### 1 The following data applies to a single phase transformer a Peak Flux density in the core

1. The following data applies to a single phase transformer :

(a) Peak Flux density in the core = 1.41 T

(b) Net core area = 0.01 m2

(c) Current density in the conductors = 2.55 MA/m2

(d) Conductor diameter = 2.0 mm

(e) Primary supply voltage (sinusoidal) = 240 V, 50 Hz

Calculate the rating (in kVA) of the transformer and the number of turns on the primary

winding.

2. A 200 kVA, 3300 V/240 V, 50 Hz single phase transformer has 80 turns on the secondary

winding. Assuming an ideal transformer, calculate :

(a) The primary current (I1) , secondary current (I2) on full load

(b) The max. value of flux

(c) The no. of primary turns (N1)

3. The net cross-sectional area of a single phase, 50 Hz transformer is 500 cm2 and is having 400

and 100 turns in primary and secondary windings respectively. If it is connected to a 230 V,

50 Hz supply, calculate :

(a) Emf induced in the secondary.

(b) Max. value of flux density in the core.

4. A step-down transformer, operating from a 50 Hz mains supply of 220 V, is used to provide a

reduced voltage to a secondary power supply. The latter delivers 302.5 W of power to a load

having an equivalent resistance of 10 ohms. The transformer must have a total of 12,500

turns. Determine :

(a) The secondary voltage of the transformer

(b) The no. of turns on each winding.

(c) The current supplied to the transformer by mains

5. The ratio of no. of turns of primary and secondary windings of a transformer is 5. The load

current on the secondary is 120 A at 0.8 lagging power factor. The primary current is 27 A at

0.72 power factor lagging. Find

(a) The no-load current I0

(b) Its phase with respect to the voltage

6. The secondary winding of a transformer has a terminal voltage of v2(t) = 282.8 sin(377t) V.

the turns ratio is 100:200. If the secondary current is i2(t) = 7.07 sin(377t- 36.87o) A. The

impedances of the transformer referred to the primary side are Req1 = 0.2 ohms, Rc = 300

ohms, Xeq1 = 0.75 ohms and Xm = 80 ohms. Calculate

(a) The primary current I1

(b) The voltage regulation

(c) Efficiency

7. For a 4:1 transformer, the resistance and leakage reactance of high voltage (HV) winding are

1.4 and 6 ohms respectively. The resistance and leakage reactance of low voltage (LV)

winding are 0.06 and 0.04 ohms respectively. If 200 V, 50 Hz is applied to HV winding and LV

winding is short-circuited, find :

(a) Current in LV winding (I2sc)

(b) Copper loss (Pcu)

(c) Power factor

8. The primary and secondary windings of a 500 kVA transformer has resistance of 0.42 ohms

and 0.0019 ohms respectively. The primary voltage and secondary voltages are 11000 V and

400 V respectively. Core loss is 2.9 kW, assuming power factor of load to be 0.8, Calculate

efficiency at :

(a) Full load

(b) Half load

9. The primary of a certain transformer takes 1 A at a power factor of 0.4 when connected

across 240 V, 50 Hz supply and the secondary is on open circuit. The number of turns on the

primary is twice that on the secondary. A load taking 50 A at a lagging power factor 0.8 is

now connected across the secondary. Sketch and explain briefly the phasor diagram for this

condition, neglecting the voltage drops in the transformer (i.e neglect leakage reactances and

winding resistances). What is now the value of primary current?

10. A 50 kVA, 6360/240 V transformer is tested on open and short circuit to obtain its efficiency,

the results of the test being as follows:

(a) Open circuit test : primary voltage = 6360 V; primary current = 1 A; power input = 2 kW

(b) Short circuit test : primary voltage = 180 V; secondary current = 175 A; power input =

2kW

? Draw the equivalent circuit diagram of the transformer under both the tests.

? Find out all the transformer parameters.

? The efficiency of the transformer when supplying full load at a power factor of 0.8

lagging.

? Phasor diagram for the above condition

rahul
27-Jul-2020