The Armature reaction of Alternator at different power factors

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

Ф = load power factor angle