Transformer Turns Ratio, Current and Efficiency Calculator

What is the turns ratio?

The ratio of the turns on the primary and secondary windings sets the ratio of the voltages. For an ideal transformer:

N1 / N2 = V1 / V2 = I2 / I1

Voltage follows the number of turns and current goes the opposite way. The side with fewer turns has the lower voltage and the higher current. Apparent power in equals apparent power out.

Example

Apply 230 V to a transformer with 1000 primary turns and 100 secondary turns and the secondary gives 230 × 100 / 1000 = 23 V. With a 100 VA load the primary current is 100 / 230 = 0.435 A and the secondary current is 100 / 23 = 4.35 A.

Rated currents

The nameplate kVA and voltages give the currents. Use I = S / V for single phase and I = S / (√3 × V) for three phase. A 1000 kVA, 34.5 / 0.4 kV distribution transformer has a rated secondary current of 1,000,000 / (1.732 × 400) = 1443 A.

Efficiency

A transformer has two main losses. The core loss is there whenever the transformer is energised and does not change with load. The copper loss rises with the square of the load current. Efficiency is η = P2 / (P2 + P_fe + x² × P_cu), where x is the loading. It peaks at the load where the copper loss equals the core loss.

Voltage regulation

The secondary voltage drops as the transformer is loaded. Regulation is the difference between the no load and full load voltage divided by the full load voltage. Lower is better.

Limits

The turns ratio calculations assume an ideal transformer. Designing a transformer from turns also needs the core cross section, flux density, frequency, wire size and window fill, none of which this tool covers. In a three phase transformer the line voltage ratio can differ from the turns ratio by a factor of √3, depending on the vector group.