| Hi Finn
Really appreciate your reflections and design ideas. I had graded the risk of my smallish ~45amp alternator going HV as low, and had ‘assumed' that in such an event the 40Ah battery bank and ~20amp system load would ballast the system voltage for some time (minutes) before a critical engine failure was imminent. My engine monitor will let me know ‘audibly’ as soon as the system voltage exceeds 15 volts.
‘Assumptions’ right! I hadn’t got real confirmation of the real voltage/current over time in such an event.
The truth is I have always struggled to get a handle on how a three wire automotive alternator works electrically. The 'Sense', I thought was simply the feedback for the regulator which I have from memory via a 5 amp fuse to the engine buss. But what is this ‘lamp/field’ wire?? In mine it’s not connected, and again I ‘assumed’ that as my idle is ~1900 rpm this was spinning enough to excite the system and self sustain the magnetic field at that point. Perhaps I ought diode feed this from the Engine Buss?
I understood that because of the above characteristic once these alternators were generating power you couldn’t turn them off by de energising a ‘field’ etc. wire and therefore your plan to disconnect its output/ B terminal is required.
I’m thinking that in your circuit, when your Over Voltage relay is de-energised by the OVP popping the CB, like disconnecting an alternator from its battery, (at high RPM, not simply idle) might damage the alternator diodes/regulator though this is an acceptable outcome if it keeps the engine running in a critical situation.
What I would like to know in order to make an realistic risk assessment is: 1. How these alternators actually work electrically 2. What a High Voltage on the B lead will do over time. A. In the immediate mSec of an event and B Over the next 5 minutes
How is your RV4? I hope you can get back in the air soon.
Thanks again for the reflections.
Steve Izett
On 3 Jan 2025, at 11:35 am, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:
And this is the way I should have done it! The error I made (powering the overvoltage relay and
alternator field via the 5A breaker that is pulled to ground when
OVP unit triggers) is what caused total power loss when battery
failed open. With an internally regulated alternator it needs to power itself
after startup even when disconnected from battery. Because in
case of a OVP nuisance trip, the alternator will continue
producing power. The 33,000uF (or bigger) cap on the B+ terminal
should ensure sufficiently clean (low ripple and noise) to power
itself and the engine electronics is case of no battery
(disconnected or failed open). (Size it for your engine bus
current draw, desired alternator RPM range and maximum ripple you
can tolerate. C = I / (RPM/60*3*2*Vpp) --- three-phase
bridge-rectified alternator. I my case 10 amps, 3,000 RPM and 1V
peak-peak ripple ~ 0.033 or 33,000uF.
You should probably have a way to monitor the voltage on the B+
terminal to ensure the OVP didn't trip on a real alternator
runaway, before trying to reset the 5A breaker. (BTW, a 1A breaker
should suffice -- it's only powering the overvoltage relay). Now I'm undecided whether to add a backup battery or not.
Finn
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