Marine Instruments User's Manual

Basic generatic alternator control system , Diagram 1
It is vital to understand in order to see what is going on and
to help in fault finding
Diagram 1 shows the basic circuit for alternators. the rotor (
as shown above ) rotates inside a stator ( the solid bit of the
alternator that you can see ) the rotor creates a magnetic
field which is then converted into electric by the stator and
sent to the batteries via the main positive output cable. In
order to control the output voltage we must control the
amount of magnetic flux being created by the rotor inside
the alternator. The example in diagram 1, simply has 14 volts
on one side and 0 volts on the other, this would result in the
rotor creating its maximum amount of magnetic flux, and
there fore charging the batteries at the max rate and will
adventualy destroy the batteries by over charging them.
carbon
brushes
copper slip rings
on rotor
0 volts
neg
14 volts
alternator
rotor
Diagram 1
alternator
stator
B+
main positive
output
B -
alternator case
In order to control this process then we
must introduce a regulator which looks at
the battery voltage and controls the rotors
voltage in order to reduce or increase the
alternators performance. The regulator
looks at the alternators output voltage and
controls the rotor field current to increase
or decrease the current of the alternator,
to maintain a constant output voltage.
Now this is where things get a little bit
difficult. With regard to controlling the
rotor current ( and as such the alternator
output voltage ) it does not matter if you
control the voltage on the way into the
rotor ( point A ) or on the way out of the
rotor at ( point B ) . from the alternators
point of view it does not matter which side
the rotor is controlled , either side is
equally effective,
If the regulator was in position A then it is
on the positive side of the rotor, and is
controlling the positive going into the
rotor, it is hence called a positive field
control alternator.
If the regulator was fitted into position B
then it would be controlling the voltage
coming out of the rotor on the negative
side, down to negative, this would be
referred to a negative field control
alternator.
As explained in the fitting instructions,
most European and Japanese alternators
are negative field control, where as most
American alternators are positive field
Basic alternator control: Diagram 2
however it matters a lot
to you when fault finding.
0 volts
neg
14 volts
Diagram 2
B+
main positive output
B -
alternator case
A
B
Understanding
alternator
field control
carbon
brushes
0 volts
neg
14 volts
Diagram 3
B+
main positive output
2 x wires fitted
by the installer as
per the fitting instructions
and brought outside
the alternator case
14 volts0 volts
Confirming negative or positive field control
Diagram 3
Because the regulator is usually built into the
back of the alternator, there is no way of
knowing if it is a neg or a positive field control
alternator from the outside. However if we were
to fit a wire on each brush and bring them
outside the alternator case, then we can check
the voltage at the brushes, and make some
deductions. In the above case there is no
regulator, so one brush will be 14 volts and the
other brush will be 0. In real life this would
never happen,
Diagram 4
B+
main positive output
0 volts
neg
14 volts
2-12 volts
0 volts
Positive field control alt
( most common in American alts )
A
s
0 volts
neg
14 volts
Diagram 5
B+
main positive output
14 volts
B
s
Negative field control alt
( most common in europe )
Confirming negative or positive field control
Diagram 3
Because the regulator is usually built into the back
of the alternator, there is no way of knowing if it is
a neg or a positive field control alternator from the
outside. However if we were to fit a wire on each
brush and bring them outside the alternator case,
then we can check the voltage at the brushes, and
make some deductions. In the above case there is no
regulator, so one brush will be 14 volts and the
other brush will be 0. In real life this would never
happen,