Windage loss in rotatory machines occur due to air resistance to the armature. Therefore, in order to even start a motor, the machine needs to overcome friction and windage torque and inertia due to its weight to start rotating. Windage is the churning effect of the rotating armature in the confined space inside the field coils. Even a smooth airflow would take some power to achieve, the very turbulent conditions inside the motor take much more power.
The windage loss depends on the design and shape of the rotating object. In synchronous machine, the salient pole rotors have more windage loss as compared to windage loss in non-salient or cylindrical rotor. If the fan is used in motors for cooling, the windage loss takes place in the fan as well.
The higher speed electrical machine has more windage loss.Therefore, the machines are specially designed to keep the windiage loss minimum.
It is very difficult to determine the windage loss of the electric rotating machine. The friction and windage loss is about 1 % of full load loss.
Windage Loss Formula
The windage loss of the smooth cylindrical rotor can be estimated by following theoretical equation.