When you apply load to an AC alternator, it will respond with a drop in the voltage at the terminals. Conversely removal of some load will result in an increase in voltage at the generator terminals. The magnitude of the drop depends on key factors such as the design of the alternator, its component parts, its excitation system and its AVR.
When load is applied to the AC alternator, the increase in current causes a decrease in voltage in the main stator and generator terminals. The excitation system then reacts to try and return the voltage to the preset steady state.
Sensing the reduction in voltage via its connection to the alternator terminals, the AVR responds by increasing its excitation current to the exciter stator, which in turn increases the voltage at the generator terminals.
Unlike applying load to a fixed speed engine, where you will normally hear the engine react, the alternator voltage dip is normally only visible using measuring equipment such as a multi meter or by reading from a generator control panel.
In situations where the load applied is large, the voltage may drop significantly and the effect can be seen. Lighting connected to the load may flicker sensitive electronic equipment may turn off or fail.
Here is a graph of the voltage drop on a large 1000kVA Stamford Alternator running at 50Hz. This is the locked rotor starting curve from the HCI634J, which has a PMG excitation system.
The kVA you are trying to apply to the alternator in along the horizontal X axis and the resultant voltage drop is along the vertical Y axis. The 5curved lines represent the different voltages the alternator could be running at. So for this alternator, running at 400V, if we applied 1000kVA of load in a single step we would get a voltage drop of 16%. You can check this by finding 1000kVA on the horizontal axis, following it up to third curved line (which is the 400V line) and then reading this across to the vertical axis, where you should get 16%.
In this case, we were running at 400V, so we would take 16% from our 400V, which would leave us with 336V. This assumes that the engine speed hasn't fallen by more than 4%, then you should allow for an additional voltage drop.
As the engine speed falls, the frequency of the supply will also fall, as the alternator and engine are coupled together (either single bearing or twin bearing) they must both turn at the same speed. So for the 50Hz example above, if the engine speed drops from 1500RPM by 4% to 1440 RPM, the frequency will drop from 50Hz to 48Hz also.