Page 229 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

DC Motors- power stages

Losses in a dc motor

Losses in dc machines appear as heat.They raise the temperature of the machine. Losses also lower the efficiency
of the machine.
Losses include:

1 Copper losses.
2 Iron losses.
3 Mechanical losses
Copper losses
• Occur due to currents in the various windings.
• Armature copper loss = Ia Ra
2
• Field copper loss = I shRf / VI sh
2
• Also P = Series field copper loss = I se R se
2
Magnetic loss or core loss
• Occur in the armature core of a dc machine.

• Types: Hysteresis & Eddy current loss.
Hysteresis loss
Hysteresis Loss occurs in the armature of the dc machine. Magnetic field reversals occur as it passes under
successive poles. Small amount of power wasted in reversal of magnetism is called hysteresis loss. In other
words, due to magnetization & de-magnetization of core.
• P = K f V B max W/m 3
1.6
h
h
Eddy current loss
An emfis induced in armature conductors due to the rotation of armature. In addition, small amount of voltage
is also induced in the armature core. Thiscause the circulating current in the armature core, called eddy current.
Power loss due to this current is called eddy current loss. Eddy current can be reduced by making the core
resistance high. Core resistance can be increased by making the core of thin, round iron sheets called laminations.
Laminations are separated by varnish which has high resistance to avoid the flow of current from one lamination
to the other.
Pe = B max f t Watts
2
2 2
Mechanical losses
Friction loss e.g., bearing friction, brushes friction etc. Windage loss e.g., air friction of rotating parts.

216

CITS : Power - Electrician & Wireman - Lesson 30-37