mcculleyja(at)starpower.n
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 Posted: Tue Jun 20, 2006 3:45 am Post subject: SD-8 PM Alternator |
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Bob N,
Here is some in-flight data I obtained while waiting for your comments
on my 5/7/06 response to your questions. The flight configuration is the
same as the ground test set-up with 15k resistor at the diodes and 3k
resistor across a capacitor of 56k mf. All RPM is that of the engine,
with the alternator turning at 1.3 times the engine RPM.
Engine RPM 600 800 900 1000 1100 1200 1800 2700
Capacitor Volts 5.98 8.47 10.4 12.61 13.31 14.4 14.42 14.42
Upon engine start with only the Master on for cranking and then
immediately turned off, the voltage across the capacitor rises very
slowly over several minutes to stabilize at the above listed values.
As soon as the voltage across the capacitor eventually rises above
0.601, the alternator will come alive if the alternator switch is
activated. If the alternator is not turned on the capacitor voltage
will track the above listed values as the engine RPM is varied up or
down, but there is a time lag dependent upon the size of the capacitor.
An interesting observation is that the initial very slow rise after
engine start to the 0.601 capacitor voltage then takes an immediate jump
from 0.601 to 5.98 volts (or any above listed voltage based on the
engine RPM at that moment) and thereafter never again falls below 0.601
volts unless the engine is shut down. Therefore, the alternator is
always available to come on line by activating the alternator switch
once the 0.601 condition has been reached and the engine has not been
shut down.
Tests were conducted using cockpit adjustable pots for both the
resistance at the diodes as well as across the capacitor. A capacitor of
only 1kmf was also tested. The diode resistor seems optimum at 10k to
15k with 3k across the capacitor. The 1kmf capacitor is too little to
prevent voltage excursions up to 16 volts and probably higher if not
carefully monitored. This may not be a problem since the alternator is
not on line at that time, but it might be if the alternator were to be
switched on during that condition, even though the regulator recovered
quickly in the one instance when I did switch it on at that point.
At this time, I am comfortable with the system when using the component
values as listed in the first paragraph above. Some lower value
capacitor could be used with good results I would think, but my guess is
it probably should be at least in the ball park of 10 to 20kmf,
My personal opinion at this time is that this set-up provides the SD-8
with an automatic capability to be brought on line at any time without
depending on a voltage source being present such as the battery or a
powered buss that the SD-8 is being switched onto. If the engine is
running or being motored above 600 RPM (780 alternator RPM) the pilot
can activate the alternator switch and bring power to even a dead buss
if he chooses. Also, for anyone wanting an alternator power source but
without a battery or starter, this will work if hand propping is the
planned mode.
Please review and comment when your time permits.
Jim McCulley
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