EEE 3: Elementary Electrical Engineering
Practice Problems for Transformers (single-phase AC)
Instructions: For these problems, draw the circuit diagrams before solving.
1. A 1000-VA 230/115-V transformer has been tested to determine its equivalent circuits. The test
parameters are as follows:
• Open-circuit test, on the LV side: V = 115 volts, I = 0.11 amperes, and P = 3.9 watts
• Short-circuit test, on the HV side: V = 17.1 volts, I = 8.7 amperes, and P = 38.1 watts
Determine the equivalent circuit of this transformer, referred to the low-voltage (LV) side of the
transformer.
2. A 30-kVA 8000/230-V distribution transformer has an impedance of 20 + j100 ohms, referred to
the high-voltage side. The components of the excitation branch are Rc = 100 kiloohms and Xm =
20 kiloohms, referred to the high-voltage side.
a. If the voltage at the HV side is 7967 volts and the load impedance connected at the LV
side is ZL = 2.0 + j0.7 ohms:
i. What is the secondary voltage of the transformer?
ii. What is the voltage regulation of the transformer?
iii. What will be the transformer efficiency under these conditions?
b. If the load is disconnected and a capacitor with impedance -j3.0 ohms is connected in its
place:
i. What is the secondary voltage of the transformer?
ii. What is the voltage regulation of the transformer?
Reference: S. J. Chapman, Electric Machinery Fundamentals, 5th edition, New York McGraw-Hill, 2012.