If they're used at all, it's not on account of their efficiency.
The classic "DC Generator" generates AC internally which is
electromechanically converted into DC by the brush/commutator assembly
which delivers the DC output.
The carbon brushes typically account for about a 1 to 2 volt drop per
brush (accounting for a 2 to 4 watt loss per amp of output) as well as
an extra drag. The rotating armature is effectively one big solenoid
made up of several seperate solenoid windings on each pole of the
armature wired in series and forming a closed loop with each joint made
via a commutator segment.
A simple DC generator using permanent magnets for the stationary field
would generate a no load voltage in proportion to its speed. This same
electrodynamic rule applies to a DC Motor where its no load speed is in
proportion to the applied voltage (ignoring windage and other losses).
The classic automotive DC generator uses a field winding instead of
permanent magnets and is fed a varying current from the voltage
regulator so as to maintain a reasonably constant voltage over the
normal RPM range provided by the engine.
One of the main problems with the classic DC generator design is the
limited strengh of the armature assembly limiting its maximum RPM rating
which, in turn, decides the gearing ratio that can be used for any given
engine it is coupled to. In most cases, this means the output at
tickover or idle rpm is either very low or so low as to not cut in at
all, meaning very low or no charging current until the engine rpm
increases when actually propelling the vehicle along the road.
The advent of the silicon rectifier diode capable of handling 30 or
more amps allowed the generator designers to divorce the mechanically
bound commutation function from the generator and replace it with an
electronic commutation circuit in the form of a rectifier pack of
The electrical losses in such a diode pack were usually lower compared
to the electro mechanical commutator/brush system (about 2 volts versus
3 or 4) and the considerable mechanical drag of such brush gear was
replaced by the much lower drag of the field brushes and slip rings
arrangement in the modern alternator.
The benefit of this development (as far as the automotive DC generator
was concerned) was that the armature could be replaced by a much
stronger field rotor assembly and the output taken from windings on a
laminated stator assembly.
Effectively the static field/rotating armature coils arrangement had
been replaced by a rotating field/fixed coils one. Since the need to to
use a mild steel based magnetic circuit made up of laminations to reduce
eddy current losses in the rotating part had disappeared, a much
stronger two piece rotor assembly made up from solid steel could be used
enclosing a single solenoid winding wound around the axis of spin, a
winding arrangement with its axis of maximum strengh aligned with the
axis of maximum force.
The ends of this rotating field coil were connected to two slip rings
which were also much stronger than a segmented commutator and only
needed to carry a maximum of 4 to 6 amps (requiring smaller brushes with
lighter spring pressure which, in the absence of segmentation on the
slip rings, meant reduced drag losses) versus a 20 amps and upwards
load current in the classic DC generator design.
All in all, the AC generator/DC rectifier pack offered a whole host of
improvements, not the least of which was the much greater operating
speed range which meant they could be geared up to produce useful output
even at tickover or idle speeds.
The main problem with using either of these types of generator for wind
turbines is the power consumed by the field current which will be at its
maximum at minimum useful rpm. The field current, for a given generator
load, decreases with increase of rpm above this minimum speed.
The permanent magnet generator/alternator designs remove this
'auxilliary' demand, but require an alternative means of voltage
regulation. In this case, a simple analogue voltage regulator designed
to handle the output current and maximum generator voltage would be a
more ideal solution even if you do end up wasting the unused voltage as
an energy loss perhaps even greater than the useful energy output at
maximum operational wind speeds
An alternative form of electronic voltage regulator for a permanent
magnet type of generator is the switching type of regulator. This
reduces current demand as the generator voltage rises above requirement
for a given loading which translates to less drag on the generator's
prime mover, essentially providing a similar mechanical loading
characteristic to the standard field regulated automotive alternator
(but the system needs to withstand much higher voltages in the generator
windings, rectifier pack and the switching regulator).
Each of the two methods (analogue or switching) have their own merits.
Which is best depends on your needs and how much electrical storage
capacity you have available.
 Even the minimal losses of slip rings could be eliminated, if
needed, by using a high frequency magnetic coupling technique as used by
cordless shaver chargers and the like. However, since the slip ring
losses are so slight in this case, the technique is only likely to be
applied to very large generators where issues other than the mere 1% or
less overall efficiency improvement are being addressed (such as
increasing uptime between servicing intervals).
 Such 'waste energy' could be used to supplement heat energy demands
elsewhere (such as water heating), especially useful during the cold
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