nuckolls.bob(at)aeroelect
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 Posted: Mon Sep 24, 2012 10:58 am Post subject: Misprint in Z-16, v.12/ OV with Rotax generators? |
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At 12:28 PM 9/24/2012, you wrote:
Hi Sacha
I might be a bit optimistic with that comment. My thoughts are that
when my OVM activates it immediately shorts out the relay but it is
still connected to the buss and is clamping the buss voltage until
the circuit breaker pops.
Actually, it's a healthy battery that "clamps
the bus". During OVM reaction time the alternator
goes into max v current limited output with the
ship's accessories taking part of the load
and the battery working to soak up the rest.
Once the OVM trips, the current draw on the bus
WILL go up to 50+ amps . . . so I suppose there's
some additional clamping going on. A 5A breaker
with typical OVM loads opens in 15 to 50 milliseconds
so it's all over in a blink of an eye.
So you are correct that with a slow opening relay and a fast acting
CB there could be a few milliseconds delay during which time the
voltage will be rising again.
Relay, cb and ovm timing 'races' don't figure into
system functionality at all. Your electro-whizzies
manufactured to DO-160 design goals will stand off
20 volts for 1 second. The battery will keep bus
volts down to 17 volts or so. This give the OVM and
associated components all the time in the world to
do their intended job.
Probably shouldn't mention it since I can't claim they are
significant but I also have a few TVS diodes on the buss. Since they
are fast acting and cheap they seemed like a good idea (several years
ago) with an electrically dependent engine.
They don't hurt a thing . . . don't do anything
useful either. The TVS is designed to mitigate
low energy, fast rise, high voltage transients
that do not exist on your ship's DC bus . . . and
again, devices built to DO-160 intent are designed
to stand off the classic 'transient spike test'.
Years ago I spent a fist full of the boss's money for
my man Cecil to build a test fixture required for
GENERATION of the DO-160 test spikes. Cool gadget.
When we got it finished and calibrated, I began to
explore vulnerability for my favorite bus power
conditioning circuits.
I was astounded to find that when the device under
test had a simple capacitor of 1uF or more across
the incoming line . . . Cecil's Evil Spike Thrower
couldn't put a spear past it.
From that time onward, the fixture was never again
pulled down from the shelf . . . 'cause all our products
already featured the necessary capacitor. Had I
endeavored to analyze the fact mathematically
(or simply brass-boarded the circuit) I could have
used Cecil's time more productively.
Short answer is that the much dreaded and poorly
understood 'spikes' are already being run to ground
by MULTIPLE input capacitors that already exist
in your system. I have chased the elusive 'spike'
in airplanes from C-150 to Beechjets and have
never been able to capture one in the wild. The
ones I generated on the bench turned out to be
wimps.
Bob . . .
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