The Armature reaction of
Alternator at different power factors
Armature reaction is meant that the effect of magnetic field set up by armature current on the distribution of flux under the main poles of a generator.
•As in d.c generator armature reaction is the effect of armature flux on the main flux.
•In case of alternators the p.f. of load has considerable effect on armature reaction.
When load power factor is unity.
•When load power factor is zero lagging.
•When load power factor is zero leading.
Synchronous reactance: For the same field excitation terminal voltage decreased from its no load value E0 to V (for a lagging p.f )
This is due to
Drop due to armature resistance, IRa
Drop due leakage reactance, IXL
Drop due to armature reactance , IXa
The leakage reactance XL and the armature reactance Xa may combined to give synchronous reactance Xs,
hence Xs = XL + Xa
Xa varies with p.f of the load, because armature reaction depends on load p.f
•Total voltage drop in an alternator under
load = IRa +
JIXs
I ( Ra + JXs ) =
IZs
where “Zs” is
known as synchronous impedance.
•
When
the synchronous reactance is combined with the armature reactance Xs and
armature resistance Ra, the quantity obtained is called
synchronous Impedance.
•
Zs = Ra +
JXs Ω/ phase
•
Ra is usually very small compare to Xs.
Therefore
Zs may assumed equal to the Xs for
many purposes.
Voltage
variation on load
As the load on alternator is varied, its
terminal voltage "V" also varies due to the following reasons
1)
Voltage drop due to armature resistance Ra
2)
Voltage drop due to armature reactance
XL
3)
Voltage drop due to armature reaction.
•We
know that terminal voltage decreases on load.
•Eo = no
load emf ,
that is also equal to maximum value of induced emf.
•E
= load induced emf,
•E
is vectorially less
than Eo / IXa
•V
= terminal voltage, it is vector ally less than Eo / IXs
•Z2 =
Ra2 + XL2
•I
= Armature current per phase
