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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
Homepage: http://www.flyrotary.com/
Archive:
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