Help troubleshoot blown alt field fuse/breaker

[alpinelakespilot2000]

Like many others here on VAF, I'm having a bit of trouble with what appears to be nuisance trips of my Plane Power alt field. I've read through many other posts, but am hoping my specific issues will help someone tell me where I might start...

Relevant details:

1. I have the alt field protected in two places: at the fuse block and also with a 5A breaker on the panel accessible in flight. I've added the aux. fuse at my main bus fuse block in order to protect the wire between the main bus and the alt switch. (Nuckoll's shows a fusible link here, I just used a fuse.) [See wiring diagram picture or link below.]

2. It has happened 3 times in my first 10 hours of flight. Once just before my third flight. Started up, turned on my alt. field. Blew my 5A fuse, but not the 5A c/b since the latter is downstream of the fuse. Replaced that 5A fuse, no further issues.

3. Fast forward to beginning of flight #10. Started up, blew my 5A fuse when I started up and just after turning my avionics bus on. Again, did not trip the 5A c/b. Changed the fuse to a 10A (since it is only protecting a very short length of 18AWG wire), started up again and it promptly blew the 10A fuse after turning on my av bus but did not trip 5A c/b.

4. It has never happened in flight, only at startup. In flight I get good alternator output.

5. I can find no evidence of worn insulation anywhere on the alt field wire, but in places where it's buried deep in a bundle, who knows.

6. One potential culprit is a weak PC680 battery. It's the one that has suffered and been abused through the 4-5 years of building and testing. It still cranks the engine very well, but it's resting state voltage is down to 12.1V which Odyssey literature suggests means that its is down to a 40% state of charge. By the time I add strobe lights and maybe wig wag lights I'm down to about 11.7V before the alternator gets turned on. Thus, I'm wondering if the weak battery is causing the alternator to intentionally trip the field breaker. I know I should get a new battery anyway, so that is the first place I'll probably start.

7. Anyone have any other ideas?

Kind of frustrating to have the plane grounded when everything else on it works nicely! Thanks for any suggestions.



Here's a link to a diagram that might be easier to read...
http://tinypic.com/r/2jct10o/6

[Neal@F14]

What does it do if you leave the alt field switched on before ever starting up the engine?

Suddenly engaging the field circuit when you've prepared a large load on the system with the engine already turning is going to put a large in-rush current demand on the alternator... kinda like the electricity version of slamming a car into gear suddenly while at a dead stop with its engine already revved up to high rpms.

[MauiLvrs]

Originally Posted by alpinelakespilot2000

... Started up, turned on my alt. field.
7. Anyone have any other ideas?

Leave the Alt field turned on...

[alpinelakespilot2000]

Originally Posted by Neal@F14

What does it do if you leave the alt field switched on before ever starting up the engine?

Suddenly engaging the field circuit when you've prepared a large load on the system with the engine already turning is going to put a large in-rush current demand on the alternator... kinda like the electricity version of slamming a car into gear suddenly while at a dead stop with its engine already revved up to high rpms.

Good question, Neal. I am aware that there is heated debate about whether the alt switch should be turned on or off independently. The first two times were with the alt switch off before startup, which was how Mike Seager taught me. The last time was with the alt field switch on before startup to see if that made any difference. Blows either way. Thanks though.

[Neal@F14]

Originally Posted by alpinelakespilot2000

The last time was with the alt field switch on before startup to see if that made any difference. Blows either way. Thanks though.

In that case, then if the wiring itself has proven to not have any intermittant shorts, I'd begin to suspect that something may be wrong internally with the alternator. Perhaps the integrated voltage regulator is in the process of failing or an intermittent short-circuit condition exists somewhere inside the alternator case. Do you ever hear any abnormal crackling/popping sounds electronically induced thru your headset while flying or taxiing?

[alpinelakespilot2000]

Originally Posted by Neal@F14

Do you ever hear any abnormal crackling/popping sounds electronically induced thru your headset while flying or taxiing?

No, nothing out of the ordinary, though what "ordinary" is after so few hours on the plane is still being learned.

Any thoughts on whether my weak battery could be a/the culprit?

Thanks again for your help.

[Neal@F14]

Originally Posted by alpinelakespilot2000

Any thoughts on whether my weak battery could be a/the culprit?

Thanks again for your help.

Well, theoretically it could... considering that if the battery's voltage is low but still putting out at least 10-11 volts or more, and the alternator's integrated regulator want to feed as much juice as possible to get the field coils (actually the rotor) energized fully , then there will be maximum current draw that the regulator plus field coil circuit could muster.... but in all my experience, I've never ever seen that total be more than 5 amps on any 12 volt alternator commonly used on our planes.

