EETC Transformers 2 — Questions and Answers
Question 1: A transformer has 500 primary turns and 100 secondary turns. If the primary voltage is 240V, what is the secondary voltage?
- 48V (Correct answer)
- 1200V
- 240V
- 24V
Correct answer: 48V
Vs = Vp × (Ns/Np) = 240 × (100/500) = 48V.
Question 2: What happens to the magnetizing current in a transformer when the core becomes saturated?
- It increases sharply and becomes non-sinusoidal (Correct answer)
- It decreases proportionally
- It remains constant
- It becomes zero
Correct answer: It increases sharply and becomes non-sinusoidal
Core saturation causes a sharp, non-linear rise in magnetizing current and introduces harmonic distortion.
Question 3: Which type of transformer winding connection introduces a 30-degree phase shift between primary and secondary voltages?
- Delta-wye (Δ-Y) (Correct answer)
- Delta-delta (Δ-Δ)
- Wye-wye (Y-Y)
- Open delta
Correct answer: Delta-wye (Δ-Y)
A delta-wye (or wye-delta) connection inherently produces a 30-degree phase displacement between primary and secondary voltages.
Question 4: The impedance of a transformer expressed as a percentage (% Z) is primarily used to determine:
- Short-circuit current available at the secondary (Correct answer)
- Core loss under no-load conditions
- Magnetizing reactance
- Efficiency at full load
Correct answer: Short-circuit current available at the secondary
Percent impedance directly determines the available fault current: I_fault = I_rated / (%Z/100).
Question 5: In a transformer, the eddy current losses are proportional to:
- The square of the frequency and the square of the flux density (Correct answer)
- Frequency only
- The square of the load current
- The cube of the flux density
Correct answer: The square of the frequency and the square of the flux density
Eddy current losses follow Pe ∝ f² × Bm², making them sensitive to both frequency and flux density.
Question 6: What is the purpose of a conservator tank on a large oil-filled transformer?
- To accommodate the expansion and contraction of the cooling oil (Correct answer)
- To provide additional fault current capacity
- To house the tap changer mechanism
- To filter impurities from the oil
Correct answer: To accommodate the expansion and contraction of the cooling oil
A conservator tank allows the oil volume to change with temperature while minimizing the oil's contact with air and moisture.
Question 7: When two single-phase transformers are connected in open-delta, the available three-phase kVA capacity compared to a full delta bank is approximately:
- 57.7% (Correct answer)
- 66.7%
- 50%
- 86.6%
Correct answer: 57.7%
An open-delta bank delivers 57.7% (1/√3) of the kVA rating of a full three-transformer delta bank.
EETC Transformers 2
A transformer has 500 primary turns and 100 secondary turns.
If the primary voltage is 240V, what is the secondary voltage?