Page 642 - Basic Electrical Engineering

the machine as shown in Fig. 7.24.
The various losses that take place in a dc machine are described as follows.

Figure 7.24 Shows the relationship between input, output, and losses

2
(a) I R loss in the armature winding

Due to current flow in the armature winding a good amount of power gets

lost as R , where I is the armature current and R is the resistance of the
a
a
a
armature circuit. As load on the machine changes, I also changes. Hence,
a
R is called the variable loss as this loss varies with the variation of load on
a
the motor.

(b) Core loss or iron loss in the armature

Iron loss or core loss consists of hysteresis loss and eddy current loss. The
core is made up of magnetic material and is subjected to variations in

magnetic flux. When the armature rotates it comes under North and South
poles alternately. Hysteresis loss occurs due to the alternate magnetization of

the magnetic material. Hysteresis loss depends upon the flux density, the
frequency of variation of flux, and the volume of the core material.
Eddy current loss is due to the presence of circulating current in the core

material. When the armature rotates in the magnetic field EMF is induced in
the armature core also. This EMF causes a circulating current i in the core
c
2
which is wasted as i r and produces heat. To reduce eddy current loss in the
c
c
core, the core is made up of varnished, laminated steel sheets instead of a
solid core. This causes increase of resistance, r through which the eddy
c
current flows. Eddy current loss depends upon flux density, frequency of