Waitaminute.... Going back and re-reading your original post, I payed a bit more attention to your type of alternator... you've a Plane Power, which has an internal built-in overvoltage protection circuit as well as an internal voltage regulator.

Hmmmmm..... I wonder if the overvoltage detection circuit (crowbar circuit) is sensing some brief, temporary overvoltage condition and tripping itself, which, by intentional design, causes a brief deliberate short circuit in the field coil feed in order to trip your breaker or blow a fuse to prevent damage to avionics, etc, on the bus. If your battery is failing (intermittant internal connections between the cells or the cells themselves briefly opening up while the alternator is running full-tilt trying to charge them, etc) this could be tripping your OV detector inside the alternator which would definitely pop the breaker in a heartbeat.

Food for thought... do you have any other small 12v lead-acid battery (even a motorcycle or lawn tractor battery) you could temporarily connect in place of your suspected Odyssey, and crank the engine up on the ground and see if it still trips the breaker?

[alpinelakespilot2000]

Originally Posted by Neal@F14

Waitaminute.... Going back and re-reading your original post, I payed a bit more attention to your type of alternator... you've a Plane Power, which has an internal built-in overvoltage protection circuit as well as an internal voltage regulator.

Hmmmmm..... I wonder if the overvoltage detection circuit (crowbar circuit) is sensing some brief, temporary overvoltage condition and tripping itself, which, by intentional design, causes a brief deliberate short circuit in the field coil feed in order to trip your breaker or blow a fuse to prevent damage to avionics, etc, on the bus. If your battery is failing (intermittant internal connections between the cells or the cells themselves briefly opening up while the alternator is running full-tilt trying to charge them, etc) this could be tripping your OV detector inside the alternator which would definitely pop the breaker in a heartbeat.

Yeah, that was basically my idea based on some earlier threads. I went out and picked up a new PC680 and will try that tomorrow or the next day. I'm not holding my breath, but even if the new battery doesn't solve my alternator field problem, at least I'll know I've got longer battery life in the event my alternator did happen to fail in flight.

[Walt]

Fuses are fast acting devices, not what you want in this circuit so I would put a slow blow fuse in this spot. Not sure what kind of breaker you have but many breakers are spec'd around 20% tolerance and the field will draw very close to a full 5 amps with a low battery or heavy load. So my suggestion would be install a slow blow 7.5 or 10a fuse (or eliminate it) and a 7.5 amp Klixon 7274-7.5 breaker.

And never switch the alternator off/on when its under load, it should be on prior to start. If you want to test the field circuit do it at very low engine speeds (idle) to avoid load surges.

[alpinelakespilot2000]

Originally Posted by Walt

Fuses are fast acting devices, not what you want in this circuit so I would put a slow blow fuse in this spot. Not sure what kind of breaker you have but many breakers are spec'd around 20% tolerance and the field will draw very close to a full 5 amps with a low battery or heavy load. So my suggestion would be install a slow blow 7.5 or 10a fuse (or eliminate it) and a 7.5 amp Klixon 7274-7.5 breaker.

Thanks Walt. I saw your comment in one of the earlier threads about bumping everything up to 7.5A. It is on my list of possible solutions. I wired the alt field circuit with 18AWG per Plane Power drawings. AC43.13 shows 18AWG can carry 7.5A continuous over only about 10 feet (which, from bus to alternator, I'd probably be pushing), so I wasn't sure if I might have to rewire the entire circuit with 16AWG. Still, going up to 7.5A is a possibility. In the meantime, I will definitely see if I can find a "slow blow" 7.5 or 10A for that fuse I have in my main bus. I currently have a Klixon 7277-5A from Stein or B&C for the c/b in the panel. Not sure if that's different from the 7274 number you note above as far as being "fast" or "slow."

One other question... is the fact that the fuse is blowing rather than the circuit breaker telling me something about where a problem might be? My circuit looks like this:

Main bus (fuse block) --> 10A fuse --> 18AWG wire --> 5A panel C/B --> 18AWG --> Split Cessna Master Switch -- > 18AWG --> Alternator Alt Fld terminal.

If I do have a problem (short) in my wiring, since it is blowing the fuse at the bus and not the circuit breaker on the panel, the short would have to be either in the wire between the 10A fuse and the C/B or in the C/B itself, correct? Anything problem farther downstream would first trip the C/B, wouldn't it? On the other hand, if the problem were in the alternator itself, which would blow/trip first? The fuse or the c/b?

Anyway, thanks to everyone for the help. Will try to install the new battery later today, try to find a slow blow fuse, check all connections to make sure they're solid, and see what happens.