The correct answer is: A. Stray losses.
Stray losses are the losses that occur in a machine due to the magnetic flux that is not linked to the windings. These losses are caused by the leakage flux, which is the flux that does not pass through the air gap and the stator and rotor iron. The leakage flux can cause eddy currents in the stator and rotor iron, as well as hysteresis losses in the stator and rotor iron.
The retardation test is a test that is used to determine the stray losses in a machine. The test is performed by applying a constant voltage to the field winding and then gradually reducing the speed of the machine. The power input to the machine is measured during the test, and the stray losses are calculated from the power input and the speed of the machine.
The other options are incorrect because:
- Eddy current losses are caused by the eddy currents that are induced in the stator and rotor iron by the leakage flux.
- Field copper losses are caused by the resistance of the field winding.
- Windage losses are caused by the friction between the air and the rotating parts of the machine.