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24v Step down

50Pesos

Active Member
I would like to put an all electric air conditioner in my plane. The electric compressor does much better on 24v. The rest of everything else on the plane is 12v. So what are my options to cover this and which is the best?

1. 12v alternator and system with a 24v step up converter?

2. 24v primary alternator with 12v step down to run the rest of the ship?

Thanks
 
3. use 24V on the whole ship. Smaller and lighter wires, more juice, no step either direction :)
 
3. use 24V on the whole ship. Smaller and lighter wires, more juice, no step either direction :)

Smaller and lighter wires is certainly true but really not a player in our aircraft. Most wiring is 16, 20, 22 AWG which is pretty small to start with...
 
#2. 24v primary alternator with 12v step down to run the rest of the ship

24v alternators run similar amps to 12v alternators. Meaning you get twice the power. Running an AC will take a lot of power. Do you want a 60a 24v alternator with a step down, or try to find a 120a 12v alternator?

Many of the components we use cost a lot more or simply don't exist in 24v.
 
Option 3:
28 volt belt driven alternator for air conditioning, with limited step down to 14 volt capability for essential buss.
14 volt alternator on vacuum pump pad for normal buss.
 
Good suggestion Bob T.
Nobody has mentioned yet that many modern avionics are designed to operate on either 24 or 12 volts. In fact, both supply voltages could be connected to dual-voltage loads using isolation diodes, thus providing automatic backup.
 
I have also looked at this. Running a 24V alternator with step down creates a LOT of risk without a good amount of redundancy, as it introduces a single point of failure (the step down converter) for the complete system and they are not known to be the most reliable parts. Many of the Buck converters use high freq switching and at those current level could introduce a lot of noise, so that eliminates the low cost parts. Also, MANY options for 12V alternator. 24v, not so much. Good luck finding an affordable 100 A version that fits in your cowl. I did find a 12V, 90A alt in the S-10 form factor that would fit and suspect a 100A version is out there somewhere. If you are willing to cut and form the cowl, many options exist.

If you can squeeze in a 100A 12V alternator, it may be enough. Be advised though that the process of going from 12 -> 24 volts will only net about 75-80% efficiency, so factor that into your electrical planning. You can expect a 12V 100A alternator to produce 35-40A at 24V after conversion. I found that a mininun of 8K BTU is required (12K is better) and those draw around 40-45A @ 24V. Don't forget that the rest of your plane will require around 20A for the first 5 minutes after start to recharge and probably 12-15A after.

re-engineering the plane for 24V is no small undertaking, as many things are not available inexpensively. Try finding a boost pump that runs on 24V, and so on. Things like Pmags or surefly are likely out of the question. Where do you get 24V trim servos, etc.

Other big problem is the need for battery capacity. AC is most needed on the ground. That 100A alternator might only produce 50A or less at Idle. Therefore need a lot of battery capacity to insure you are not launching into the air with depleted batteries and suffer an Alt failure. Maybe ok in VFR with Mags. NOT OK in IFR or with electrical dependent engine. If you need 80A and are only producing 50A, that PC680 will be dead in a few minutes and will have no real indication that it has happened, as the alternator will not be producing 14V at loads beyond it's capacity. The 680 is marketed as an 18AH battery, BUT that is at a 2 amp discharge rate. You will be shocked when you study the discharge performance chart and see what happens if you are drawing 18 amps. Gets much worse if you are pulling 25A out of it.

Larry
 
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Option 3:
28 volt belt driven alternator for air conditioning, with limited step down to 14 volt capability for essential buss.
14 volt alternator on vacuum pump pad for normal buss.

This is my favorite. Lower chance of consequential failures and better and failure modes.
 
These days there's only a few items that are 12V only. For example, on one 24V RV-10 the only 12V items are:
- Van's flap motor
- AFP fuel pump
- Ray Allen trim servos
- Cigarette lighter

Everything else runs on 24V without a converter. There are also some other small other benefits to 24V such as not needing any kind of boost device to keep the avionics alive during cranking. However, some devices are more expensive for 24V applications than 12V (eg: alternators).
 
I would like to put an all electric air conditioner in my plane. The electric compressor does much better on 24v. The rest of everything else on the plane is 12v. So what are my options to cover this and which is the best?

1. 12v alternator and system with a 24v step up converter?

2. 24v primary alternator with 12v step down to run the rest of the ship?

Thanks

Pretty sure this looks better in theory than in real life, I've talked to a few folks who started out with the same idea but ended up abandoning the project.
 
Pretty sure this looks better in theory than in real life, I've talked to a few folks who started out with the same idea but ended up abandoning the project.

You can add me to that list. Once you analyze all the details, it doesn't look good, even on paper.
 
Yep, I agree that doing a 28 volt air conditioner would be a tough thing to do on several levels as prev mentioned.

Exploiting the half voltage point of a 28 volt alternator that has a Y connected stator might be doable, though. That would, at least be a ready 14 volt source for normal loads. Maybe some big gotchas to do that. ??

Also, can a standard Lyc V belt setup handle the very high demand of a 28 volt X ~100 amp alternator at low RPM's?

Ron
 
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