Page 623 - Basic Electrical Engineering

The value of the critical field resistance can be determined by drawing a line
tangent to the OCC and finding its slope. The students should notice that the
OCC is initially linear, but later becomes somewhat horizontal. This shows

that with increase of field current, I the induced EMF increases linearly but
f
later the core saturates. Further increase of I does not give rise to much
f
increase in induced EMF.

7.5.2 Load Characteristics

The load characteristic is drawn between the voltage available across the
output terminals, V when the generator is loaded, against the load current I .
L
The equation relating E, V, and I for a dc generator is given as
a

V = E − I R (7.2)
a
a

Where R is the armature winding resistance and I is the current flowing
a
a
through the armature winding.
Thus, as the generator is loaded, the voltage available across the load will
be somewhat reduced due to I R drop. It may be noted that R is very small
a
a
a
and is of the order of less than 1 Ω. Therefore, I R drop although small will
a
a
reduce the terminal voltage with increase of load current or armature current.
In addition, some more amount of voltage drop will be due to the reduction of

the magnetic field strength when current flows through the armature winding
which is called armature current reaction or simply armature reaction. This is
explained as follows.

Effect of armature reaction

When the generator is loaded the armature current will produce a certain

amount of flux that would come into existance in the air gap. This would
create a demagnetization and cross-magnetization effect. The
demagnetization component of the armature flux will work in opposition to

the main field flux, thereby reducing the EMF induced, E and as a