ELL101
TUTORIAL 3 (TRANSFORMERS)
1. A 40kVA transformer with a ratio of 2000/250 V has a primary resistance of 1.15 ohm
and a secondary resistance of 0.0155 ohm. Calculate,
(a) the total resistance in terms of the secondary winding.
(b) total copper loss on full load.
2. A 30kVA, 2400/120V, 50-Hz transformer has a high voltage winding resistance of
0.1Ω and a leakage reactance of 0.22Ω. The low voltage winding resistance is 0.035Ω
and the leakage reactance is 0.012Ω. Find the equivalent winding resistance, reactance
and impedance referred to the
(a) high voltage side
(b) low voltage side.
3. Open circuit test is conducted on a single-phase 230V/440V , 50 Hz transformer. The
no load current measured is 4.5 A. The wattmeter reads 700 watts. Calculate:
(a) No load power factor
(b) Core loss parameters (Rc & Xm)
(c) If the operating frequency is changed to double the original value, what are the new
values of wattmeter and ammeter reading?
4. 15 kVA , 440V/110V, 60 Hz single phase transformer subjected to short circuit test.
Wattmeter reads 1200 W. 45 V on the HV side is required to circulate full load current
in both the windings.
(a) Calculate the power factor in this test
(b) What is the equivalent leakage resistance and leakage inductance
(c) If the core loss of the transformer is 500 Watts and the transformer is working at
50% load with 0.8 lagging pf, calculate the efficiency of the transformer in this case.