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Lynn is exactly right on in his analysis of the
starter solenoid's behavior in limiting in-coming
current, as well as the large output voltage and
current the solenoid (coil) produces when the contacts
open.
While a capacitor will help to prevent the contacts
from arcing when opened another, prehaps a better
solution is a diode which is normally reversed biased.
When the switch is opened the collapsing field creates
a voltage of reverse polarity which will forward bias
the diode causing it to conduct. This creates an
effective short across the switch which becomes the
path of least resistance where the current will flow
harmlessly to ground. The aircraft ignition switches
from Aircraft Spruce come complete with just such a
diode along with instructions on how to install it.
Any power diode rated at greater than 50volts and
about 10 amps will do the trick. They are generally
available for $2-$3 from any electronics store.
By the way any solenoid in the airplane, including the
Master solenoid should have such a diode to protect
the switch that controls it. Otherwise you will find
yourself welding your switches closed.
Jim Maher
Dyke Delta 13B (almost done)
--- Original Message ---
From: Lehanover@aol.com
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Starter Problem
>In a message dated 6/18/2003 8:58:38 AM Eastern
Daylight Time,
>msteitle@mail.utexas.edu writes:
>
>> The referenced article states that current through
the starter switch
>> >will
>> >be somewhere between 25-35 amps. Most switches
I've seen are rated at
>> >10-15 amps. Pegasus Auto Racing, Inc. has a 40
amp push-button starter
>> >switch (p/n 4546) for $8.99, if that's what you
>> >prefer.
http://www.pegasusautoracing.com/pdfs/047.pdf
>
>The rather long and small diameter wire that makes up
the solenoid windings
>limit current flow in two ways. Plain resistance and
inductive reactance. The
>closing of the starter button applies power to this
coil. The current flow
>generates a flux field (that makes the armature move)
and the expanding flux field
>cuts through those same windings and in doing so,
generates a current flow
>that opposes the battery generated current flow. So,
that the current flow
>increases very slowly
>(in electrical terms) .
>
>There would seldom be any damage from latching up the
solenoid. However,
>since the solenoid is a coil of many turns with a
fully developed flux field
>expanded through it, when you release the starter
button, that flux field collapses
>and this solenoid coil acts exactly like the ignition
coil. Now the
>collapsing flux field re-cuts all of the coil
windings in a very short period, and
>starts an arc across the slowly (in electrical terms)
opening contacts of the
>starter button. Eventually the contacts will become
so damaged and dirty that
>current flow cannot be established.
>
>You can protect this set of contacts (points)? Just
as the points in an old
>car are protected. Just place a capacitor across the
contacts. You can add a
>high value resister to bleed down the capacitor if
you like. You don't need a
>high current switch at all. In my GTP Lola we used a
micro switch. The wires to
>the switch weighed as much as the switch. Or, if you
worry about this kind of
>thing, put in a 30 amp rated starter button.
>
>Any time you power up an inductive loop you should be
aware of the very high
>voltages and arcing that my occur when the circuit is
opened.
>
>Lynn E. Hanover
>
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