A real electrical machine does not convert all input power into useful output. Losses appear as heat, sound and mechanical drag throughout the electromagnetic and mechanical structure.

Copper losses

Current through stator, rotor, field or armature winding resistance produces

$$P_{\mathrm{Cu}}=I^2R.$$

These losses rise strongly with load current.

Magnetic-core losses

Alternating magnetic flux produces hysteresis and eddy-current loss in iron cores. Their magnitude depends on flux density, frequency, material and core construction.

Mechanical and stray losses

Bearings and seals produce friction, while rotating surfaces and fans produce windage. Real machines also have additional stray-load losses caused by leakage fields, harmonics and nonideal current distributions.

Power flow

A useful machine model follows power through successive stages:

input power → internal conversion → output power
                 ↓
               losses

In a motor, electrical power enters and mechanical shaft power leaves. In a generator, the direction is reversed.

Efficiency

Efficiency is

$$\eta=\frac{P_{\mathrm{out}}}{P_{\mathrm{in}}} =1-\frac{P_{\mathrm{loss}}}{P_{\mathrm{in}}}.$$

Different losses dominate at different operating points. Loss analysis therefore determines not only efficiency but also heating, cooling requirements and allowable machine rating.