I read that if we increase the load at the terminals of a generator, the current driven from the generator will increase which will slow down the generator. But for this to happen the generator should output constant voltage. How does it achieved? And if there is no voltage regulation (just a generator and we increase the load step by step), what would happen?
I am going to assume you are referring to wound rotor Synchronous generators.
There are two main aspects to consider when you are dealing with such generators.
- Field control
- Damper Bars/Windings
To generate electricity the shaft obviously has to be rotated but there also needs to be some form of field on the rotor. This is provided by either brushes or a brushless (rotating diode) arrangement. This produces a DC excitation field on the rotor that can be controlled from the stators mechanical frame of reference (ie non-rotating)
What method is chosen is a system specific consideration but both provide means to control the rotor flux. Such a controlling system can be referred to as a Generator Control Unit (GCU)
A certain rotor field for a given rotor speed will produce a certain magnitude sinus waveforms at the stator terminals (the frequency of that is related to the speed of rotor rotation & the number of pole pairs on the stator).
The terminal voltage can thus be controlled via the strength of the excitation field. By providing a closed-loop controller that measures the terminal voltage w.r.t. a desired value, the terminal voltage can be maintained across the load range simply by varying the rotor's excitation field strength.
Damper bars/windings are a secondary means to help facilitate the terminal regulation. They are buried deep within the stator winding slots and essentially they aim to short-circuit the magnetic circuit in the event of high di/dt which may occur during load applying or shedding. This help to mitigate massive terminal over-voltage (load-shedding) or machine stalling (load applying) until the GCU control loop can "catch up"
An induction wind turbine generator has to be connected to a grid to generate power. (It could be self excited with a capacitor bank, but that's beyond the scope of this answer.)
Below a speed which would generate useful power, a control system cuts the turbine off so it doesn't run as a motor. Above that range it will run near synchronous speed with the slip being proportional to the torque produced by the wind. Higher electrical loads will increase the torque load on the blades causing some slowing. The grid usually will supply most of the load and buffers the effects of varying wind speed.
In an engine driven generator the mechanical engine speed is usually regulated so that loads that would ordinarily slow the machine down only do so for the transient case while the control system brings the motor back to nominal speed.
It's also possible to take the variable speed/voltage output of a non grid-connected synchronous machine (wind turbine for example), rectify it to DC and invert it to something that can be grid attached, or convert it to charge batteries.
Other schemes are possible, such as the one described by JonRB above.