TRANSFORMERS
1) A certain transformer has a core loss of 100 watts at 30 Hz and core loss of 300 watts at 60 Hz.
What is the hysteresis loss at 60 Hz?
a. 100 c. 200
b. 130 d. 250
2) A 37.5 kva, 20,000/240 volts, single phase transformer has rated core loss of 350 watts and full
load copper los of 500 watts. Calculate the load to achieve maximum efficiency
a. 32.5 kva c. 31.4 kva
b. 37.5 kva d. 26.5 kva
3) A 20kv/ 7.97 kv auto-transformer draws 100 A at its secondary. What is the current at the
common winding?
a. 139.85 A c. 60.15 A
b. 100 A d. 39.85 A
4) Three transformers are delta connected. One connection has been cut, what is the percent
reduced?
a. 33% c. 67%
b. 57.5% d. 42%
5) What is the equivalent resistance of the transformer primary having turn ratio of 5.0 when its
equivalent referred to secondary is 0.2 ohm?
a. 5c. 0.04
b. 0.08 d. 1.0
6) The primary winding of a 4600/230 volts transformer has 1000 turns. How many turns are there at
the secondary?
a. 1000 c. 50
b. 100 d. 20,000
7) A 60 Hz transformer has a primary turns of 2400 and secondary turns of 2400 and secondary turns
of 240. If the maximum mutual flux is 2.5 x 10^6 maxwells, calculate the primary induced emf in
volts.
a. 3600 c. 3000
b. 15984 d. 1598
8) A 2000 kva, 34500/240 volts, 60 Hz 3-phase oil filled transformer has an impedance of 5.75%.
Determine the secondary full load current.
a. 5102 c. 5350
b. 5280 d. 4811
9) Material used for construction of transformer core is usually
a. Wood c. Aluminum
b. Copper d. Silicon steel
10)Device use by transformer to adjust load voltage if line supply is above or below nominal voltage.
a. Thermal trip c. differential relay
b. Tap changer d. bucholz relay
11)Transformer laminations are insulated from each by
a. Mica strip c. paper
b. Thin coat of varnish d. rubber
12)Which of the following is common to insulator, bushing potential transformer and transformer
winding?
a. Noise level c. current rating
b. Insulator d. BIL rating
13)The output current corresponding to the maximum efficiency for a transformer having core loss of
200 W and equivalent resistance referred to the secondary of 0.5 ohm is
a. 5 A c. 15 A
b. 10 A d. 20 A
