# Transformers Questions and Answers – Three Phase Transformer Connections

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

1. When does star/star transformers work satisfactorily?
c) On balanced as well as unbalanced loads

Explanation: With the unbalanced load connected to the neutral, the neutral point shifts thereby making the three line-to-neutral (i.e. phase) voltages unequal. The effect of unbalanced loads can be illustrated by placing a single load between phase (or coil) a and the neutral on the secondary side.

2. When does delta/star transformer work satisfactorily?
c) On balanced as well as unbalanced loads

Explanation: Large unbalanced/balanced loads can be handled satisfactory. The Y-D connection has no problem with third harmonic components due to circulating currents in D(delta). It is also more stable to unbalanced loads since the D partially redistributes any imbalance that occurs.

3. Scott connections are used in _______________
a) three-phase to single phase transformation
b) three-phase to two-phase transformation
c) single phase to three-phase transformation
d) all phase transformations

Explanation: Scott connections are used to convert three-phase to two-phase conversion, to start two phase motors and two phase furnaces. It requires two single phase transformers with adjustable tappings, one transformer is main transformer which is centre tapped through teaser transformer primary with a suitable number of turns to get a balanced two-phase supply.
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4. In a three-phase star – delta transformer, what is the angle difference between primary and secondary phase voltages?
a) Delta side leads by 300
b) Delta side lags by 300
c) Star side leads by 300
d) Star side lags by 300

Explanation: This is a vector group and has + 30° displacement. Therefore, delta side leads by + 30°. So, it can be stated that delta side is having lead of 300 over star side because of the connections made.

5. Which can be also called as 00 /1800 connection?
a) Star/star
b) Direct star
c) Delta/star
d) Star/delta

Explanation: Star connection is formed on each side by connecting together phase winding terminals. The voltages of the corresponding phases (and thus of the corresponding lines) are in phase. This is known as the 0°-connection. If the winding terminals on secondary side are reversed, the 180°-connection is obtained.

6. What is the ratio of transformation of star/star connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation 2x:1
c) Phase transformation x:1, line transformation x/3:1
d) Can’t say

Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/star connection we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x.

7. Delta/delta connection is also called as ____________
a) 00-connection
b) 900-connection
c) 1800-connection
d) 00/1800-connection

Explanation: Delta/delta connection is also called as 00-connection as seen from the phasor diagram that primary and secondary line voltages are in phase with each other. By reversing connection, we can get 1800 phase shift.

8. What is the ratio of transformation of delta/delta connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation 2x:1
c) Phase transformation x:1, line transformation x/3:1
d) Can’t determine

Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the delta/delta connection also similar to the star/star, we will get the same ratios, as at a particular point, voltage reading on primary will be proportional to secondary with x.

9. Open delta connection has VA rating of _______________
a) √3 times delta/delta VA rating
b) 1/√3 times delta/delta VA rating
c) 3 times delta/delta VA rating
d) 1/3 times delta/delta VA rating

Explanation: When one of the transformer in delta/delta connection is removed we get open delta connection. This connection can handle the power of √3VI. While on the similar line delta/delta connection can handle the power of 3VI.

10. Star/delta connection is also called as ____________
a) 300-connection
b) 00-connection
c) -300-connection
d) 300/-300-connection

Explanation: Star/delta connection is also called as +/-300-connection as seen from the phasor diagram that primary and secondary line voltages are either ahead or below by 300 phases with each other. By reversing connection, we can get another condition phase shift.

11. What is the ratio of transformation of star/delta connection?
a) Phase transformation x:1, line transformation x:1
b) Phase transformation x:1, line transformation √3x:1
c) Phase transformation x:1, line transformation 3x:1
d) Can’t determine with information available

Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the √3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with √3x.

12. x/√3:1 ratio is obtained in ______________
a) Star/delta
b) Delta/star
c) Delta/delta
d) Star/star

Explanation: The phase transformation ratio is given as x:1, where x simply denotes the turns ratio of a transformer given, thus, in the star/delta connection we will get the 1/√3 factor in ratios, as at a particular point, voltage reading on primary will be proportional to secondary with 1/√3x.

13. Which both connections have the same line transformation ratios?
a) Star/star and delta/delta
b) Star/delta and delta/star
c) Star/zig-zag star and delta/zig-zag star
d) Star/star, delta/delta and star/delta, delta/star

Explanation: Star/star and delta/delta both connections have phase transfer ratio of x:1 and line transfer ratio also equal to x:1, while star/zig-zag star and delta/zig-zag star connections have line transformation ratio equal to 2/√3x:1.

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