Page 775 - Basic Electrical Engineering

The armature reaction will, therefore, have an effect on the magnitude of
the induced EMF. The more is the load current, the more will be the effect of
armature reaction. At no load, there is no effect of armature reaction.

When the synchronous generator is loaded, there will be a voltage drop in
the windings as well as an armature reaction effect. At unity power factor

load, the voltage drop due to loading will be less than at lagging power factor
load. For capacitive load, since the armature flux will aid the main field flux,

the air-gap flux will increase, and hence the EMF induced will go on
increasing as the capacitive loading increases.

10.11 SYNCHRONOUS IMPEDANCE AND VOLTAGE DROP DUE TO SYNCHRONOUS
IMPEDANCE

The armature winding, i.e., the stator winding of a synchronous machine has
a winding resistance of R Ω. When the machine is working as a generator
a
supplying some load, current will flow through the windings causing some
I R voltage drop. Some of the armature flux which does not cross the air gap
a a
is called the leakage flux. This leakage flux will lead to leakage reactance,
X , of the windings. There will be voltage drop due to leakage reactance of
1
the windings. Further, the change in terminal voltage due to the armature

reaction effect can also be viewed as a reactance voltage drop. This is a
fictitious reactance voltage drop. This reactance due to the armature flux is
called X . The reactance due to the armature leakage flux is called X . The
1
a
voltage drop due to resistance, R is in phase with the armature current, I .
a
a
The reactance voltage drops are in quadrature with the armature current, such
that

E = V + I R + j I (X + X )
a
l
a
a a
where, E is the induced EMF per phase at no-load, I is the armature current
a
flowing through each phase, R is the armature resistance per phase, X is the
1
a
leakage reactance of the armature winding due to the leakage flux, and X is a
a