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Automatic Load Shedding

Tall_Order

Active Member
Patron
Instead of “rolling my own” circuit for this, has anyone seen an off-the-shelf module that would automatically energize/de-energize an electrical bus based on excess supply capacity?

(Considering a dedicated bus for items like heated seats, etc...)
 
I had thought of something like this. It might be simpler than you think. If all non essential shed loads were on a separate bus like you mention, it could be fed by a solid state relay. One could then make a simple circuit that only enables the relay above say 13.5 volts. If you lost the alternator, voltage goes to 12, the bus drops out.
 
A different way to look at it

Modern avionics and LED lights have really reduced normal load. I have a max'd out panel (dual G3X, GTN 750, dual comm's, autopilot, audio panel, TX & a G5). My normal amp draw with No lights is 11 amps.

My old Whelen strobes drew about 5 amps, my new LED strobes and nav lights draw 1 amp. I rarely fly a night, so I haven't replaced my landing lights but I'm sure new LED lights don't draw much current. The only other big current draw item I have is pitot heat.

So if I wanted to shed load, the first thing I would want to do is make sure my lights are off and pitot heat is off. After that, I could pop the breaker for 1 G3X screen and the 2nd comm - there is not much else I would want to turn off.

I have an 8 amp backup alternator which I have never used in over 1,000 flight hour. I run a B&C 40 amp alternator which has worked flawlessly for over a 1,000 hours.

So put in a B&C alternator and take good care of your battery (I replace my PC 680 every 2 or 3 years) and you may never get into a situation where you have to shed electrical load.
 
I have a separate Endurance Bus for my fallback configuration in case of failure of my primary alternator. I have to manually shutdown my main bus and primary alternator if I decide to go into the mode where I just have my Endurance Bus operating off of my standby alternator. I don't think I would want this to be automatically switched. Some loads on my main bus are lights and etc. that I can decide to shed separately before having to result to turning off the main bus for example. In an automatic switchover you don't have any options on what you keep running or for how long. As Rick mentioned some of the loads for current devices are very low. Other loads are either selectable like lights or possibly not used like transmitting on your com. For me my COM1 is on my Endurance Bus, but lights are on the Main Bus so once I make the switch over I'll be running in a limited, but no so limited configuration.
 
As someone who used to operate some very complicated aerospace vehicles, I can tell you that automatic load sheds can be very frustrating - especially when they happen due to a failed transducer, or if the conditions for a load shed are met unexpectedly and it isn’t really necessary. truthfully, unless you are needing to shed many tens (or hundreds) of individually switched loads, my feeling is that it is more trouble than its worth.

In our simple airplanes (even the most complex RV is still fairly simple), it just isn’t that hard to turn off un-needed loads if you feel that you have a supply side failure. Turn off the pitot heat, seat heaters and lights, an extra radio or screen, and you’re done. Takes longer to write than to do.

Paul
 
Copy bigger airplanes with decent load shedding designs.

Beechcraft has a triple fed bus system. In essence there's 3 primary busses. A left main bus, right main bus and essential bus. Each main bus carries less important loads. Each main bus and the battery feeds the essential bus through diodes. Loose a generator (alternator) and the associated main bus goes to zero, shedding all those loads. The essential bus continues to be powered by the operating main bus and battery. Loose both generators (alternators), a bunch more stuff gets shed and the essential bus is powered by the battery alone.

In our planes we could put critical avionics, EI/EFI on the essential bus. Seat heat, pitot heat, secondary radios and backup avionics go on a main bus.

Would be a bit trickier with alternators because they need a exciter field. Generstors can turn on and off without assistance.
 
Loose both generators (alternators), a bunch more stuff gets shed and the essential bus is powered by the battery alone.

I understand the concept - and I understand alternators sometimes fail (I've had it happen) - but I'm very curious how many people here can say they've seen a dual alternator failure in a single flight. Two completely separate mechanical/electrical alternator failures - anyone?

