Page 598 - Basic Electrical Engineering

6 hours : full-load at 0.9 power factor

The full load copper loss and core loss is 5 kW and 2 kW, respectively. Calculate the all-day
efficiency of the transformer.

[Ans 92.5 per cent]

47. A 12 kVA, 200/400 V, 50 Hz single-phase transformer gave the following readings on the
open-circuit test and the short-circuit test:
open-circuit test : 200 V, 1.3 A, 120 W
short-circuit test conducted on the H.V. side: 22 V, 30 A, 200 W
Calculate the equivalent circuit parameters as referred to the low voltage side. Also calculate
the magnetizing component of the no-load circuit.

[Ans R = 333 Ω, X = 174 Ω, R′ = 0.055 Ω, X′ = 0.175 Ω, I = 1.15A]
m
e
c
m
e
C. Multiple Choice Questions

1. A transformer having number of turns in the primary and secondary winding of 1000 and 500,
respectively, is supplied with 230 V at 50 Hz. The induced EMF in the secondary winding will
be

a. 460 V at 50 Hz
b. 115 V at 25 Hz
c. 115 V at 50 Hz
d. 500 V at 50 Hz.

2. The core of the transformers is made of laminated steel sheets so as to

a. Reduce hysteresis loss
b. Reduce eddy current loss
c. Increase output voltage
d. Reduce both hysteresis loss and eddy current loss.

3. The EMF induced in the windings of a transformer

a. Lags the core flux by 90°
b. Leads the core flux by 90°
c. Is in phase with the core flux
d. Is in opposition to the core flux.

4. To reduce the core losses in a transformer

a. The core is made of silicon steel laminations
b. The core is fastened very tight so that the core flux do not fly away