ESO 210
Introduction to Electrical Engineering
Lecture-23
Three Phase Transformers
2
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
a)
b)
c)
d)
Compute the following:
Core loss
Full-load copper loss
Efficiency at full load, 0.85 lagging pf
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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10
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.
11
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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13
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