Transformers and Distribution Systems Flashcards

What is the turns ratio of a single-phase transformer with a 480V primary winding and a 120V secondary winding?

Answer: 4:1

Turns ratio equals primary voltage divided by secondary voltage: 480V ÷ 120V = 4:1.

A 25 kVA, single-phase, 480V/240V transformer is operating at full load. What is the approximate rated secondary current?

Answer: 104A

Secondary current = kVA × 1,000 ÷ secondary voltage = 25,000 ÷ 240 ≈ 104A.

Which three-phase transformer connection provides a neutral conductor on the secondary side?

Answer: Delta-Wye

A Delta-Wye transformer provides a neutral from the center point of the wye-connected secondary windings.

According to NEC 450.3(B), a 480V transformer with a rated primary current of 9A requires primary overcurrent protection not exceeding what value?

Answer: 15A

NEC allows 125% of 9A = 11.25A; since no standard device matches that value, the next standard size of 15A is permitted.

What is the primary purpose of a K-rated transformer?

Answer: To handle harmonic currents from non-linear loads

K-rated transformers are built to withstand the additional heat generated by harmonic currents produced by non-linear loads such as computers and variable frequency drives.

A 75 kVA, three-phase, 480V/208Y/120V transformer is operating at full load. What is the approximate full-load secondary line current?

Answer: 208A

Full-load secondary current = kVA × 1,000 ÷ (secondary line voltage × √3) = 75,000 ÷ (208 × 1.732) ≈ 208A.

What type of transformer is designed to make small voltage adjustments, typically within ±5–20% of the supply voltage?

Answer: Buck-boost transformer

Buck-boost transformers are designed to raise (boost) or lower (buck) supply voltages by a small percentage to compensate for voltage variations.