Home' Afloat : October 2013 Contents 58 AFLOAT.com.au October 2013
cool or warm white
• G4 back or sidepin: 8 SMD $15.00
10 SMD $20.00
15 SMD $25.00
• Festoon 37mm or 42mm
6 SMD $15.00
MR11- 6 SMD $15.00
MR16-15 SMD $25.00
• BAY15D Tower 18 SMD $35.00
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by Kurt Küpper*
The alternator on boat engines is in many
cases the primary or even only source
of charging for the boat's batteries, so it
is important to understand a bit about it.
Alternators, generators, electric
motors as well as items like solenoids make
use of the fact that a current flowing in a
wire generates a magnetic field around the
wire, and conversely, if a wire is moved in
a magnetic field, a current is generated
inside it. The more wires are run and/or
the greater the magnetic field, the greater
the effect. When a wire in a magnetic field
is moved towards the magnet, a current
flowing one way is induced into the wire.
When the wire is moved away from the
magnet, a current flowing the opposite
way is induced.
The other effect that comes
into play is that like magnets
attract and unlike magnets
repel each other. These are the basic
principles that are harnessed to create
movement from current (e.g. in a motor
or solenoid) and current from motion (e.g.
in an alternator).
Before the advent of alternators,
generators were used to charge batteries
on boats and cars. These generators were
just like electric motors, but instead of
using electricity to create rotary motion,
they converted spinning motion into
Coils of wire were placed around a
shaft to form what is called an armature.
A belt drive from the engine was then
used to spin the armature inside a
magnetic field created by placing fixed
wire windings in the outer case of the
generator. The strength of the magnetic
field was controlled by varying the current
applied to the outer windings, called the
In order to conduct the current
generated in the rotating armature to the
fixed outer case, the armature windings
were connected to slip rings on the
armature shaft. Carbon brushes fixed to
the outer case ran against these rings to
conduct the electrical power away to the
Because the windings of the armature
were alternately moving towards and then
away from the electro magnets placed
around them, the direction of the current
induced was continually alternating. The
slip rings were thus sectioned and the
sections connected to the windings with
opposing polarity (called a commutator).
The alternating current generated was thus
converted to direct current.
The voltage and current generated in
a generator depended on the strength of
the field, the number of armature windings
and the speed at which the armature was
rotating. The power generated was thus
fed to an external regulator to control
the voltage and current to desired levels.
These were usually mechanical devices.
The problems with the design of
the generator were that it was heavy
and that in order to get a higher output,
an ever increasing rotating mass and
increasingly large brushes to carry the
higher current were required. The breaks
in the commutator slip rings also led to
high brush wear. Because the output was
significantly dependant on speed, often
very little or no charging occurred at engine
The solution to the problem was to
reverse the functions of the fixed and
rotating components. Instead of having
the windings rotating in a fixed outer
magnetic field, it was decided to rotate a
lighter electromagnet (rotor) inside fixed
outer windings (stator). Now only much
smaller brushes were needed, as only
the relatively small field current had to be
conducted. It was also no longer necessary
to have the breaks in the slip rings, further
reducing brush wear.
However, what had been lost was
the rectifier (i.e. the commutator). This
was easily overcome by using a cheap-
to-produce, solid state diode rectifier
mounted on the outer case.
And so the alternator as we know it
today was born. The lower mass of the
rotor meant that it could be spun much
faster, so that even at engine idle speed
there was usable output.
There are usually at least three sets
of windings in the stator, and they are
offset against each other so that after
rectification the peaks and valleys of
the current generated is fairly constant.
Some alternators have a terminal that is
connected to an unrectified output to give
a signal for driving tachometers (labelled
'W' on German alternators).
Regulating alternator output will be discussed in the
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