LOSSES IN A DC MACHINE

The losses that occur in a DC Machine is divided into five basic classes.

1. Copper losses
2. Iron losses
3. Mechanical losses
4. Brush losses
5. Stray load losses.

Copper losses:

· These losses occur due to currents flow in the armature and field windings of the machine.

· The cause of copper loss is the resistance of the various windings.

Copper losses is divided into three classes:

1. Armature Cu loss (I²_aR_a)

2. Shunt field Cu loss (I²_shR_sh)

3. Series field Cu loss (I²_seR_se)

Iron or core losses: 

· These losses occur in the armature of a d.c machine.

· These losses occur due to the rotation of armature in the magnetic field of the poles.

Iron or Core losses is divided into two classes:

           Hysteresis loss :

· This loss is due to the reversal of magnetism of the armature core.

· Consider a small piece ab of the armature.

· When positive half cycle current flow in a small piece ab is under N pole , the magnetic lines pass from a to b.

·  And after some time negative half cycle current is applied in the same piece of iron is under S pole and the magnetic line pass from b to a .

· Magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, therefore produces molecular friction, so produces heat.The heat causes armature resistances to increase.

Eddy current loss:

· According to Faraday’s law of electromagnetic induction, when an iron core rotates in the magnetic field, an emf is also induced in the core .

· Similarly, when armature rotates in the magnetic field, so an emf is induced in the core.

· These voltages produce circulating current in the armature core , these are called Eddy currents.

Brush losses:

· Power loss at the brush contact between the copper commutator and the carbon brushes.

· The magnitude of brush losses depends on the voltage drop at the contact point and on the armature current.

Mechanical Losses :

These losses are due to friction and windage loss.

1. Friction loss e.g., bearing friction , brush friction etc

2. Windage loss i.e., air friction of rotating armature. These losses depend upon the speed of the machine.

Stray Losses: 

· Stray losses is nothing more than the combination of Core losses and Mechanical losses.

· Stray losses are also known a rotational losses.