Transformer Efficiency

Formula

Description

Transformer efficiency is the ratio of output power to input power, where the input power equals the output power plus all losses. The two main loss mechanisms are core (iron) losses and copper (winding) losses. Core losses include hysteresis loss (proportional to frequency and B²) and eddy current loss (proportional to f² and B²); these are approximately constant regardless of load. Copper losses are I²R losses in the windings and increase with the square of the load current. Maximum efficiency occurs when core losses equal copper losses.

Variables

• Pout — Output (load) power (W)
• P_iron — Core loss from hysteresis and eddy currents (W)
• P_copper — Winding I²R loss (W)

Practical Notes

Typical efficiencies: small (<10 VA) transformers 80-90%, medium (10-1000 VA) 90-95%, large power transformers (>1 MVA) 97-99.5%. The result is a ratio (0 to 1); multiply by 100 for percentage. Core losses can be found from manufacturer data at the operating flux density and frequency. Copper losses can be measured or calculated from winding resistance and load current.