Runaway overvoltage Need help!

[rv7charlie]

Some of the newer cars are using the system computer to control the alt (and everything else in the car), but not really relevant.

An externally regulated alt would shut down with the master, because field voltage would be removed.

An internally regulated alternator *with proper overvoltage protection* would be *disconnected* with the master, because the OV protection relay would drop out with the master.

Neither would have been a problem with proper overvoltage protection. With a lead-acid battery, even manual OV protection would be quick enough to protect avionics (assuming the pilot is awake).

Charlie

[lr172]

Originally Posted by climberrn

Turning the master off would not have disconnected the alternator from the system.

Are you sure about this? I wired mine with alternator B lead (i.e. output) wired on the battery side of the main contactor. This was just logical for me and I didn't research much, but I thought this was standard practice. In my opinion the master should remove ALL power sources (batt, alt, etc.) from the main buss. If wired as I mentioned, it is not possible for the Alt to supply power to your buss.

If yours is truly wired as you describe, I suggest changing it. How else would you stop an electrical fire if you can't remove the power source, not to mention the situation you experienced.?

Larry

[longline]

There are two sources of power that might need to be removed from the system in an emergency. The battery is one, of course, and the "B" lead (output) from the alternator is the other.

The master switch allows you to shut off power applied to the Big Wire running to your starter contactor and system bus. This is the solution to the first issue.

Having positive control of the alternator, with a switch, in conjunction with an OV relay, contains the second issue. The perennial conversation, with regards to alternators, is if an "internally" regulated alternator is sufficient to protect the system in the event of an internal alternator short.

My research appears to indicate that there are two styles of internally regulated alternators. One essentially functions as if the regulator is external, while the other is self propagating once power is applied to "turn it on"... the second variety cannot be turned off by a switch. How can you tell the difference? Dunno, except by testing.

Connecting the B lead of an internally regulated alternator directly to the battery side of the Master contractor appears to be an invitation to a "Welding 101" demonstration, should the necessary failures occur. That you, or others, have not witnessed that demonstration in XXXXX hours does not give me comfort.

I am not convinced with your argument. Educate me.

[444TX]

I was going to write a long post, but have decided not to. There are so many problems with this failure scenario and the resulting chain of events that I could use ten pages and get flamed for a month.

Just one thought, did an internal alternator problem start the overvoltage event or the battery.

Internally regulated alternators, lithium batteries, dual electronic ignitions, efis installs with no backup gauges, no independent backup systems, electronic buses, etc. There are a many who stray from the norms and hide under the blanket of "experimental" without thinking of the consequences.

A guarantee Joel and others are thinking seriously about what happened here. He is lucky the engine stayed running. If IFR the outcome would likely have been bad.

George Meketa
RV8 , 1550+ hours

[climberrn]

We all build the best we know how. I followed the plans for 99+% of the build.

I had a PC-680 go out in less than 60 hours. Decided to try something different. In hindsight, that was not the best choice.

As for the alternator, most of the RV owners I know prefer the automotive based ones, so they can pick up a new one in town, for cheap. I was going to make the decision to switch to a better one at a later date. Alternators go out frequently with nothing more than an inconvenience. Both Dynons have backup batteries, my ignition doesn't rely on external power to run, and my plane is strictly VFR at this time.

As for the PMags, the main reason I chose those was the way they are powered. According to their documentation, they run on internal generated power above about 700 rpm. I wanted something that would keep running without outside power. They worked exactly as advertised.

All circuit protection for essential functions and all instruments are through certified circuit breakers.

Thankfully we have a choice to try new products with experimental aviation and sometimes there are setbacks. Not too long ago glass panels were looked down upon. Now we have STC's to install a D10 and soon a G5 in many certified planes. I am glad we have these options. I can't afford a Cirrus and don't want to pay a lot more than I do now to fly a 110Kt, 50 year old plane.

If I followed all the "advice" that is given in this community the plane would never be flying. I made the best decisions I could, even though others might have done it differently. Hindsight is 20:20.

George: I put this thread up to help others, and get some constructive ideas about how to prevent this from happening again. You might not agree with the way my plane is set up, but we all have a choice. Enough pointing fingers.