Class: BEE2-A
Time Allowed: 10 mins
Maximum Points: 20
Reg. No:
QUIZ THREE - SOLUTION
NOTE: Please draw labelled circuit diagrams where appropriate, show complete calculations, and reasoning
to get maximum marks.
1. A 1000 VA , 230/115 V transformer has been tested to determine its equivalent circuit:
Open Circuit Test: Poc = 30W;Voc = 230V;Ioc = 0.45A
Short Circuit Test: Psc = 42.3W;Vsc = 19.1V;Isc = 8.7A
All data were taken from the primary side of the transformer. [20]
a. Draw the transformer equivalent circuit referred to low voltage side, and label all the impedances, reac-
tances, voltages and currents, as appropriate. [5]
b. Using phasor diagram, find the transformer’s voltage regulation at 0.6 PF leading. [5]
c. If the transformer were supplying rated load at 115 V and 0.8 PF lagging, what would be the transformer’s
input voltage? [5]
d.What is the transformer’s efficiency under the conditions of part c?[5]
SOLUTION
a. Using open-circuit test values, we can compute the impedances of the excitation branch.
θ= cos−1(Poc
VocIoc ) = cos−1(30
230×0.45 ) = 73.15olagging.
Yoc =Ioc
Voc =0.45∠−73.15
230 = 5.67 ×10−4−j1.875 ×10−3Ω−1
Zoc =1
Yoc = 1763 + j533.09Ω
→Rc= 1763Ω, XM= 533.09Ω
Using short-circuit test values, we can compute the series impedances:
θ= cos−1(Psc
VscIsc ) = cos−1(42.3
19.1×8.7) = 75.3olagging.
ZSE =Vsc
Isc∠−75.3o= 2.19∠75.3=0.557 + j2.118Ω
→Req = 0.557Ω, Xeq = 2.118Ω
Since the test data were taken from primary side (high voltage side), we need to transfer the impedances
to secondary side so as to get the equivalent circuit referred to the low voltage side.
The turns ratio, n=230
115 = 2
1
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