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Posted for "Al Gietzen" <alventures@email.msn.com>:
My further thoughts regarding using a starter contactor:
First of all, reading the Bob Nuckoll's article makes me wonder. I have not
seen a car in the last 30 years that has not used a relay between the switch
on the dash and the starter solenoid circuit. So the issue of the high
initial current to the solenoid burning up the starter switch is ancient
history. The point is that the relay handles the current to the solenoid;
not the starter motor current. I planned the same for the plane; an
automotive relay rated at maybe 40 amps; activated by the push button switch
on the panel - which powers the starter solenoid, which closes it's heavy
duty contactor to switch the motor current; which can be very high. He doesn't mention another real issue; that is the collapse of the solenoid
coil field upon disconnect produces a pretty good arc at the contacts; so a
diode to ground to bleed off the back current is important to the longevity
of the relay. He doesn't give any reason for switching the starter motor
current with a starter contactor; other than B&C suggests doing it that way.
I did learn that the steady state current draw while the solenoid is engaged
is about 10 amps; so my initial question answered. That I appreciate.
I think in aircraft use it is primarily for safety reason. If the solenoid
should fail closed then the starter would remain engaged with the engine
running and cause damage. With a switch activate contactor you remove all
power to the starter thereby precluding that type of mishap.< Ed
Bob's approach is to tie the solenoid activation terminal to the starter
motor terminal. Power to the starter activates the solenoid, engaging the
starter gear, and closes the motor contacts allowing the starter to operate.
Then place another contactor in series with this that switches the combined
load of the solenoid and motor. This doubles the probability of a stuck
contactor causing the starter to fail to disengage. It may more than double
the failure probability because the return spring in the starter solenoid is
much more robust than the one in the contactor; so which is more likely to
fail closed? Maybe the concern is that the additional mechanical mechanism
in the solenoid may cause a failure to open?
I believe that the purpose of the starter contactor in Bob Nuckol's drawings is to isolate the starter cable from the battery.
If the starter cable were connected directly to the battery and it shorted to ground there would be no way to disconnect it.< Jim
This may have some validity, depending on the configuration. There is still
the battery contactor to disconnect it; and is it any more likely that the
short would occur between the battery contactor and the starter contactor,
than on the starter side. Nuckoll's also recommends placing the starter
contactor at the engine; and using the heavy starter lead for the alternator
charging circuit. For me; with an engine in back and the batteries up
front; it's the only logical way to go. So for me a short in the starter
circuit can only be broken by the master switch (battery contactor). Since
I have an 'engine critical' buss on the battery of the contactor I can still
fly the plane.
Unless I'm still missing something, my conclusion is use the contactor in
the starter solenoid, to switch the starter motor current, switch the
solenoid circuit with a 40 amp automotive relay in the engine compartment,
and have some nice little #22 wires running to my starter push button
switch. I noticed this morning that Paul Messinger came to the same
conclusion in his article in Contact!
Sorry to go on about this. The good thing is - I did get my question
answered! Thanks for your comments.
Best,
Al
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