The main purpose of exciter in a generator(alternator) is to provide stationary rotating magnetic field.Which is used to induce the e.m.f in the armature coil.
So, DC power is given to exciter and the exciter is nothing but a coil, and the exciter creates a magnetic field.
If a mechanical supply is given to the exciter then there will be a rotating magnetic field which cuts the stationary armature coils and induces emf in the stationary armature coil.
Exciter is a small generator, mounted on the same shaft as that of the main generator, which produces DC power for the main generator field winding.
Many variety of exciters are available, but the most popular today is brushless exciter.
In a brushless exciter, excitation power is generated by a small alternator whose armature is driven by the main generator shaft. The armature output leads are connected to a rectifier, which is also shaft mounted and rotating. The DC output of rectifier is fed to the rotating field winding of the main generator. As no slip rings or commutator or brushes are involved in this system, it is called brushless excitation system.
A generator must have relative motion between a magnetic field and a coil of wire. The magnetic field is produced either a permanent magnet or an electro-magnet (A current flowing through a coil of wire wound around a suitable magnetic core).
In large generators they use a small generator to provide the electrical current to feed into this coil to produce the magnetic field in the main generator rather than a permanent magnet.. This small generator is called an exciter generator. An old power house will have a couple of small generators in tandem to produce enough current to excite the main magnetic field of this generators.
Excitation current (and the voltage that drives it) is controlled by the generator?s voltage regulator, which is connected to the output terminals through instrument transformers and voltage meters that monitor the output voltage. When the output voltage shifts, the input from the meter changes the signal to the voltage regulator. The voltage regulator (whether automatic, electronic or manual) increases (or decreases) the voltage to the field windings installed in the rotor, either through a set slip rings or a brushless exciter (that uses even more field and armature windings and a rotating three phase rectifier, but I won?t get into that). The increased voltage at the field windings increases the current in the field coils, which makes the magnetic field stronger, increasing the output voltage of the generator ? which is again closely monitored by the voltage meters through instrument transformers.