(3) Load Characteristics of Synchronous Generator:
While the exciting current and the speed remain constant, the terminal voltage changes with the load current in the armature and the relationship between the terminal voltage and load current of an alternator is known as its load characteristics.
When the armature current increases, the terminal voltage drops. This is mainly due to
(a) Resistance and reactance of armature winding, and
(b) Armature reaction.
The load characteristics of an alternator is shown in the figure.
Phasor diagram of synchronous generator under three types of leading conditions :
Simplified equivalent AC circuit (per phase) for synchronous generator:
AT POWER FACTOR LAGGING:
(A) When RA is very small:
α = torque angle
P = 3 VφE0 sin α/ Xs
Torque induced,
T ind = 3 VφE0 sin α/ Xs ωm
where ωm = speed.
(B) General case:
P = 3 E0/Zs [ E0cosθ – V (cosθ + α) ]
where cosθ = Ra / Zs
:. Small Ra implies θ = 90.
For maximum power output:
cosφ = E0/√ (E20 +V2φ)
α = 900
3 P max = (3 Vφ I max E0)/√ (E20 +V2 φ) = 3 Vφ E0 / Xs
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