# Questions & Answers of Three Phase Transformers: Connections, Parallel Operation, Auto-transformer

## Weightage of Three Phase Transformers: Connections, Parallel Operation, Auto-transformer

Total 13 Questions have been asked from Three Phase Transformers: Connections, Parallel Operation, Auto-transformer topic of Electrical Machines subject in previous GATE papers. Average marks 1.77.

A 3-phase $900\;\mathrm{kVA},\;3\;\mathrm{kV}/\sqrt3\;\mathrm{kV}(\triangle/\mathrm Y),$  50 Hz transformer has primary (high voltage side) resistance per phase of $0.3\;\Omega$and secondary (low voltage side) resistance per phase of $0.02\;\Omega$. Iron loss of the transformer is 10 kW. The full load % efficiency of the transformer operated at unity power factor is _______ (up to 2 decimal places).

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A three-phase, three winding  $\triangle/\triangle/Y\;(1.1\;kV/6.6\;kV/400\;V)$ transformer is energized from AC mains at the 1.1 kV side. It supplies 900 kVA load at 0.8 power factor lag from the 6.6 kV winding and 300 kVA load at 0.6 power factor lag from the 400 V winding. The RMS line current in ampere drawn by the 1.1 kV winding from the mains is _________ . (Give the answer up to two decimal places.)

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If the primary line voltage rating is 3.3 kV (Y side) of a 25 kVA, Y-$\triangle$ transformer (the per phase turns ratio is 5:1), then the line current rating of the secondary side (in Ampere) is____________.

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In a constant V/f induction motor drive, the slip at the maximum torque

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The starting line current of a 415 V, 3-phase, delta connected induction motor is 120 A, when the rated voltage is applied to its stator winding. The starting line current at a reduced voltage of 110 V, in ampere, is _________.

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The primary mmf is least affected by the secondary terminal conditions in a

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A 200/400 V, 50 Hz, two-winding transformer is rated at 20 kVA. Its windings are connected as an auto-transformer of rating 200/600 V. A resistive load of 12$\mathrm{\Omega }$ is connected to the high voltage (600 V) side of the auto-transformer. The value of equivalent load resistance (in Ohm) as seen from low voltage side is _________.

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Find the transformer ratios a and b that the impedance (${\mathrm{Z}}_{in}$) is resistive and equal 2.5$\mathrm{\Omega }$ when the network is excited with a sine wave voltage of angular frequency of 5000 rad/s.

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A balanced (positive sequence) three-phase AC voltage source is connected to a balanced, start connected through a star-delta transformer as shown in the figure. The line-to-line voltage rating is 230 V on the star side, and 115 V on the delta side. If the magnetizing current is neglected and $\overline{{I}_{s}}=100\angle {0}^{°}$ A, then what is the value of $\overline{{I}_{p}}$ in Ampere?

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A three-winding transformer is connected to an AC voltage source as shown in the figure.The number of turns are as follows: ${N}_{1}$ = 100, ${N}_{2}$ = 50,= 50. If the magnetizing current is neglected, and the currents in two windings are $\overline{{\mathit{I}}_{\mathit{2}}}=2\angle {30}^{°}$ A and $\overline{{\mathit{I}}_{\mathit{3}}}=2\angle {150}^{°}$A, then what is the value of the current $\overline{{\mathit{I}}_{\mathit{1}}}$ in Ampere?

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Two three-phase transformers are realized using single-phase transformers as shown in the figure.

The phase different (in degree) between voltage ${\mathrm{V}}_{1}$ and ${\mathrm{V}}_{2}$ is ________.

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The figure above shows coils 1 and 2, with dot markings as shown, having 4000 and 6000 turns respectively. Both coils have a rated current of 25 A. Coil 1 is excited with single phase, 400 V, 50 Hz supply

The coils are to be connected to obtain a single phase, 400/1000 V, auto transformer to drive a load of 10kVA. Which of the options given should be exercised to realize the required auto transformer?

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The figure above shows coils 1 and 2, with dot markings as shown, having 4000 and 6000 turns respectively. Both coils have a rated current of 25 A. Coil 1 is excited with single phase, 400 V, 50 Hz supply

In the autotransformer obtained in Question 57, the current in each coil is