Transformers Flashcards

A transformer has 500 primary turns and 100 secondary turns. If the primary voltage is 240V, what is the secondary voltage?

Answer: 48V

Vs = Vp × (Ns/Np) = 240 × (100/500) = 48V.

What happens to the magnetizing current in a transformer when the core becomes saturated?

Answer: It increases sharply and becomes non-sinusoidal

Core saturation causes a sharp, non-linear rise in magnetizing current and introduces harmonic distortion.

Which type of transformer winding connection introduces a 30-degree phase shift between primary and secondary voltages?

Answer: Delta-wye (Δ-Y)

A delta-wye (or wye-delta) connection inherently produces a 30-degree phase displacement between primary and secondary voltages.

The impedance of a transformer expressed as a percentage (% Z) is primarily used to determine:

Answer: Short-circuit current available at the secondary

Percent impedance directly determines the available fault current: I_fault = I_rated / (%Z/100).

In a transformer, the eddy current losses are proportional to:

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.

What is the purpose of a conservator tank on a large oil-filled transformer?

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.

When two single-phase transformers are connected in open-delta, the available three-phase kVA capacity compared to a full delta bank is approximately:

Answer: 57.7%

An open-delta bank delivers 57.7% (1/√3) of the kVA rating of a full three-transformer delta bank.