nuckollsr(at)cox.net
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 Posted: Tue Jun 12, 2007 5:49 am Post subject: starter/battery contactor wiring and choices |
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| Quote: | Comments/Questions: Bob,
I would appreciate your making a statement summarizing the reasons for
running starter current through a "Battery Contactor". I have tried to
glean an answer to this question from your posts on this subject, but it
seems to be related to other considerations that I found confusing.
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| Quote: | I am planning to run starter current from battery through a Tyco LEV200
contactor that has a 500A continuous rating and a peak rating of 2500A.
The starter is a Jabiru 1500W motor. Jabiru has very little information on
its characteristics, so I am not ready to accept their statement that they
manufacture it. The battery is an Odyssey PC680.
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If you believe that you NEED that battery contactor
based on an assessment of its stellar RATINGS, know
that the battery contactor of choice in light aircraft
for nearly 70 years has been a $20 device with a 70A
rating. Contactors and switches are rated for their
ability to make and break certain kinds of loads at
various voltages. A battery contactor seldom needs to
make and break a significant load and then only once
per flight cycle. Hence, what appears to be an
"under-rated", el-cheeso contactor is a choice that
scores well on the cost-of-ownership study.
Now, if your focus is on running current, the Tyco
product has some appeal for its built in electronics
designed to reduce holding current to a fraction of
that needed to pull the contactor in. This too may
be a driving consideration in you selection of parts
but know that builders have reported that various
systems on their airplanes have be influenced by the
noise generated by the pull-in/hold controller. This
part has not been looked at in the lab to evaluate
suitability for use aboard aircraft.
The real ratings for Jabiru's starter are relatively
immaterial. Like all small starter motors, they'll
generate inrush currents that will scare your socks
off and draw running currents that are pretty spectacular
when compared with the demands of other accessories
aboard the airplane. These numbers can be largely
ignored when you understand that ALL starters do
this, all batteries have to DELIVER to this demand,
battery contactors have to CARRY the loads, and
starter contactors have to SWITCH the loads. This
is discussed in the chapters of AeroElectric Connection
on Batteries and Contactors.
The reason for putting the battery contactor in
series with the cranking circuit is simple. The
battery contactor (or battery switch) has a first
duty of being able to make 99.7% of the ship's
wiring go 'cold' when the master is off. This is why
the battery contactor is mounted right at the battery.
The second task is to back up the starter contactor
should the starter contactor "stick". The battery
contactor allows you to bring order to a starter
run-on situation that is pretty exciting to control
by any other means.
Unless you're really hard over on the current draw
thing, it's my best recommendation that you not depart
from those products suggested in the Z-figures
and other writings on aeroelectric.com website. It's
been my observation that reasons given by most
builders for breaking new ground are not well
researched or confirmed by repeatable experiment.
If it's your goal to become a good researcher, then
by all means, try anything you like. However, if
it's your goal to achieve first daylight under the
wheels with a minimum of time and risk to future
cost of ownership, then you're on solid ground by
sticking with parts and architectures proven in
the field on thousands of aircraft.
I'll recommend you join us on the AeroElectric-List
for still more comprehensive support of your questions.
Best yet, the answers can be shared with hundreds of
other builders.
Bob . . .
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