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-   -   Wiring - Which Approach is better and why (https://vansairforce.net/community/showthread.php?t=135837)

rv7charlie 03-21-2016 11:21 AM

Quote:

Originally Posted by Bavafa (Post 1063972)
Mine is wired based on option 3. The added cost is about 4-5 feet of wire (in terms of weight) and one CB but the added protection was worth it for me.

What does it protect?

N941WR 03-21-2016 01:18 PM

Quote:

Originally Posted by RV6_flyer (Post 1063970)
...
One of my good very respected friends suggested that I look into not using a belt driven alternator and look at using one off the vacuum pad. Just passing on his idea to you. You may be able to use a lighter alternator that way and still have the output capacity needed.

I was thinking like you to have a backup alternator on my RV-8 project. I keep asking myself is it worth the extra cost and weight to have one? How often does a good B&C Alternator system fail? Part of me wants the backup but the cost, weight, and need for one keep saying that I do not need one.

....

Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

Low Pass 03-21-2016 01:36 PM

Quote:

Originally Posted by N941WR (Post 1064011)
Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

Another question one needs to ask themselves is, "Am I going to return home, cancel or alter my trip when one of my two alternators fail?" Having two gives you (roughly) twice the chance of experiencing a failure. So, do you then take a spare with you??

tectweaker 03-21-2016 01:42 PM

I now have both SD8 and plane power 60 A sitting in my hangar. Both of them were sourced from the local field.

I realized that I don't have a gear assembly on the back of the engine. It was capped off. Looks like PP might be the only option now. Otherwise; SD8 might be totally sufficient for my needs.

If anybody has a vacum gear assembly with housing by chance for o320A1A (Narrow Deck) then please let me know...

Carl Froehlich 03-21-2016 01:45 PM

Quote:

Originally Posted by N941WR (Post 1064011)
Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

Three things:
- Two batteries are more reliable than two alternators - if and only if you incorporate the one of the various simple power distribution schemes to have them independent from each other. Most two alternator schemes have at least one common failure point that would take out both units. The easiest way to do this is to have two identical batteries that combined provide the cranking amps you want. That way nothing is just along for the ride "in case".
- Dynon and others have backup batteries and such, but this does not support things like XPDRs, radios or electric boost pumps.
- Do not use a BNC 'T' to connect a handheld to an existing antenna connected to any other radio. You will be feeding RF energy into the panel radio - and a little bit can do a lot of damage. Use a BNC double female connector and first disconnect the antenna lead from the panel radio and then attach to your handheld.

On the original question, a 70 amp alternator breaker mounted on the firewall in between the alternator and the battery side of the starter solenoid protects from the unlikely but "real bad outcome" of discharging a battery to a failed alternator - or a broken off alternator lead that is shorted to engine ground. If you have a 60 amp alternator use a 70 amp breaker. If the breaker trips you will not be able to reset in flight - but if it does something bad has happened and you don't want to reset it. Two batteries like the PC-625 provide at least 3 hours of IFR flight.
http://www.aircraftspruce.com/catalo...clickkey=37667

Carl

Bavafa 03-21-2016 01:47 PM

Quote:

Originally Posted by rv7charlie (Post 1063986)
What does it protect?

protection and isolating the power from the alternator via a CB. I know many of the alternators have OV protection built-in, including my alternator but I have had experience in the past that the built-in OV protection did not kick-in quick enough. The extra cable was actually about 3 feet for me from the master solenoid to the panel where I have a CB and that is little cost in terms of $ or weight for the added protection.

rv7charlie 03-21-2016 02:42 PM

Hate to tell you, but a CB will not protect your equipment from an overvoltage event. All it will do is open if you have a catastrophic short to ground (with current supplied by the *battery*), or, if you rate it too close to the alternator capacity (which is almost always higher than its rating), it will 'nuisance trip' if the alternator tries to meet heavy loads while recharging a 'soggy' battery.

The only way you get overvoltage protection is with a dedicated overvoltage circuit that is designed to either open the alternator field, or open a contactor between the alternator and its load(s). If the alt goes overvoltage without dedicated protection, the voltage will go up until {voltage divided by the load resistance = alternator amps capacity} stops the voltage increase. This will take a looong time to happen in electronics time units, if at all.

edit: What I mean is that if the CB is rated higher than the alt's max capacity, as it should be to prevent nuisance trips, then it will *never trip* due to an overvoltage event, because the alternator 'self-limits' its output, by nature. It cannot produce more current than it can produce. :-) If its rating is, for example, 60 amps, then it will likely be able to produce 65-70 amps in the real world. And it will do that, if asked, at 14 volts, or if you have a regulator failure, at whatever voltage it can get to & still produce that maximum current. So, if you supply a CB that's safely above the alt's capacity to avoid nuisance trips, then current never rises high enough to trip it. The CB has basically no interest at all in voltage, until it exceeds the insulation capacity of components inside. Then you'll get internal arcing; not a trip.


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