aerobubba(at)earthlink.ne
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 Posted: Wed Jun 04, 2008 5:50 am Post subject: AeroElectric-List Digest: 14 Msgs - 06/03/08 |
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glen matejcek
aerobubba(at)earthlink.net
| Quote: |
Time: 10:49:57 AM PST US
From: "Larry L. Tompkins, P.E." <tompkinsl(at)integra.net>
Subject: Re: SD-8 vs. SD-20 choice
Ernest:
It is my understanding that the SD-20 has both a wound stator and a
wound coil, i.e., no permanent magnets. Therefore it is "off" until
power to the field windings is switched on.
Let me try to do a better job asking my question. The SD-8 is
continuously producing output.
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That is a misleading statement. Under normal ops, the SD-8 is creating a
voltage, but no current. Much like a battery sitting on a shelf.
| Quote: | Does that mean it also has the
continuous potential to produce electrical noise?
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Not really. As I recall it, the regulator for this device is a 'switcher'.
It uses pulse width modulation to take whatever the output voltage of the
dynamo is and match it to the needs of the system. When the SD-8 is used
as a back up, it's regulator should be set to a voltage greater than
battery and lower than the main alternator's. In normal operations, this
arrangement causes the SD-8's regulator to sense a high system voltage
state, and responds by maintaining a constant 'off' state. No current, no
noise.
Once the system voltage sags to the SD-8 regulator's set point, it starts
switching the dynamo's output on and off as required to take up the slack.
This is a pretty noisy situation, and is why there is such a physically
large capacitor used as a filter on it's output. I occasionally fly a
plane that is equipped with a Com radio and an SD-8 as the only engine
driven source of electrons. I've never experienced any electrical noise in
the audio.
| Quote: | On the other hand,
the SD-20 is spinning continuously, but does not produce output unless
the field circuit is switched "on." What is the potential for the SD-20
to generate electrical noise when it is spinning and not producing
output?
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Nothing noticeable. No current, no noise.
| Quote: | My question about "wear" was exceptionally poorly phrased. I am
actually interested in knowing whether there is more mechanical wear
(bearings) because the SD-8 is "on" all the time vs. the SD-20, which
would only be "on" during a primary alternator failure (I suspect this
may have more to do with the relative mechanical robustness of the
devices than anything else).
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The SD-8 is not only not producing power when the main alternator is
functioning properly, but there are no brushes to wear. The only wear
items I am aware of are the seal in the oil pump pad, and the bearing in
the SD-8. If B&C has been true to their nature, the bearing in the SD-8
will be the last one under your cowling to fail.
| Quote: | Further, I was trying to understand if
mechanical wear on the SD-8 would increase its "noise signature" over
time
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No.
| Quote: | and if mechanical wear on the SD-20 would eventually result in an
electrical "noise signature" even with the alternator off.
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No
| Quote: | I hope I did
a better job of asking my questions this time.
Larry
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The one thing that I could foresee being a problem would be if one ran a
system wire, especially a sensor wire from a thermocouple, right next to
the SD-8 installation. The spinning magnetic field of the SD-8 could
impose a signal over the weak signal from an EGT (or other) sensor. The
twisted pair of wires on these sensors should help mitigate that situation.
Also, I don't recall ever hearing anyone mention such an issue in this
forum, so I suspect it's a pretty low order threat.
FWIW, and I hope this helps-
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