Alternator cut off switch

I am designing my electrical system for my Kitfox SS7. I have a Rotax 912S and I am installing a master cut off relay for the battery. I have looked at several wiring diagrams others have used and see some people put an additional relay in to cut off the alternator power. On conventional aircraft they cut off the field wire to stop the output of that device so no power even exits in the cable coming out of it. On the 912 it installs a relay to stop the output to the battery. I am thinking this is really not doing much and is it really needed? Unless the relay is mounted right next to the regulator to eliminate the presence of a high amperage hot wire running in your engine compartment, what advantage does this really have? I was thinking of running the regulator directly to the battery which is already protected by the master cut off... Anyone have any thoughts on this and am I way off track? Your thoughts please... Ken

Ken,

My less than scientific guess tells me that most do not have a cutoff relay for the alternator.

Having some kind of output cutoff for the rotax integral alternator (since it does not have a field circuit option as installed) probably has some merit to the extent that an option is then available to cut out the alternator if it goes nuts and does an overvoltage or short out so you can still run off the main buss on the battery while the master battery relay is still on.

Our series 7 has a dual electrical system with the essential buss and battery running off the integral alternator and a separate aux 40 amp alternator run off a belt which also has it's own buss and battery for the high current draw stuff.

I do have alternator disconnects for both systems using a manual pull out circuit breaker for the integral alternator output to the buss; and, output and field pull out circuit breakers for the Aux (which does have the field circuit). Either alternator can be disconnected and a cross tie permits a person to run both sides off of either system with the offending alternator cut out.

Check the photo of the panel with the link below - the pull out circuit breakers can be identified as having black rather than white ends - the one for the integral alternator is on the left bottom of the panel immediately to the right of the three toggles. The output and field pull out breakers for the aux alternator are just below the map box on the co-pilot's side.

http://www.teamkitfox.com/Forums/alb...&pictureid=345

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

Nice looking panel Dave and it sounds like you are set up very nice with your electrical system. I agree with the ability to turn off the alternator (usually powering a primary bus) and switching over to an essential bus that is basically just going to get you on the ground ASAP using a direct battery line. I know I have beat this one to death here, but take a look at the Aero Electric Connection book sold on the web site by the same name. There is a very nice schematic laid out for several engines (Rotax included) that lay out this approach.
Best Wishes
Dan

I guess that for a cost of a a couple parts that I can't find a reason to not do it. Better to be safe than sorry! Nice looking panel! Ken

Check out this thread:

http://www.teamkitfox.com/Forums/showthread.php?t=3275

I looked in to installing a "crowbar". I talked to several Rotax mechanics and most do not use a crowbar when using the internal alternator. However, it totally would not hurt to add one. I opted not to since the problem I encountered was caused by a bad crimp. As soon as I tuned off the master, the load dropped and the wire quit melting. I thought about putting a switch in one of the field coil wires before the regulator but that just makes for more crimps and more places to cause resistance and burnt wires.

When I was building my first Model IV in the early to late 90s the Bob Nuckolls of Aeroelecrics connection was an active participant of the Kitfox List, as I suppose, he was active on other lists. At the time Skystar was supporting the NSI EA81 Subaru. Being a very persuasive guy, I put in circuitry for the essential buss, but never completed it, nor did I ever think about it in flight. The completion issue was the resistance needed across the rectifier in the event the switch was ever pulled to avid frying it - just never put it in. My understanding is that without the exciter circuit of an alternator, the risk of a runaway alternator was simply a non issue with the basic Rotax engine. If the charging circuit ever failed, it would just cut out exactly like opening a switch or pulling a breaker. Like my buddy would say - Crowbar behind a Rotax, "Heavy". My take on the whole subject is that there are apples and oranges - alternators and charging coils.
Lowell