We have one in our Glasair and I have never been really impressed
with it's output. Seems to run around 13.1 volts most of the time
except for
right after engine start. The ammeter does indicate a net flow of
current INTO the battery. That might seem all right but when I start
putting some
serious loads on the system (nav lights, landing light, etc) the
voltage
drops to 12.8 or so and the low voltage light starts to glow.
The way the voltage regulator is wired to the alternator is a
little "different" than conventional. Instead of the voltage sense
terminal of the regulator being connected directly to the B+ terminal
of the alternator and thereby also electrically to the aircraft bus
bar and battery, it is connected to a separate "reg" terminal of the
alternator, which is fed by a diode trio parallelling the three
positive-conducting diodes from the 3-phase stator going to the output
(B+) terminal of the alternator. These diodes have the effect of
isolating this regulator terminal from the aircraft bus. Why would you
want to isolate the regulator from the load that it is supposed to
regulate? The voltage at that terminal is also quite a bit higher
than at the B+ output terminal. It seems to be about 15.8 volts.
That is the voltage that the regulator reads and tries to hold.
A resistor does connect across from the B+
terminal of the alternator to the "reg" terminal, probably to bleed
some small voltage before the alternator starts spinning (it is about
1 volt) to the regulator to provide a small
amount of initial field current to bootstrap the alternator into
providing voltage when first starting. I notice that with the battery
master
turned on but the engine not running, the resistor gets hot. Probably
due to
the 10 volt or so drop across it. When the alternator gets running,
the drop
should only be a couple of volts and it should run cool, I imagine, no
way to
put my hand there while the engine is running.
But, why don't they just connect the voltage regulator to the B+
terminal and program it for 13.8-14.2 volts? Don't you want to sense
the voltage at the battery
and the aircraft bus, not some isolated output voltage of the
alternator? What if I just connected the regulator to the B+ ? I
tried it.
I disconnected the regulator from the regulator terminal at the
alternator and connected it directly to the battery.
Since the battery/bus will read much lower than the 15.8 volts the
regulator
expects to see, it will try to increase the field current until the
bus voltage
builds up to 15.8. At least, that is what I expected. I started up
the engine and briefly saw the bus voltage climb over 15 volts and
then the OverVoltage relay activated, disconnecting the field circuit
from
the alternator just like it is supposed to. So, you cannot connect
the regulator as is to the battery/bus and have it work properly,
since the
set point is set at around 15.8 volts or so it seems. Way to high for
normal operation.
Could I use some sort of voltage divider between the regulator
terminal at the alternator and the connection at the voltage regulator
to pad down the input to the regulator to fool it and get it to supply
enough field current to get that padded down voltage up to the 15.8
that it is programmed to do and thereby raise the battery/bus voltage
with it to around 14.2 volts from the 13.1 or so that it now is? If
so, then the divider could be adjustable and make the non-adjustable
InterAv regulator adjustable. Any thoughts about all this?