The losses in a d.c. machine (generator or motor) may be divided into three types

1. Copper losses: These losses occur due to currents in the various windings of the machine.

2. Iron or core losses: These losses occur in the armature of a d.c. machine and are due to the rotation of armature in the magnetic field of the poles. They are of two types

a. Hysteresis loss: Hysteresis loss occurs in the armature of the d.c. machine since any given part of the armature is subjected to magnetic field reversals as it passes under successive poles.

b. Eddy current loss: The voltages induced in the armature conductors produce circulating currents in the armature core known as eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency.

3. Mechanical losses: These losses are due to friction and windage. These losses depend upon the speed of the machine. But for a given speed, they are practically constant.