Transformer Calculator

The Transformer Calculator computes key operating parameters of a three-phase power transformer including turns ratio, primary and secondary full-load currents, and output power. Transformers are the workhorses of the electrical power system, enabling efficient long-distance transmission at high voltage and safe distribution at utilization voltages.

The fundamental transformer equation is V₁/V₂ = N₁/N₂, where V represents voltages and N represents the number of turns on each winding. A step-down transformer with a 11,000V primary and 400V secondary has a turns ratio of 27.5:1. The secondary voltage is determined purely by this ratio (in an ideal transformer), while current transforms inversely: I₁/I₂ = N₂/N₁.

The kVA rating defines the transformer's apparent power capacity. For three-phase transformers, the full-load line current is calculated as I = kVA × 1000 / (√3 × V_L), using the √3 factor that relates line-to-line voltage to the per-phase voltage in balanced three-phase systems.

Transformer efficiency is very high — typically 97-99% for distribution transformers — because transformers have no moving parts. Losses consist of core losses (eddy currents and hysteresis, roughly constant with voltage) and copper losses (I²R in windings, varying with load squared). Modern amorphous core transformers achieve over 99.5% efficiency under loading conditions.

Understanding transformer ratings is essential for load planning. A 100 kVA transformer with unity power factor loads can supply 100 kW. With 0.85 power factor loads, it only supplies 85 kW of real power while the transformer is fully loaded. The kVA rating constrains apparent power, not real power — hence the importance of power factor correction at transformer level.