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What happens if

RONSIM

RONSIM

Well Known Member
on a standard electrical system setup --- single alternator, battery and standard split master switch, if, for whatever reason, the main (battery) relay drops out. If the master/alternator switch stays "on", will the alternator continue to power the bus, provide field current, and keep all the electrical stuff going (I know that the battery would no longer be charging)and, if so, how does the voltage regulator sense needed voltage output?.

This is just one of those "sitting around the hangar with nothing else to think about" moments.

Ron
 
az_gila said:
If the Battery (Master) relay goes open, no power will be provided to the voltage regulator. The alternator will not put out a voltage.

Details here for the generic system you describe -

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z11M.pdf

Another reason for having a seperate back-up battery for any EFIS/EMS system - you can safely fly to a nearby airport and trouble shoot on the ground.
Click to expand...

In the schematic you show, if the battery contactor opens when the alternator is already energized and producing power, the voltage to the regulator will still come off the main bus which is being powered by the alternator...

I suspect that if there is significant electrical load from lights and avionics, this won't be a problem, but if there isn't any load, alternator voltage ripple may become uncontrollable causing it to drop out, and at that point there will no longer be any power to the regulator to bring the field voltage back up.
 
If you've got overvoltage protection (most of us do), then when the master drops out and battery is no longer there to smooth the supply/demand ripples, the regulator will "hunt" a bit and the voltage will swing. First time it hits the overvoltage trip point, the field will be shut off and that's it. It won't take long for that to happen, maybe a few seconds.
 
Thanks for the replies -

Pretty much confirms my suspicion that the master (battery) contactor is probably a "single-point" failure for the electrical system in the airplane.

Ron
 
There are several "single-point" failure items in our planes. Some of them you just have to accept and deal with.

I only have one prop, one crankshaft, one nose gear, one rudder, etc etc.
 
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