Synchronous Generator : Concepts -- 2 ( sai saikumar jn)

Synchronous Generator : Concepts

 

(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 R_A is very small:

α = torque angle
P = 3 V_φE₀ sin α/ X_s

Torque induced,
T _ind = 3 V_φE₀ sin α/ X_s ω_m

where ω_m = speed.

(B) General case:

P = 3 E₀/Zs [ E₀cosθ – V (cosθ + α) ]
where cosθ = R_a / Z_s

:. Small R_a 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 V_φ E₀ / X<sub>s

from ur's -- Bellapuri saikumar
                         ( sai saikumar jn)</sub>