nuckolls.bob(at)aeroelect
Guest
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 Posted: Thu Jun 25, 2009 1:49 pm Post subject: Alternator N-Terminal |
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At 10:49 AM 6/25/2009, you wrote:
| Quote: |
<bob(at)rvbuildershotline.com>
Hi there. I'm posting this on behalf of a reader of a newsletter I
write. Any help you can give this gentleman would be appreciated:
"Have fitted a Bosch externally regulated alternator which has
terminals marked F, E, and N. I was advised that when the
alternator is giving the correct output there should be 7V
potertial at N which I have routed through a relay to extinguish an
LED. When the engine is ticking over at 1000 revs the LED is
flickering on and off but the battery shows 14 volts. Have I got the
principle wrong? what is the purpose of terminal N? "
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To understand the answer to your question we need to
go back about 42 years to when the first alternators
(Ford derivatives) were being bolted to airplanes
for the first time. The regulators were stock
automotive, 2-relay, electro-mechanical devices
shown in this figure . . .
http://www.aeroelectric.com/Pictures/Regulators/Legacy_EM_Regulator_(Ford).pdf
Terminal N is the center tap on a "Y-wound" stator.
Yes, it has an output nominally 1/2 that of the
B terminal. It was originally used to sense the fact
that the alternator was being turned by an engine and
was ready to produce power to the vehicle's systems.
A field control relay was powered from the N terminal
as shown in the attendant drawing. If the altenrator
control switch was open, field excitation through the
alternator fail light was zero. . . OR . . . if the
alternator was not turning, then N-terminal voltage
was zero.
If the alternator is turning and the control switch
is closed, then a small current (100 mA or so) places
a small excitation current on the alternator's field
terminal through the normally closed contacts of the
v-reg relay. This causes the stator windings to produce
voltage that is essentially unloaded (open circuit)
until high enough to begin to conduct the diodes and
deliver energy to the system. In this very lightly
loaded state, the N-terminal voltage rises and energizes
the field control relay.
When the field relay closes, full bus voltage is
available through a low impedance source and causes
the alternator to wake up and power the ship's
systems.
If the engine is stopped, or a belt breaks, N-terminal
volts goes to zero. Field control relay drops out
removing a short across the ALT WARN lamp causing it
to illuminate assuming that the control switch is
still closed.
This opens the high current loop for supplying
alternator field current thus stopping internal
generation of heat in the alternator and depleting
the battery with no-value-added load of field
current.
I don't recall that Cessna ever used the N-terminal
or field control relay for this purpose. We chose
to apply bus voltage directly to the regulator's
"S" terminal through the alternator control switch.
In later years, the OV control module was placed
in series with the field control relay as shown
in . . .
http://www.aeroelectric.com/Pictures/Schematics/Cessna_OVModule.gif
Years later in the employ of Electro-Mech, I
was designer of some rather crude voltage level
sensors that watched the N-terminal voltage with
the idea that it was good place to detect an
alternator failure. We supplied these modules
to Cessna and Beech. After working this industry
for several years, I discovered that there were
failure modes that would cripple the alternator's
ability to power the system that COULD NOT BE
DETECTED by watching the N-terminal.
I recommended and developed a low-voltage
warning module that was adopted by Beech. I
don't recall if we ever sold any to Cessna.
The short answer to your question: I recommend
that you ignore the N-terminal of your alternator
and install some form of active notification of
low voltage set up to warn for any condition
where bus voltage drops below 13.0 volts. This
is one of several solutions to the low voltage
warning task.
http://www.aeroelectric.com/DIY/LV_Warn_Fab_and_Install.pdf
Bob . . .
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