Generators may be divided into two groups: dynamos (direct-current generators) and alternators (alternate-current generators).
Dynamos
Their scheme of work is based on a rotating turn (rotor) in a magnetic
field (generated by permanent magnets that compose the stator).
Movement can only be obtained through an external, mechanical energy source
(e.g., a water turbine).
A positive voltage V=F(t) can be measured (at the ends of two immobile
brushes) by commutator segments (two separate conductors that are
integrated with the turn itself and that offer the possibility of sliding
contact).
By increasing the number of turns and closing the circuit with a utilizer,
a voltage V (and hence also a current I) can be obtained which becomes
increasingly uniform to the point that it may be considered continuous over
time.
It possible to use windings, in place of the permanent magnets, which
create a magnetic field when crossed by direct current (according to the
excitation, and supplied by external batteries or the machine itself).
The operation of generators in direct current is defined by characteristic
curves of the type V=f(I).
The dynamo is reversible and thus can also function as an engine (with
current I sent to the rotor).
Alternators
Alternators are similar to dynamos, but the, difference between them
consists of the fact that the ends of the turn are connected to two sliding
slip rings on two immobile brushes.
The same result can be obtained by making the magnetic field rotate and by
keeping the coil immobile. In this case, it is possible to avoid sliding
contacts.
The model described generates single-phase currents.
Multiphase currents (normally three-phase) are obtained with the use of
several coild and pairs of poles.
Alternators are also characterized by V=f(I) curves.
Generator efficiency
Efficiencies greater than 0.95 can be obtained.
In practice, performance is connected with machine size; with small,
medium and large capacities, μ is ≃0.70–0.80, 0.80–0.95 and 0.95–0.98,
respectively.
