10.9 Armature Reaction in Alternator
When an alternator is running at no-load, there will be no
current flowingthrough the armature winding. The flux produced in
the air-gap will be only dueto the rotor ampere-turns. When the
alternator is loaded, the three-phase currentswill produce a
totaling magnetic field in the air-gap. Consequently, the
air-gapflux is changed from the no-load condition.The effect of
armature flux on the flux produced by field ampere-turns (i.
e.,rotor ampere-turns) is called armature reaction.Two things are
worth noting about the armature reaction in an alternator.
First,the armature flux and the flux produced by rotor ampere-turns
rotate at the samespeed (synchronous speed) in the same direction
and, therefore, the two fluxesare fixed in space relative to each
other. Secondly, the modification of flux inthe air-gap due to
armature flux depends on the magnitude of stator current andon the
power factor of the load. It is the load power factor which
determineswhether the armature flux distorts, opposes or helps the
flux produced by rotorampere-turns. To illustrate this important
point, we shall consider the followingthree cases:(i) When load
p.f. is unity(ii) When load p.f. is zero lagging(iii) When load
p.f. is zero leading
(i) When load p.f. is unity
Fig. (10.10 (i)) shows an elementary alternator on no-load.
Since the armature ison open-circuit, there is no stator current
and the flux due to rotor current isdistributed symmetrically in
the air-gap as shown in Fig. (10.10 (i)). Since thedirection of the
rotor is assumed clockwise, the generated e.m.f. in phase R
1
R
2
isat its maximum and is towards the paper in the conductor R
1
and outwards inconductor R
2
. No armature flux is produced since no current flows in
thearmature winding.
