It’s been my experience that an alternator will produce more current than it is rated for, sometimes much more.  So in the scenarios I’ve seen, it goes something like this:

 

1. Reg fails w/ full field
2. Charge current goes very high (pegged ammeter)
3. Battery voltage starts to increase, but it doesn’t usually ‘spike’.  Bus voltage increases over the course of a few minutes.  (The battery doesn’t really want to be at a higher voltage and therefore resists rapid increases in voltage – it acts like a gigantic capacitor.)
4. During this period the current is considerably higher than it should be, which should pop the breaker within 30 to 90 seconds.  This assumes, of course, that the alternator breaker is properly-sized.

 

Now during that 30-90 seconds the bus voltage would climb (as the battery gets charged like a son-of-gun).  On 12-volt systems, I’ve rarely seen it go above 16-18 volts in that time frame. (YMMV)

 

Most modern electronics will tolerate that level of over-voltage for that duration.

 

 

 

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Jeff Luckey
Sent: Tuesday, September 01, 2009 12:33 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: More Charging Circuit Info

What about the circuit breaker in the alternator output?  In the case of a full-field regulator failure, you simply open the circuit breaker (if it doesn’t do it on its own due to the fact that it’s charging current will go to max)

 

With the circuit breaker open nothing gets fried or boiled. (sounds like a cooking show;)

 

 

 

 

-------------- Original message from "Mike Wills" <rv-4mike@cox.net>: --------------



Hi Kelly,

 

 Thanks for the link. Some interesting info, but... What is lacking is any sort of statistical info to indicate that this is a real problem. I'm not convinced that using a Cherokee as an example system is all that applicable. And I dont know how old this article is, but gather it is pretty old given the reference to problems with alternator failures in cars resulting in nothing more than burning out lights. I'd venture to say that for virtually any car built since the mid 80s, a runaway alternator output would result in far more than a few burnt out lights.

 

 Since virtually all cars these days use internal solid state regulators, if these things were prone to failure, and since a runway over volt failure on any modern car would likely toast some pretty expensive electronics, I'd expect a fair number of really expensive repairs, warranty issues, etc.... My conclusion is that this type of failure is actually quite rare, but if you can point to relevant statistical data that says otherwise, I'm all ears.

 

 Some folks are more risk averse than others and a 1 in a million possibility is enough to justify making a change. But sooner or later you have to stop engineering for every possible risk no matter how remote the failure mode and just get on with it.

 

 Meanwhile, the important points I take from this reference are, dont overload your alternator, ensure that the field is de-energized while cranking the engine, and keep it cool.

 

 Off topic, another 1.2 hours closer to completing my Phase 1. This was the first flight that after I landed I didnt have anything on my list to fix before the next flight. 95 degrees OAT at takeoff today and the oil temp got to 200 at the top of the climb to 5000' but backed off to 192 after I leveled. H2O temp never above 180. Starting to gain some confidence in it. Oh, and I have a DNA muffler on order. Should have it by next weekend but may put off installing it so I can fly some more.

 

Mike Wills

RV-4 N144MW

----- Original Message -----

From: Kelly Troyer

To: Rotary motors in aircraft

Sent: Sunday, August 30, 2009 1:21 PM

Subject: [FlyRotary] FW: More Charging Circuit Info

 

  Group,

       I am still on the bandwagon about modifying our flying alternators for an external

regulator as noted in my past posts...........After supplying group member Tim Holt &

also the group with info how to modify the Mazda alternator I remembered the link

below which explains in language even the electrically challenged among us will be

able to understand...............This info may have been already seen by some of you

but I believe all of us should file it and also copy it to have available when wiring

our aircraft electrical systems.............IMHO!!..........

--
Kelly Troyer
"Dyke Delta"_13B ROTARY Engine
"RWS"_RD1C/EC2/EM2
"Mistral"_Backplate/Oil Manifold

 

-------------- Forwarded Message: --------------
From: "Kelly Troyer" <keltro@att.net>
To: tntholt23@bellsouth.net (Timothy Holt)
Subject: More Charging Circuit Info
Date: Sun, 30 Aug 2009 20:06:30 +0000

Tim,

 For your informaton here is additional justification in my opinion for going to an external

regulator in our aircraft and having the ability to either automaticly or manualy cut power

to the field windings in case of a shorted regulator............Note that solid state regulators

tend to fail "Shorted" about 50% of the time and all current internal regulators are solid

state.............Link Below..................."Nuff Said" !!...............IMHO

 

 

http://www.nflite.com/ChargingSystem.html

--
Kelly Troyer