I know there was the Austro-powered twin that went in a while back from the drained battery that gave it up when the gear was retracted - that wasn't the alternators though, not for the root cause.

Anyone?
 
I understand the concept - and I understand alternators sometimes fail (I've had it happen) - but I'm very curious how many people here can say they've seen a dual alternator failure in a single flight. Two completely separate mechanical/electrical alternator failures - anyone?

I know there was the Austro-powered twin that went in a while back from the drained battery that gave it up when the gear was retracted - that wasn't the alternators though, not for the root cause.

Anyone?

The continued singular focus on a failed alternator as the only thing that can go wrong with power distribution is, as I’ve posted in the past, short sighted.

Alternator failure is the most likely thing that can go wrong, but it by no means represents the worst outcome - just land before the battery(s) go flat.

To the specific question - yes, a two alternator ship can loose all electrical power. In addition to your example, I read an NTSB report of a twin (two engines each with an alternator) that lost all electrical at night in IFR. The engines and alternators were fine, getting power to where it was need was not. A common buss junction failed.

My point - look for such single point failures in our ships and mitigate them.

Carl
 
I know of a situation where both alternators failed. It was belt driven alternators on a twin, one on each engine. The first belt broke, taking that alternator. The remaining alternator's load doubled and that belt subsequently broke as well. Night IFR in icing conditions too. Prop heat and windscreen hear are big electric hogs. They were already on approach, and landed in about 5 minutes. Very lucky because the battery ran out in 6 minutes, before they even got to the after landing step of turning off the deicing systems.

The other time was after a lightning strike, but they were generators again. The old "turn it off and back on again" trick restored both generators.

We typically have a gear driven secondary alternator, so no belt to break. However I could see that situation happen in reverse. Backup alternator fails for any other reason (regator, brushes, etc). Primary alternator takes the extra load and breaks the belt.

The other failure mode to consider is a short circuit on a bus. A screw rattles loose on something and lands across a big connector or on the bus. Shorts the whole bus to ground. Doesn't matter how many alternators feed the bus, it's going to stop working. This is an extremely rare failure mode, and most small aircraft don't even consider it in the design. Not sure we need to either.

I'm leaning towards full EFI, so need decent electrical redundancy. The best there is will be dual independent electrical systems. Two alternators, two batteries, two busses. Keep them independent with no cross ties or anything. Do whatever failure on one and the other is unaffected. Putting one EFI, one fuel pump and the taxi light on the smaller bus. The bigger bus gets to start the engine and run everything else. Heated seats, avionics, flaps, etc. The backup bus only needs a very small, lightweight battery since it's not starting an engine or requiring endurance. I suppose it load sheds in the sense that if it's not super important (engine computer), I only have one. After a complete failure of one electrical system, there is one EFI computer and one fuel pump still working.

A multi screen IFR panel could feed the backup avionics from the smaller bus.
 
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- but I'm very curious how many people here can say they've seen a dual alternator failure in a single flight. Two completely separate mechanical/electrical alternator failures - anyone?

I know there was the Austro-powered twin that went in a while back from the drained battery that gave it up when the gear was retracted - that wasn't the alternators though, not for the root cause.

Anyone?

I guess I'll sheepishly raise my hand. 2 B&C externally regulated, a couple minutes after takeoff headed out to sea for the next couple of hours. Lost the main, then about 90sec later lost the backup. It was an epic set of circumstances that had us immediately return to the runway in a foreign country.

Sometimes stuff does happen out in the world. I try not to loose sleep over it.
 
Thanks everyone

Great info. Certainly tradeoffs involved with designing allowances for human factors into the electrical system- thanks for sharing preferences. Voltage tiers, simplicity, bus routings, etc.

What started this:

Bench test/experimenting with a couple (salvage) heated seats - these particular ones, without being throttled down via a controller, can pull 9 amps each at 12v (4A base cushion, 5A back cushion). Probably a similar draw to the rest of my aircraft systems combined...
 
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