on a Cessna Skycatcher, is state-of-the-art
for electrical power generation.
Alternator or Generator?
What’s best for your aircraft?
BY RICHARD KOEHLER
PERHAPS THE MOST DRAMATIC change in homebuilts over the past 50
years has been the increased use of sophisticated avionics, and all
these new systems need reliable electricity to function. Because of
the unreliability issue of dry vacuum pumps, some aircraft have all
critical systems for flight dependent on electricity. From GPS to attitude and heading reference systems to integrated engine indicating
systems, all you need a reliable source of power to work. In most
cases options for the generation of that power are either an engine-driven generator or alternator, or two alternators.
To understand the advantages of each it’s important to know
how a generator and alternator work. Electricity is generated by
moving a conductor across a magnetic field, or vice versa. The
electrical current is induced in the conductor (wire). The strength of
the output is proportional to the strength of the magnetic field,
length of the conductor, and speed of the relative motion between
them. On engine-driven generators and alternators, the speed of the
relative movement is changed by engine-speed changes, so the only
way we can control the system is by varying the strength of the
magnetic field.
HOW A GENERATOR WORKS
Looking first at the generator, the outer case contains windings for
one or more electromagnets. Their magnetic fields are regulated by
the amount of electricity allowed to flow through them. Spinning
inside the generator is one or more windings of wire (the
conductor). As these coils of wire pass through the magnetic field,
electricity is generated. Unfortunately, the electricity is alternating
current, and we want direct current. Also, the spinning armature
cannot have a wire connected directly to it due to the rotation. Both
of these problems are solved by use of a
segmented ring (commutator) on the
armature and carbon brushes that are
mounted on the case but ride on the
commutator. The commutator segments are
connected to the armature coils of wire in
just the right way to pick off only the
positive pulses of the alternating current,
and by using several coils, these can be
blended together to provide a fairly smooth
direct current output. The commutator
“rectifies” the alternating current to direct
current and removes it from the rotating
armature. Obviously, the carbon brushes are
critical to this operation and require regular
maintenance. So, for the generator, the
variable magnetic field is stationary, and the
conductor is moved within it.
HOW AN ALTERNATOR WORKS
An alternator is kind of an inside-out generator. The outer fixed case contains the
conductor windings, and the magnetic field
is rotated on the inside on the “rotor.” Again,
the magnetic field is controlled or regulated
to vary the output, and that must be done
through brushes and slip rings, but the rings
are not segmented (they are smooth), and
the current is much lower, so much smaller
