Faculty of Engineering
Electrical and Computer Engineering Department
ECED 2001 Circuits Analysis
Assignment 5
Dr. Hamed Aly
Due Mar 6/ 2021
Problem 5
A 23 KVA, 2300/230 V, 60 Hz, step down transformer has the following resistance and leakage
reactance values: R1 = 4 Ω, R2= 0.04 Ω, X1 = 12 Ω, and X2 = 0.12 Ω. The transformer is operating
at 75% of its rated load. If the power factor of the load is 0.866 leading, determine the efficiency
of the transformer at rated voltage.
Problem 6
A simple power system is shown in Fig.5. This system
contains a 480-V generator connected to an ideal 1:10
step up transformer, a transmission line, an ideal 20:1
step down transformer, and an inductive load. The
impedance of the transmission line is 20+j60 Ω, and the
30
impedance of the load is 10
Ω.
a) Find the power supplied to the load in this
system.
b) Find the power lost in the transmission line.
Fig. 5
Problem 7
A 15-KVA, 2300/230-V transformer is to be tested to determine its excitation branch components,
its series impedances, and its voltage regulation. The following test data have been taken from the
primary side of the transformer:
Open circuit test: Voc = 2300 V, Ioc = 0.21 A, Poc = 50 W
Short circuit test: Vsc = 47 V, Isc = 6.0 A, Psc = 160 W
a) Find the equivalent circuit of this transformer referred to the high voltage side.
b) Find the equivalent circuit of this transformer referred to the low voltage side.
c) Calculate the voltage regulation when the transformer delivers rated load at 0.8 lagging
power factor, 1.0 power factor, and at 0.8 leading power factor at rated voltage.
d) What is the efficiency of the transformer at full load with a power factor of 0.8 lagging?
e) Find the loading at the maximum efficiency and the maximum efficiency at 0.8 power
factor lagging.
Problem 8
These data were obtained when open circuit and short circuit tests were performed on a single
phase, 50 KVA, 2,400:240 V, 60 Hz distribution transformer:
With high voltage winding open circuited: Voc = 240 V, Ioc = 4.85 A, Poc = 173 W
With low voltage winding short circuited: Vsc = 52 V, Isc = 20.8 A, Psc = 650 W
a) Determine the equivalent circuit of the transformer referred to its high voltage side and to
its low voltage side.
b) Draw the phasor diagram corresponding to the circuit referred to the high voltage side.
c) Compute the efficiency of the transformer corresponding to a full load, 0.8 power factor
lagging and one half load, 0.6 power factor lagging.
d) Determine the fraction of full load at which the maximum efficiency of the transformer
occurs, and compute the efficiency at that load.