>There are two more considerations in connection with solenoids that you
>should be aware of.
>1) If you have low battery voltage, like too much resistance in the line,
>or maybe one of the old-style lead-acid batteries, or do too much
>cranking, you can weld the contacts together.
Quite true . . . and it can happen to ANY style contactor.
>2) The commonly used solenoids (Wicks et al) have bolt heads inside for the
>contacts. If you twist the posts by tightening the electrical cables too
>tight on the outside, the bolt head on the inside can turn, so instead of
>the contacts being the flat of the bolthead, it is one of the points, and
>that greatly reduces the contact area, and could cause welding of the
>contacts.
Not sure about what Wicks is selling if it looks like:
http://www.aeroelectric.com/Catalog/switch/s701-1l.jpg
then IT IS important to hold the nut up next to the contactor
shell and keep it from turning while you tighten the wiring
nut. ANY rotation of the threaded stud will render the contactor
trash.
If the contactor looks like:
http://www.aeroelectric.com/Catalog/switch/s702-1l.jpg
Then the studs are captive in the contactors molded housing.
With this style, twisting the stud will break the housing
but at least you KNOW when you've cranked down on it too hard.
>B and C Specialties has a special contactor where the posts can't turn, and
>it avoids this problem. Benoit, would you like Bill Bainbridge to send you
>one of these which is garaunteed not to stick?
Don't know about "guaranteed" but they're really hard to stick.
However, starters with failed wiring inside or low battery
voltage can weld the best contactor . . .
> To amplify what I said in one of my earlier posts, the battery
>solenoid is energized when you turn on the master switch, and nothing else
>should be on at that time, so you aren't switching any current.
Most airplanes have several amps of things that are on all the
time but a battery contactor is generally rated to SWITCH 70
amps or better. Normal bus loads don't errode the battery
master contactor to any significant degree.
> . . . .But when
>you energize the starter solenoid, you could be switching as much as 600
>amps, which is the cranking power of the battery. That is why the starter
>circuit goes directly from the battery to the starter . . .
This is not recommended. All of our drawings and virtually every
certified airplane takes starter current through the battery master
contactor . . . if the starter contactor DOES stick, then the
battery master gives you a way to shut things down. While a battery
master contactor is rated to SWITCH 70 amps, it will nicely CARRY
the 200-250 amps common to starting an engine. I've had builders
try to use one of my S701-1 battery contactors as a starter contactor and
didn't get very good service life . . . however the battery contactor
was ALWAYS there to bring a potentially bad situation under control.
> . . . and not through the power bus and the 50 amp circuit breaker.
Not sure what breaker this is . . . some folks have a fat breaker
in their alternator b-lead output to the bus. By-in-large, there's
no practical application for fat breakers anywhere else in the system.
Bob . . .
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( Knowing about a thing is different from )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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