>Benoit
>
>There are two parts to a relay and these must not be confused.
>
>1) a coil - to activate a high current switch
>2) the high current switch
>
>The diode is used on the coil to stop the reverse current pulse, this only
happens when the coil is released and does not effect the switch part of
the relay.
>
>If the switch part of the relay cannot handle the current being drawn
>by the starter it will have a tendency to weld close or stick.
Not the whole story. It's true that contacts overheat
and degrade when loaded with too much current but 90%
of the time it's not an overcurrent issue. As I write
these words, I've got a test setup running on my workbench
to deduce the reason for sticking relays in the roll trim
system of a bizjet . . . current rating: 5A, motor draw: 1A
life of relay in service: 100-200 hrs and then they stick.
>The best option my be to get a relay with a higher current rating.
>Check the current rating of the starter and multiply this by 1.5 to 2
>(Safety factor), then look for a relay to handle this current.
I've had customers force me to drop a 5A relay in a slot
where I had tested and qualified a 2A relay to tens of thousands
of cycles over full temperature range for the system. My protests
went unheaded and the government blessed DER's signed off on the
change without further testing. They reasoned that if 2A was
"good", 5A was "better".
Within weeks of delivery of first units to the customer and before
the airplanes could be delivered to customers, 5A relays began to
stick . . .
Here's what started this thread . . . .
>>>> Benot LECOQ <lecoqben@club-internet.fr> 10/25/00 11:30AM >>>
> Hello from France
>
> I still get lots of fun flying around my new Cozy and everything is
> fine.
>
> Nevertherless there a few squawks that come up from time to time.The
> biggy
>for me is the starter relay that is sticking after releasing the start push
>button after engine start from time to time.This of course keeps the starter
>engaged with the engine running and you have to shut everything down to
>avoid any damage.Up to now, I had no real trouble with coping with that but
>it starts to be a real hassle.
> Here is what I tried:
> - Apply aeroelectric connection recommandations: big cable gauge and a
>diode to minimize the spark.
> - Change the relay a few time but after a while it starts again.
> The configuration is :
> - Lycoming O360 A3a,
> - Lightweight BC starter, wiring for the start part per Nat's
>recommandations including aeroelectric connection features,
> - 2 switches for the ignition system: one is grounding a magnetoand the
>other one switching on and off a Jeff rose electronic ignition.
> - A push button triggering the starter relay.
> I hear from a friend that the BC starter is pulling a lot more amps on
the
>battery than other starters and occasionnally triggers a big spark inside
>the relay that would lead quickly to a sticking relay.
> As I said I included a diode between the ground and the output of the
relay
>toward the starter (obviously in the inverted direction )to try to minimize
>the spark inside the relay. It looks like it does not play its role.
I don't have enough information yet.
Scenario 1: Is the sticking "relay" a starter contactor with fat wires
to carry starter current and light wiring to accept control voltage via
the start switch? What kind of contactor? Did you purchase it from either
B&C or from AeroElectric Connection? Is the system wired such that the
built in into the B&C starter is jumpered to the main terminal as-supplied
from B&C?
If the the "relay" being discussed is a starter contactor external
to the B&C starter then I suspect it's not a device purchased from
either Bill or myself and is not rated for the task. The contactors
we sell have been installed by the hundreds and I'm yet to be notified
of any sticking problems . . . although it CAN happen, we would not
expect it in less than hundreds of hours of service.
Scenario 2: Is the "relay" an auxiliary device used to take advantage
of the starter's built in contactor? Many folk have used some 30A
rated, plastic relays as an amplifier to buffer the built in starter
contactor's horrible inrush currents. See:
www.aeroelectric.com/articles/strtctr.pdf
Even tho these relays are rated sufficiently to withstand the high
inrush requirements, the load they switch is VERY inductive and has
a strong potential for burning the contacts of the aux control
relay. Going to a 70A rated relay would not solve the problem.
Missing or in-appropriately applied arc suppression is the
cause of problems.
Both problems are easy to fix. Benot, it would help if you could
fax me a sketch of your wiring so that I can accurately deduce
the problem and recommend a solution. Send it to 316.685.8617
and I'll publish the diagnosis and prescription on the lists.
Bob . . .
--------------------------------------------
( 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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