LOSSES IN A DC MACHINE

 No-Load Rotational Losses

These losses are constant even as armature current varies. These losses are composed of the following losses:

1. Friction and Windage Losses: In a DC Machine friction is present between brushes and the commutator and due to bearings also there is friction. The ventilation losses are counted as Windage Losses.

2. Hysteresis and Eddy Current Losses: Similarly to Transformer in the field and armature core of the machine iron losses occur. Hysteresis Loss is proportional to speed of the machine and Eddy Current Loss is proportional to square of speed.


Variable Losses

The following losses depend on the current flowing in the machine and hence termed as variable losses.

1. Stray Load Losses: These losses are proportional to square of current flowing in the armature winding and occurs in iron parts of the machine. These are usually negligible.

2. Armature Copper Loss: The copper loss in armature winding of the machine is termed as Armature Copper Losses. It is equal to (Ia^2)Ra.

3. Field Copper Loss: In case of Shunt Machine as long as Terminal Voltage is constant these losses are treated as Constant Losses as they do not depend on Armature Current. But for series field winding these losses are taken as Variable Losses because in a series machine armature current flows through the field winding. These Losses are equal to (If^2)Rf.

4. Brush Contact Loss: Brushes are generally made of Carbon so they have negative temperature coefficient and hence as current through the brush increases the temperature of the brush rises and resistance goes down so the product of current and resistance that is Brush Voltage Drop remains constant. So, Brush Contact Loss is given by, VBDIa