# Electrical Machines Questions and Answers – Three Phase Transformers

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Three Phase Transformers”.

1. A V-V connected transformer can be connected in parallel to delta-delta connected transformer but not to ___________
a) delta-star
b) star-delta
c) star-V
d) all of the mentioned

Explanation: The V-V connected transformer and D-D connected transformers have same phase displacement, so they only can be connected in parallel to each other.

2. Three units of single phase transformers and one single three-phase transformer rating ___________
a) will be same for one rating
b) can never be made same
c) may be same
d) none of the mentioned

Explanation: Three single phase transformers and one single unit of three phase transformer will be same only. this is done to reduce the cost and spacing.

3. The transformer which is more feasible to use in the distribution ends should be ___________
a) star-delta
b) delta-star
c) scott
d) delta-delta

Explanation: Star-delta will have lower voltage at delta end.
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4. For the given circuit below, the voltage across the terminal ‘a’ and ‘b’ is? a) 1.73*V
b) V/1.73
c) V
d) 3*V

Explanation: From delta to star, the voltage between the lines will increase by 1.73.

5. The voltage across the terminal ‘r’ and ‘y’ is? a) 1.73*V
b) V/1.73
c) V
d) 3*V

Explanation: From star to delta, the line voltage will reduce by 1.73.

6. The delta-delta connections are used in applications of ___________
a) large l.v. transformers
b) small h.v. transformers
c) large h.v. transformers
d) small l.v. transformers

Explanation: Delta connected winding handle line voltages so it needs more turns in the winding but thin wires.

7. Open delta transformers can be obtained from ___________
a) delta-delta
b) star-delta
c) delta-star
d) any of the mentioned

Explanation: If one of the transformers is removed from the bank of only delta-delta, then it behaves with 58% power delivery.

8. If one of the transformers is removed from the bank of only delta-delta, then it behaves with 58% power delivery.
a) True
b) False

Explanation: It is true as the circuit will still be closed and the transformer will operate will lesser operating point.

9. Below is the circuit diagram for delta-delta transformer have a transformation ratio ‘k’. The current flowing in the windings of phase r-y in the below diagrams is? a) k*I/1.73
b) I/k*1.73
c) 1.73*k*I
d) 3*k*I

Explanation: The phase current will be reduced by 1/1.73.

10. If para magnetic core is used in the place of the ferromagnetic core of the transformer, then magnetostriction will ___________
a) be vanished
b) reduce
c) increase
d) not be affected

Explanation: If the core is not ferromagnetic then the transformer will not operate at all so no humming sound.

11. When we magnetize the ferromagnetic core of the transformer, core length_______ and it ______ when demagnetized.
a) decreases, increases
b) decreases, decreases
c) increases,increases
d) none of the mentioned

Explanation: Due to aligned magnetic dipoles in the material, the size will reduce by a small margin and vice versa while demagnetization.

12. A 400 V, 10 KVA transformer at 50 Hz, is operated at the frequency of 40 Hz, then the humming ___________
a) increases
b) decreases
c) remains same
d) increases to very high

Explanation: If the frequency is reduced, the core flux density increases, so the noise also will increase.

13. While using three phase transformers, it is taken care about the third harmonics current in the system. This is present when transformer set is delta on either side.
a) True
b) False

Explanation: Third harmonic current are co-phase in nature and require closed path which is present in delta connected only.

14. A recording of the output of the emf induced for star and delta are recorded. Then shape of the emf induced in Y-connected 3-phase transformer is non sinusoidal in nature due to ___________
a) 3rd harmonic component of currents is absent
b) 3rd harmonic component of currents is present
c) negative sequence component of current is present
d) none of the mentioned

Explanation: In a star connected transformer, closed path to generate 3rd harmonic is not there. So the emf shape will be peaky in nature.

15. Shape of emf generated by delta connected transformer is not always sinusoidal.
a) True
b) False

Explanation: No, for a delta transformer, closed path is present to generate 3rd harmonics current and so the emf will be sinusoidal always.

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