nuckolls.bob(at)aeroelect
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 Posted: Sat Jan 04, 2014 4:53 am Post subject: Relays on Lear jets |
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At 12:55 PM 1/3/2014, you wrote:
Bob,
On an earlier post today, you mentioned a project where you energized
a relay before any current flowed thru it, and de-energized it after
current flow stopped.
Can you tell me how you did this and why. If the relay was not used
as a switch, then why was a relay used in this solution?
This was a design requirement unique to a system
that moves flight control surfaces with a motor.
This is common to trim systems, autopilots, flaps
and more recently fly-by-wire systems.
In the conduct of an analysis for failure mode
effects, any fault that causes anomalous or unintended
motion of a flight control surface needs to be
fitted with a means for rapid manual or perhaps
automatic shutdown.
In a flap system design for Eclipse some years
ago, we incorporated a crowbar system that opened
the motor supply breaker to effect a shutdown.
In the trim system for the Lear, a hard-contacts
relay was used in series with the motor power path
to the solid state electronics that controlled motor
speed and direction.
The fault monitoring system needed to watch
the both control and shut down components for
failure so it was necessary that the series relay
be exercised to demonstrate functionality. Obviously,
if the relay fails to close, the trim doesn't run.
But if it failed to open, then some means for
detecting and annunciating the event was called for.
The technique chosen in this application was to
have the disconnect relay operate every time the
pilot called for trim. But in the interest of
long relay life, it was advantageous to close
the relay milliseconds before the motor was
energized through solid state electronics. Similarly,
the motor was de-energized through solid state
switching milliseconds before the relay was
commanded open. Hence, the relay contacts were
preserved for carrying out a single, important
task . . . offer mechanical disconnect of trim
system power when commanded by the pilot's
Wheel Master Disconnect system.
By making the relay offer both closed and open
conditions for each trim event, the monitor system
could watch for and detect both failure to close
and failure to open events.
Also, since solid state relays are immune from the failure modes of
mechanical relays, what are the "typical" causes of failures in solid
state relays?
They short, they fail to close. They are no
different than their electro-mechanical counterparts
for how failure manifests . . . different
only in the physics that precipitates the failure.
Solid state relays are immune to the effects of
contact arcing that produces erosion and/or
sticking . . . but they can still be degraded
by inappropriate application and/or extra-ordinary
external stresses.
Thanks for all your efforts with the aeroelectric list, I'm not there
yet but pedaling as fast as I can.
You're welcome . . . I'm pleased that you find
value in the effort.
Bob . . .
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