Electrical Machine - IES Academy

India’s No. 1
IES Academy
Transformer
Chapter-1
OBJECTIVE QUESTIONS (GATE, IES)
Previous Years GATE Questions
Transformer Losses and Efficiency
GATE-1.
A single phase transformer has a maximum efficiency of 90% at 111 load
and unity power factor. Efficiency at half load at the same power factor
is:
[GATE-2003]
(a) 86.7% (b) 88.26%
(c) 88.9% (d) 87.8%
GATE-2.
A 500 kVA, 3-phase transformer has iron losses of 300 W and full load
copper losses of 600 W. The percentage load at which the transformer is
expected to have maximum efficiency is:
[GATE-2003]
(a) 50.0% (b) 70.7% (c) 141.4% (d) 200.0%
Data for Q.3 and Q.4 are given below. Solve the problems and choose the correct
answers.
A 300 kVA transformer has 95% efficiency at full load 0.8 pf lagging and 96%
efficiency at half load, unity pf.
GATE-3.
The iron loss (Pi) and copper loss (Pc) in kW, under full load operation
are:
[GATE-2006]
(a) Pc= 4.12, Pi, = 8.51
(b) PC = 6.59, Pi = 9.21
(c) Pc= 8.51, Pi = 4.12
(d) PC = 12.72, Pi = 3.07
GATE-4. What is the maximum efficiency (in %) at unity pf load [GATE-2006]
(a) 95.1 (b) 96.2 (c) 96.4 (d) 98.1
Principle of Transformer Action
GATE-5. Figure shows an ideal single-phase
transformer. The primary and secondary
coils are wound on the core as shown.
Turns ratio (N1/N2) = 2. The correct
phasors of voltages El, E2, currents I1, I2
and core flux Φ are as shown in figure.
[GATE-2003]
www.iesacademy.com E-mail: iesacademy@yahoo.com Page-30
25, 1 st Floor, Jia Sarai, Near IIT. New Delhi-16 Ph: 011-26537570, 9810958290