Which losses primarily determine transformer efficiency?

• A. Core losses (hysteresis and eddy currents) and copper losses (winding resistance)
• B. Mechanical losses only
• C. Copper losses only
• D. Friction losses only

Answer: (A)

Transformer efficiency is set by heat losses inside the device, mainly from two places: the magnetic core and the windings. The core losses come from hysteresis and eddy currents in the core material. These losses are largely determined by the voltage, frequency, and the core’s material and design, and they stay roughly constant as the load changes. The winding copper losses arise from the resistance of the coils; these losses grow with the current and follow the I^2R relationship, meaning they become more significant as the transformer carries more load.

Together, these two types of losses determine how much input power is wasted as heat for any given output, which is what sets the efficiency. Mechanical losses are not a primary factor in a stationary transformer, since there are no moving parts driving a load. So the losses that mainly determine efficiency are the core losses plus the winding copper losses.