Question

Why speed of alternator is required to be constant irespective of load?

Answer 1

Explanation:

So when the shaft is loaded(load torque increased), the speed reduces and the air gap flux is a bit more in phase with field flux. Since the rotor has to move at constant speed, the air gap flux has to go back to initial position(leading field flux),

Answer 2

ℏ✺ℓᾰ ժḙᾰԻ

The speed of rotor must be equal to stator magnetic field( meaning it is impossible for the frequency to drop ) the increase in load is compensated by increase in rotor angle (Delta).