Efficiency

The efficiency of any machine is the ratio of the output power to the input power. The input power to the generator is provided by the prime mover. Part of the input power is lost as heat and hence the efficiency is defined as 

The losses of dc machines may be classified as follows:

1.      Copper losses.  The ohmic losses (I²R losses) in the armature and field windings are known as copper losses.

Armature copper loss:             P_A = I_A² R_A

Field winding loss:                  P_F = I_F² R_F

where I_A = armature current, I_F = field current,  R_A = armature resistance and R_F = filed resistance.

2.      Brush losses: The brush drop loss is the power lost across the contact potential at the brushes of the machine. It is given by

P_BD = V_BD I_A where V_BD is the brush voltage drop.

3.      Iron losses.  As armature rotates in the magnetic field, the iron parts of the armature also cut the magnetic flux. Since iron is a conductor, the emf induced in the iron parts causes currents to flow which produces what is known as the eddy current losses. Laminating the core will reduce the eddy current losses. Another loss occurring in the iron part (armature) is due to hysteresis loss.

4.      Other rotational losses. (Mechanical losses). These are bearing friction, friction of the brushes and windage losses. Iron losses and other rotational losses are made very small by design

5.      Stray losses: Stray losses are losses which cannot be placed in one of the previous categories. No matter how carefully losses are accounted for, some always escape inclusion in one of the above categories. All such losses are lumped into stray losses.