Page 596 - Basic Electrical Engineering

B. Numerical Problems

35. A 40 kVA 3200/400 V, single phase, 50 Hz transformer has 112 turns on the secondary
winding. Calculate the number of turns on the primary winding. What is the secondary current
at full load? What should be the cross-sectional area of the core for a core flux density of 1.2
2
Wb/m ?

2
[Ans 896, 100 A, 01362 m ]

36. A 400 kVA transformer has a full-load core loss of 800 W and copper loss of 2500 W. What

will be the values of these losses at load?

[Ans 800 W, 625 W]

37. A single-phase transformer is required to step down the voltage from 1100 V to 400 V at 50
2
2
Hz. The core has a cross-sectional area of 25 cm and the maximum flux density is 5Wb/m .
Determine the number of turns of the primary and secondary windings.

[Ans 396, 144]

38. A single phase 40 kVA transformer has primary and secondary voltages of 6600 V and 230 V,
respectively. The number of turns of the secondary winding is 30. Calculate the number of
turns of the primary winding. Also calculate the primary and secondary winding currents.

[Ans 860, 6.06 A, 173.9 A]

39. A transformer on no load takes 4.5 A at a power factor of 0.25 lagging when connected to a
230 V, 50 Hz supply. The number of turns of the primary winding is 250. Calculate (a) the
magnetizing current, (b) the core loss, and (c) the maximum value of flux in the core.

−3
[Ans I = 4.35, P = 259 W, ϕ = 4.14 × 10 Wb]
m
m
c
40. A 660 V/ 220 V single-phase transformer takes a no-load current of 2A at a power factor of
0.225 lagging. The transformer supplies a load of 30 A at a power factor of 0.9 lagging.
Calculate the current drawn by the primary from the mains and primary power factor.
Resistance and reactance of the windings may be neglected.

[Ans I = 11.38 A, cos ϕ = 0.829 lag]
1
1