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From: Rick Morel (no email)
Date: Fri Nov 02 2001 - 07:57:03 EST
Wanna' bet, Colin? True, the starter motor lead has just been DISCONNECTED
from the battery, but, and a BIG but, there still is a period of a few
milliseconds when the solenoid contacts are arcing, dropping current and
collapsing the mag field in the starter field and armature coils. BTW, a
much smaller spike occurs at application of current to the starter. Stick an
ocilliscope, preferably a storage one, across the starting battery and crank
up. Move it to the starter terminal and repeat. Much longer and all, but how
much is enough? No 'scope? Grab the + and - posts of the battery and get
someone to start the engine. Make sure the EMT's are standing by in case.
Don't even try grabbing the starter + and ground and hitting the starter!!!
Yes, modern and even "old" (somehow I can't think of "old" gear with
microprocessors!) equipment have various protection schemes, and one can add
things such as Zeners, huge capacitors, etc. Actually the battery acts as a
pretty good capacitor as well. Thing is, all those protection components
generally protect by shunting to ground. Each time they do their job, they
run the risk of failure. If solid state devices, they also get "holes"
punched though here and there and will eventually fail. Like the ol'
lightning strike. Some gear will let out the smoke the factory puts in
immediately; some will appear okay and work for a few days or weeks, then
die. Add in a "gotcha!" of the starter solenoid contacts getting pitted and
cruddy and close to requiring replacement. Then you have all kinds of arcing
and sparking going on while cranking, resulting in surges and spikes and
high frequency energy. Chances are it'll have to get worse before you notice
a problem from the sound of the starter.
Real world is that each time you start with electronics on or even connected
with a lot of today's electronic power switches, you're probably doing at
least minute damage to the equipment. If you knew it would take 10 years for
the damage to add up to failure, that would be fine; but how is one to know?
Or maybe you just don't care. We ran into a guy that didn't. He was a
retired double-E, knew all about it, didn't want the bother, and said he'd
just buy new. Must be nice to have the $$$ for that attitude!!! Me cher', I
don' gots de' bucks to take any chances.
It's not a big deal to make switch-off a part of the starting routine; even
less of a deal to wire the engine direct to the starting battery. Of course
you have to remember to have the battery selector switch in the house
position when starting, or use a combiner for auto charging of "both".
Ever notice when you move the key on a car to "start", the radio and other
stuff goes off? That's the way automakers protect things, wiring all the
accessories through a terminal on the ignition switch that opens the circuit
in the "start" position.
Bottom line. My opinion, verified to my satisfaction by testing, is that
there will be voltage spikes and surges of several hundred Volts in the 12V
system during the engine startup process. At least some of the time. One can
take or not take whatever action one feels appropriate.
Rick Morel
At 10:55 PM 11/1/2001 -0500, "Colin Foster" <> wrote:
>What you are saying is true, Lee, you can get a tremendous voltage spike
>when the starter motor turns off. But the spike appears on the starter
>motor lead that has just been DISCONNECTED from the battery so the spike
>doesn't do any harm - the only thing it is connected to is the starter
>motor. None of that spike is injected into the 12 volt starting battery
>circuit. Spikes and arcing generated by the brushes will cause some noise
>but to get anywhere it has to pass the battery and the circuit to, and
>through, the battery has a very low resistance so any spikes that could
>otherwise be dangerous, are absorbed by the battery.
>
>Colin Foster,
>
>> There is a tremendous spike from the collapsing field from both starter
>> motor and solenoid, when it is disconnected. In fact, in a previous
>> discussion about an armed forces member killed by 12V (impossible), may
>have
>> been from this voltage spike. This is the same way that point type
>ignition
>> makes spark plugs spark, a collapsing field. Try holding your hand on a
>> starter solenoid (or take my word for it), You'll jump about 10 feet! In
>> addition there is likely a bunch of transient voltage spikes sent out
>while
>> the starter is running, arcing brushes look like dirty power to me.
>> Lee Haefele
--------------------
S/V Final Step
Port of Iberia, LA
http://www.morelr.com/coronado/
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