Licensed Electrical Contractor Transformers and Transformer Calculations 2

Question 1

What is the full-load primary current of a 45 kVA, single-phase transformer with a 480V primary?

Accepted Answer

93.75A

Answer

Full-load current = kVA × 1000 / primary voltage = 45,000 / 480 = 93.75A.

Question 2

A single-phase transformer has a 3:1 step-down turns ratio. If the primary current is 10A, what is the secondary current?

Accepted Answer

30A

Answer

Secondary current = primary current × turns ratio = 10A × 3 = 30A; as voltage steps down, current steps up by the same ratio.

Question 3

What is the full-load secondary current of a 25 kVA, 240V single-phase transformer?

Accepted Answer

104.2A

Answer

Full-load secondary current = kVA × 1000 / secondary voltage = 25,000 / 240 = 104.2A.

Question 4

What is the full-load primary current of a 75 kVA, three-phase transformer with a 480V primary?

Accepted Answer

90.2A

Answer

For three-phase: I = kVA × 1000 / (V × 1.732) = 75,000 / (480 × 1.732) = 75,000 / 831.4 ≈ 90.2A.

Question 5

A transformer has a nameplate impedance of 5%. If the available short-circuit current at the primary is 2,000A, what is the transformer's full-load primary current?

Accepted Answer

100A

Answer

Full-load current = Short-circuit current × impedance (decimal) = 2,000A × 0.05 = 100A.

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Licensed Electrical Contractor Transformers and Transformer Calculations 2