nuckolls.bob(at)aeroelect
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 Posted: Sat Oct 22, 2022 2:04 pm Post subject: Contactor Failure |
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At 07:22 AM 10/22/2022, you wrote:
| Quote: | --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net>
user9253 wrote:
> If the battery contactor fails open during flight (unlikely), how will the pilot know it?
When the pilot pulls the throttle to idle, voltage drop from 14 might be noticed. |
This may not even happen with belt driven alternators.
They often support light loads at normal bus voltage
at taxi idle rpm.
| Quote: | | A further point is to review the failure modes and their effects. Looking at things that way may be enlightening. With the battery contactor failed open and wired per legacy designs, if the pilot (for whatever reason) attempts a reset of the alternator, the airplane goes dark and the alternator will not be re-energized. Why would a pilot do that? It is an instructional tool used by some instructors and some pilots check alternator function this way. Not saying it is right, but it is out there. Seems to be pretty low likelihood... |
agreed. but some alternators, particularly belt
driven, will self excite at cruise rpms or even
less. The Bonanza and Baron alternators will do
it . . . in fact, it's a requirement.
| Quote: | | With the designs we find dear to us from AeroElectric Connection, we have alternate methods of repowering the fields of our main and aux alternators. Z-12 has E-Bus Alternate Field switch, Z-14 has always hot buses, etc. Failure tolerant design changes the rare failed contactor from a full blown emergency into a ramp failure, and a simple repair. |
Speaking of contactor failures:
There are two primary causes for contactor
failure (1) loss of coil current which produces
the magnetic engagement force and (2) degradation
of switched power conduction path (worn out or
corroded contacts). Perhaps we could consider
a sub-set of (2), stuck or welded contacts.
Loss of contact integrity is the most common . . .
the thing just wears out or succumbs to environmental
stresses. One such example is seen here:
https://tinyurl.com/226jcjnl
We've seen examples of coil 'failures' . . . this
one was a manufacturing defect . . . failure to
solder the joints!
https://tinyurl.com/me767pxn
In general, failure to energize is rare . . .
electro-magnets just don't 'wear'. Most
failures are due to compromise of the
current carrying contacts and those will
start to get 'fussy' before they absolutely
quit.
Had a hangar renter on our airport that
bragged about getting his airplane started
by rapping the battery contactor with
a screwdriver.
Contactors are cheap. The cognizant owner/
operator will replace one the FIRST time
it gets fussy. A teardown of the old one
is always in order too. Worn contacts?
Signs of corrosion? Perhaps evidence of moisture
ingress? Was it mounted with proper orientation
(cap down) and a drain hole added to the cap?
A failure to energize in flight is more
likely to be in the contactor's control
path. Lose hardware, bad crimp, master switch
going south? Again, age and environment
are the biggest stresses on switchgear.
The ONLY switch I ever replaced on a piece
of heavy iron was in a Beechjet where a $high$
toggle switch that carried very low signal
current went 'open'. The tech I was working with
got out his pen to write up a requisition for
the $100 switch (two hours away in a warehouse).
I got a constant current bench supply out of
the crib, set it for 10A and connected the
offending switch to 'dead short' the power
supply. I wiggled the switch a few dozen times
against the corrosion burning power source
and put the switch back in the panel where it
would probably run another ten years.
If your project's low cost toggle switches
with fast-on tabs are getting long in the tooth . . .
the cost of replacing all the power management
switches is an hour's effort and fewer dollars
than a family trip to MacD's. That's why I've
favored them from day-one.
In any case, loss of a contactor or switch
in a well considered architecture (even while
airborne) is rare and no big deal.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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