Cairo University, Faculty of Engineering
3 rd
Year Mechanical Production
EPM 312: Electrical Power and Machines
1.
A single-phase transformer has 400 primary turns and 800 secondary turns. The net iron cross-sectional area of the core is 40 cm2. If the primary winding is connected to a 600 V, 50
HZ supply. Calculate the maximum value of the core flux density and the secondary voltage.
2.
Determine the rated primary and secondary currents of a 20 kVA, 1200/120 V, 1-ph transformer. If this transformer delivers a load of 12 kW at 0.8 power factor, find the primary and secondary currents and the impedance of the load referred to the primary side.
3.
The no load current of a transformer is 4 A at 0.25 pf lag. When supplied at 250 V, 50 Hz.
The number of turns on the primary winding is 200. Calculate: a) The rms value of the flux in the core. b) The core loss. c) The magnetizing and working components of the no load currents.
4.
The parameters of a 2 KVA 200/100 V, 50 HZ, single-phase transformer are: r
1
 0.1
Ω r
2
 0.03
Ω R c
 8400 Ω
x
1
 0.4
Ω x
2
 0.1
Ω X m
 2500 
The transformer supplies a secondary load of 2 KVA at 0.8 power factor lagging with rated secondary voltage. Calculate
A.
The primary applied voltage and primary current.
B.
Efficiency and voltage regulation.
5.
A 150-kVA, 2400/240 V transformer has equivalent impedance referred to primary = 0.5 + j1.5 Ω. The magnetizing impedance is very high. At full load, the transformer delivers rated kVA at 0.8 lagging pf and the secondary voltage 240 V. Calculate: a) Voltage regulation. b) Efficiency at 600 W core losses.
6.
A 500 KVA, 2300/230 V, single-phase distribution transformer is tested to find the equivalent circuit parameters. The following data were obtained:
Open circuit test
Voltage (V)
230
Current (A)
94
Power (W)
2250
Short circuit test 100 230 9200
I.
Find the equivalent circuit parameters referred to the primary.
II.
Find the equivalent circuit parameters referred to the secondary.
III.
Efficiency and voltage regulation at full load and 0.85 lagging pf.
IV.
Efficiency and voltage regulation at full load and unity pf.
V.
Efficiency and voltage regulation at full load and 0.85 leading pf.