This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Phase Conversion”.
1. Only Scott connection is used for _________
a) Converting three-phase to two-phase conversion
b) Converting three-phase to single-phase conversion
c) Converting single-phase to two-phase conversion
d) None of the mentioned
Explanation: Scott connection is used for obtaining two-phase supply as it is needed for various special purposes like supplying two-phase electric arc furnaces and can easily obtained from three-phase supply by this method.
2. In Scott connection, according to the vector diagram two windings are placed at ______________
a) At 1200 to each other
b) Perpendicular with respect to each other
c) At 600 to each other
d) Can’t say
Explanation: For a transformer vector diagram, if one winding is kept along the perpendicular axis of another winding then one can obtain two-phase supply from three-phase supply by using Scott connection. Physically this can be obtained by appropriate turns ratio.
3. What is the ratio of voltage/turn of two windings of a transformer in operated in Scott connection?
Explanation: The transformer primaries must have √3 N1/2 (teaser) and N1 turns; this would mean equal voltage/turn in each transformer. A balanced 2-phase supply could then be easily obtained by having both secondaries with equal number of turns, N2.
4. The primaries of two transformers in Scott connection are in turns ratio of ___________
a) √3/2: 1
b) 2/√3: √2
d) Can’t say
Explanation: A 2-phase supply could thus be obtained by means of transformers; one connected perpendicular according to the vector diagram is called the teaser transformer and the other is connected across, with turns ratio √3/2: 1 respectively.
5. The secondaries of two transformers are in ratio ____
b) √3/2: 1
c) √3: 1
d) √2: 1
Explanation: Though transformer primaries are in the ratio of √3/2: 1 with respect to each other secondary windings of these transformers are in ratio of 1:1. This simply means that both of these transformers have equal voltage per turns.
6. In Scott connection neutral point is located at _______________
a) Three phase side at teaser
b) Two-phase side
c) Three phase side secondary
Explanation: Neutral point if required can be located on three phase side of a transformer at teaser in Scott connection which is used to obtain two-phase supply from three-phase supply. Neutral point is located at teaser winding dividing the winding in the ratio 1:2.
7. For single-phase to three-phase change in transformer which of the following connection is most suitable?
a) Scott connection
b) Scott connection with resistor network
c) Scott connection with some energy storing device
d) Any of the mentioned
Explanation: A single-phase power pulsates at twice the frequency, while the total power drawn by a balanced 3-phase load is constant. Thus a 1-phase load can never be transferred to a 3-phase system as a balanced load without employing some energy-storing device (capacitor, inductor or rotating machine).
8. For 3/6-phase converter transformers __________
a) Scott connection is used
b) Each secondary winding is divided
c) Each primary winding is divided
d) Can’t be done
Explanation: For converting 3-phase to 6-phase, three-phase delta connection is used on primary side. Each secondary phase is divided into two equal halves with appropriate polarity and corresponding 6-phase voltage is obtained.
9. In 3/6-phase connection of transformers _______________
a) Secondaries are two stars in phase sequence
b) Secondaries are two deltas in phase sequence
c) Secondaries are two stars in phase opposition
d) Secondaries are two deltas in phase opposition
Explanation: Six-phase voltages (characteristic angle 360°/6 = 60°) are obtained by means of two stars in phase opposition, each star being formed from three respective half-windings. This is done without Scott connection.
10. 3/6-phase circuit is employed for providing DC path in some circuits.
Explanation: In certain applications like thyristors and rectifiers six-phase supply is required for providing path for the DC current. Therefore, it becomes necessary to convert three-phase AC supply into six-phase.
11. Two single-phase furnaces let’s say, A and B are supplied at 100 V by means of a Scott-connected transformer combination from a 3-phase 6600 V system. The voltage of furnace A is leading. Calculate on of the line current on the 3-phase side, when the furnace A takes 400 kW at 0.707 pf lagging and B takes 800 kW at unity pf.
a) 110 A
b) 99 A
c) 250 A
d) 149 A
Explanation: N1/N2= 6600/100= 60
√3/2 (N1/N2) = 57.16
Furnace currents are= Ia = 400*1000/ (100*0.707) = 5658 A
Similarly, Ib= 8000 A
On 3-phase side, IA= 5658/57.16 = 99 A.
Sanfoundry Global Education & Learning Series – Transformers.
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