according to the number of turns simultaneously, we call that power is transformed, though the magnitude remains same. Frequency is kept constant.

3. Greater the secondary leakage flux ___________
a) less will be the secondary induced emf
b) less will be the primary induced emf
c) less will be the primary terminal voltage
d) cannot be determined

Explanation: Since emf induced in the transformer coils is directly proportional to the change in the flux with respect to time, we can say that if flux reduces the change in flux after some time will be less which will induce less voltage in secondary.
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4. Which of the following is not the purpose of iron core in a step-up transformer?
a) to provide coupling between primary and secondary
b) to increase the magnitude of mutual flux
c) to decrease the magnitude of magnetizing current
d) to provide all above features

Explanation: In real transformers, the two coils are generally wound onto the same iron core. The purpose of the iron core is to provide the path for the magnetic flux generated by the current flowing around the primary coil, so that as much of it as possible also links the secondary coil, with minimum losses.

5. In a transformer the tappings are generally provided on
a) Primary side
b) Secondary side
c) Low voltage side
d) Can be connected to any side

Explanation: The turns ratio is different with different tappings and hence different voltages are obtained with different tappings adjustment. The tappings are placed either on high voltage or low voltages or sometimes on both high and low voltage windings to get required output.

6. Helical coils can be used at _____________
a) low voltage side of high kVA transformers
b) high frequency transformers
c) high voltage side of small capacity transformer
d) high voltage side of high kVA rating transformers

Explanation: Helical winding is used for low voltage and high current winding of large generator transformers. Due to its distinct design that is spiral form, small number of turns and high current, some additional eddy-current losses may happen in winding.

7. In real transformer, primary winding has _________
a) Infinite resistance
b) Zero resistance
c) Some finite resistance
d) Cannot say

Explanation: For a real transformer on load, both the primary and secondary have finite resistances which are uniformly spread throughout the winding. These resistances give rise to associated copper (I2R) losses.

8. Both resistances and leakage reactances of the transformer windings are __________
a) Series effects
b) Parallel effects
c) Series-parallel effects
d) Cannot say

Explanation: Both resistances and leakage reactances of the transformer windings are series effects and for low operating frequencies at which the transformers are commonly employed (power frequency operation is at 50 Hz only), these can be regarded as lumped parameters.

9. To convert an ideal transformer into a practical transformer we add ____________
a) Primary winding resistance and secondary winding resistance
b) Primary winding leakage reactance and secondary winding leakage reactance
c) Primary winding resistance, leakage and secondary winding leakage reactance
d) Cannot be determined

Explanation: We consider all resistances i.e. of primary and secondary as series parameters of equivalent circuit of transformer, while all leakage reactances are also connected into the circuit as series parameters.

10. Parallel parameters in a transformer equivalent circuit contains ___________
a) Gi and Bm
b) R1 and X1
c) R2 and X2
d) Cannot be determined

Explanation: Primary and secondary resistances and leakage reactances are the series parameters in the transformer equivalent circuit. So, these are not included in parallel parameters. Parallel parameters contain Gi and Bm in which current Ii and Im flows respectively.

11. When does capacitor is included in equivalent circuit of transformer?
a) Transformer of very high VA rating
b) Transformer with very high frequency operation
c) Transformer with less VA
d) Never

Explanation: The passive lumped T-circuit representation of a transformer discussed above is adequate for most power and radio frequency transformers. In transformers operating at higher frequencies, the interwinding capacitances are often significant and must be included in the equivalent circuit.

12. The size of a transformer core will depend on _____________
a) frequency
b) area of the core
c) flux density of the core material
d) frequency and area of the core

Explanation: According to the frequency of transformer size of the core of transformer changes. While area of core also depends upon many parameters like operating voltage, capacity of transformer, hence all these contribute to the size of the core.

13. A single phase transformer has specifications as 250 KVA, 11000 V/415 V, 50 Hz. What are the values of primary and secondary currents?
a) Primary current = 602.4A, Secondary current = 22.7A
b) Secondary current = 202.7A, Primary current = 602.4A
c) Primary current = 22.7A, Secondary current = 602.4A
d) Primary current = 11.35A, Secondary current = 301.2A

Explanation: Primary current is defined as the ratio of rated capacity of transformer to the rated primary voltage of the transformer. Rated primary current= Rated power/voltage= 250000/11000= 22.7 A. Similarly calculating for secondary current gives secondary current = 602.4A.

14. A 25 KVA transformer is constructed to a turns ratio of N1/N2 = 10. The impedance of primary winding is 3+j5 ohms and of secondary winding is 0.5+j0.8 ohms. What will be the impedance of transformer when referred to primary?
a) 53j + 85 ohms
b) 53 + 85j ohms
c) 3.5 + 5.8j ohms
d) Can’t be calculated

Explanation: Given turns ratio is 10. Thus, secondary resistance when referred to the primary is equal to k2*Z2, so net impedance on primary is equal to primary impedance + k^2*Z2 . Substituting all the terms we get net impedance = 53 + 85j ohms.

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