The effect of a large load suddenly switched on to a small power installation such as a ships plant will be an instantaneous dip in the generator voltage. This effect, due to the transient reactance on starting, cannot be obviated either in a self regulated machine, or in a conventional generator with A.V.R.

The sluggish response of the excitation systems limits the speed of voltage recovery.

In a self excited generator the dip is less and the recovery time greatly improved. (say 0.3s against 0.7s)

In order to maintain constant voltage, under varying conditions, excitation must be varied.

Variation of voltage at constant excitation

Variation of excitation at constant voltage