ESO 210

Introduction to Electrical Engineering Lecture-23

Three Phase Transformers

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Selected Problems A 15 kVA, 2200/220V, 50 Hz transformer gave the following test results:

O.C. test from l.v side: 220V 2.72A 185W

S.C. test from h.v side: 112V 6.3A 197W

Compute the following:

a) Core loss

b) Full-load copper loss

c) Efficiency at full load, 0.85 lagging pf

d) Voltage regulation at 0.8 lagging pf

Ans: a) 185W b) 231W c) 96.8% d) 4.41%

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A 4 kVA, 200/400 V, 50 Hz, single phase transformer, of which the following are

the test figures:

O.C with 200 V applied to LV winding, current 0.8 A, power 70 W.

S.C with 17.5 V applied to HV winding, current 9 A, power 50 W.

Calculate:

a) Full-load efficiency at unity power factor at i) unity, ii) 0.8 lagging T

b) The voltage at the secondary terminals when supplying full load secondary

current at power factors i) unity, ii) 0.8 lagging.

a)(Ans:96.8% and 96%) b)(Ans:392.8% and 384%)

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A 500 kVA transformer has an efficiency of 95% at full load and at

60% of full load; both at unity power factor

a) Separate out the losses of the transformer.

b) Determine the efficiency of the transformer at 3/4th full load.

Ans : a) Pi = 9.87 kW, Pc = 16.45 kW b) 95.14%

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A three phase transformer bank consisting of three 1-phase transformers is used to step-down the voltage of a three-phase, 6600 V transmission line. If the primary line current is 10 A, calculate the secondary line voltage, line current for the following connections: (a) star-delta, (b) delta-star. The turns ratio N1:N2 is 12. Neglect losses.

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Three single phase, 50 kVA, 11000/250 V, 50 Hz transformers are connected to

form a 3 phase, 11000/433 V transformer bank. The equivalent impedance of each

transformer, referred to the high voltage side, is (300+j400) ohm. The transformer

delivers 32 kW load at 0.8 pf (lagging) at rated voltage. The magnetizing and core

loss components can be neglected. (Assume Delta-Star connection)

a) Determine the transformer winding currents.

b) Determine the voltage regulation at given loading condition

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A 7.5 kVA, 440/220 V transformer was tested for finding

the parameters in the equivalent circuit. The readings

were as follows:

OC test (l.v test): 220 V, 50 W, 1A.

SC test (h.v. test):15 V, 60 W, 17 A.

Find the equivalent circuit referred to the HV side.

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DC Motors