Questions -

Q1. A single-phase transformer rated 2.1 kV/130 V, 7.8 kVA has the following winding parameters: r1= 0.7Ω, x1 = 0.9Ω, r2 = 0.04Ω and x2 = 0.05Ω. Determine:

a. The combined winding resistance ________ Ω and leakage resistance ________ Ω, referred to primary side, as shown in below figure.

b. The value of combined winding resistance ________ Ω and leakage resistance ________ Ω, referred to secondary side.

c. The voltage regulation of the transformer when it is delivering 7.6 kVA to a load at 130 V and 0.85 power factor lagging ______ %.

Q2. A single-phase transformer rated 7.8 kVA., 1.2 kV/115 V has a primary winding of 900 turns. This transformer is delivering 8 kVA at its rated voltages and 0.85 power-factor lagging. Determine:

a. The impedance Z₂ connected across its secondary terminals: ________ Ω

b. What is the value of this impedance referred to the primary side (i.e. Z'₂): ________ Ω

c. Using the value of Z'₂ obtained in part (b), determine the magnitude of primary current ________ A, and the kVA supplied by the source ________.

Q3. A single phase transformer is rated 400/240 V, 5.5 kVA. When the low voltage side is short circuited and 34 V is applied to high voltage side, rated current flows in the windings and the power input is 120 W. If the power loss and ratio of reactance to resistance is the same in both windings, find:

a. The resistance, ________ Ω and reactance ________ Ω of high voltage winding.

b. The resistance, ________ Ω and reactance ________ Ω of low voltage winding.

Q4. A single phase transformer rated 1100 V/100 V, 7.8 kVA yields the following test result: Open circuit test (Primary Open): Voltage V₂ = 100 V; Current I₂ = 1.8 A; power W₂ = 44 W. Short circuit test (Secondary Shorted): Voltage V₁ = 10 V; Current I₁ = 7 A; power W₁ = 36 W. Determine:

a. R₁ = r₁ + a²r₂ = ________ Ω

X₁ = x₁ + a²x₂ = ________ Ω

G_c ________ mS and B_m ________ mS, when referred to the primary side, as shown in below figure.

b. The values of above parameters when refereed to secondary side:

R' = ________ Ω

X' = ________ Ω

G'_c = ________ mS

B'_m = ________ mS

c. The efficiency of transformer when it delivers 5 kVA at 100 V and 0.9 power factor, η = ________ %.