Master Solenoid

[VictorV]

I've been using the "Aircraft Wiring Guide" by Marc Ausman to learn more about electrical system design for my KF.
The basic power circuit he describes has the battery as input to the master solenoid and the input to the starter solenoid
comes from the output of the master solenoid. I'm guessing, then, that the master solenoid has to support the same
current capacity as the starter solenoid. It looks to me that most master solenoids have a much lower current
rating than starter solenoids.

For example, this starter solenoid can handle up to 300 amps continuous:

https://www.aircraftspruce.com/catal...clickkey=11134

but this master solenoid only handles 100 amps continuous:

https://www.aircraftspruce.com/catal...?clickkey=5553

Do I need to ensure that the master solenoid can handle the same current as the starter solenoid if I wire them in series?
Or do most people connect the starter solenoid directly to the battery?

Victor

[kitfoxjim]

The solenoids are not to be connected in series. Master solenoid activates the energizer circuit of the starter solenoid which connects battery directly to the starter.

[VictorV]

The solenoids are not to be connected in series. Master solenoid activates the energizer circuit of the starter solenoid which connects battery directly to the starter.

I see. Confusing though since they have diagrams like this:
Untitled.jpg
Victor

[Dave S]

Victor,

There may be differing opinions on this subject; however, I have seen wiring diagrams which show the master solenoid and starter solenoid in series for a KF 7 with a Rotax 912.

Maybe a good place to back up to is if you could provide some additional information which is absent from your original post. What engine do you have on the kitfox and what model kitfox is under consideration? This may be pertinent to clearing up any confusion.

The rotax starter solenoid does not look at all like the starter solenoid pictured from Aircraft spruce. When installing a new rotax engine, Rotax has always supplied the starter solenoid to use and the regulator for the integral alternator.

Another thing to keep in mind is intermittent vs continuous use. The Rotax 912 supplied starter solenoid is for intermittent use only, and is good for 300 amps; but, that is for no longer than 25% of a 4 minute cycle before it has to be allowed to cool down. I very much doubt that an average start ever hits 300 amps. Master solenoids are for continuous use.

FWIW, our S7 of skystar vintage showed the solenoids in series in the FWF manual. Again, if you have a different model Kitfox or a different engine things may be different.

The cut to the chase is the FWF manual for your particular engine install.

[VictorV]

The model is SS7 and the engine will be the 915is. I have the FWF kit but have not read the installation manual yet. The Rotax installation instructions recommend
a battery that can support 350 CCA so I am planning on the Earthx 900-series battery. The manual also has wiring diagrams but does not talk about the starter
wiring although I believe the starter and solenoid etc are included with the engine. There is a set of pins in one of the 915is connectors that will engage the starter
if you connect them together but none of the connectors appear to have pins heavy enough for the starter motor power so I assume it's connected directly from
the battery to the starter motor.

I have already read about peak vs continuous load on a solenoid.

Victor

[jrevens]

Victor,

Don’t connect the starter solenoid directly to the battery. If the solenoid fails or sticks closed you have no way to de-energize the runaway starter. Every factory built airplane I’ve ever seen runs the starter circuit through the master solenoid. The common master solenoids that most of us use are very capable of handling the intermittent starter load.

[Geek]

...........
For example, this starter solenoid can handle up to 300 amps continuous:

https://www.aircraftspruce.com/catal...clickkey=11134

but this master solenoid only handles 100 amps continuous:

https://www.aircraftspruce.com/catal...?clickkey=5553

Do I need to ensure that the master solenoid can handle the same current as the starter solenoid if I wire them in series?
Or do most people connect the starter solenoid directly to the battery?

Victor

Victor - Look again at your starter solenoid example. It is an intermittent solenoid and not a continuous one. Hence the higher rating in amps since it only has to do it for short durations. Agree with John Evens's statement on why you mount the starter solenoid after the master. You want to be able to shut that starter off if something should go south with the starter solenoid. Also note that 100amps continuous load is a lot of current and can't imagine any electrical load configuration for a Kitfox that would pull 100A total. Especially since the alternator is downstream of the solenoid.

The bright side of all this is that you are bringing up solid topics that some may not know or have thought of yet. Keep it up!!

Gary

[Delta Whisky]

BTW - a nice to have feature is a 12V LED wired to the motor side of the starter solenoid in your panel - to show that the starter solenoid really disconnected when you released the key/start button.

[rv9ralph]

Victor,
The Kitfox Aircraft FWF install manual for the 915is shows the circuit from the battery to the Master Solenoid to the Starter Solenoid. The master switch controls the master solenoid, the Rotax supplied starter solenoid is activated from the engine harness.

Ralph

[VictorV]

BTW - a nice to have feature is a 12V LED wired to the motor side of the starter solenoid in your panel - to show that the starter solenoid really disconnected when you released the key/start button.

Yes. I was planning on doing that.

Here's what threw me off regarding this issue. The 915is installation manual recommends a battery that can handle 350 Cold Cranking Amps (CCA) which is 350 amps for 30 seconds. It seems like
an awful lot but I have to go by what Rotax recommends.

Looking deeper into the specs for the 2 solenoids I referenced earlier, the master solenoid has the following specs:
- Max inrush is 600 amps
- Max continuous is 100 amps
- Max breaking current is 200 amps

350 CCA is for 30 seconds which is not continuous but not really short-duration either. I'm not exactly sure what "Breaking Current" means
but it's certainly less than the 350 CCA.

The 915is Operator's Manual states that you should not crank the engine for more than 10 seconds at a time with a 2-minute cool down
between attempts.

For those with nothing better to do I have attached an article by Bob Nuckolls where he discusses starter motor current and wire sizes. In that
he states that older starter motors draw about 60 amps and the newer ones draw around 50 amps. May not be totally accurate but probably not
too far off.

Victor