A single-phase transformer has 480 turns on the primary and 90 turns on the secondary. The mean length of the flux path in the iron core is 1.8 m and the joints in the magnetic circuit are equivalent to an airgap of 0.1 mm. If the maximum flux density is to be 1.1 T when a p.d. of 2200 V at 50 Hz is applied to the primary, calculate:

A.the cross-sectional area of the core

B.the secondary voltage on no-load

C.the primary current and power factor on no load.

Assume the relative permeability of the iron core is 2200. the iron loss is 1.7 W/kg at 50 Hz and the density of iron is 7800 kg/m3.

Q2

Tests on a 10 kVA 11 000/240 V, 50 Hz single-phase distribution transformer gave the following results:

Open-circuit-test 11 000 V, 70 W, 21.8 mA

Short-circuit-test 310 V, 80 W, 0.9 A

In both tests the voltage was applied to the high-voltage winding.

a)  Calculate the equivalent circuit parameters referred to the high-voltage winding and draw these on a sketch of the equivalent circuit.

b)  Sketch the phasor diagram for the transformer and determine the voltage required to be applied to the primary winding when the transformer supplies rated kVA at 0.8 pf lagging.

c)  Calculate the regulation of the transformer when supplying rated kVA at 0.8 pf lagging.