Troubleshooting Field Breaker

[Jim P]

What's the best steps for troubleshooting an alternator field issue? I haven't run the engine in about 2 months due to annual and some other maintenance. Battery was low, but just enough to start (showing 10.8V on the EIS, batter is showing 11.5). I'm running a B&C 60A alternator with the externally regulated Z-13 architecture. It was running just fine before maintenance and there's nothing that was done with the electtircal system.

On start, the engine barely turned over, but it did start, and when bringing the alternator on-line, it blows the field breaker within about 2 seconds. This has been a good system for 375 hours, and this is the first electircal issue.

Any tips for troubleshooting?

Jim

[flyinga]

I'd put a good charge on the battery and try again. It's possible that since the battery is so low the alternator is trying to charge too fast and goint into an overvoltage condition thus the breaker popping.

[Jim P]

I had the same thought, so I'll put a charge on it. I've also dug out a trouble shooting guide for the B&C regulator so that should help as well.

[Walt]

Not sure what kind of CB you have installed but I've seen the 5A tyco style cause this. Under a heavy load the field is drawing a full 5A so a weak breaker could be the problem. I only use Klixon now, on the 60A alternators I prefer 7.5A to prevent nuisance breaker popping (especially on the PP units).

[Mike S]

Originally Posted by Jim P

. when bringing the alternator on-line, it blows the field breaker within about 2 seconds. This has been a good system for 375 hours, and this is the first electircal issue.

Any tips for troubleshooting?

Jim

First, charge the battery.

Then, start the plane with the field already energized.

There has been a lot of discussion in the past concering switching the alternator on while the engine is running.......most folks seemed to think it was hard on the unit, and let to a short life.

Good luck.

[David Bell]

Been there...Get a new battery.

[Jim P]

Well darn! Swapped out the batter, checked the LR-3 external regulator and everythign seems to be within limits. Fired up the engine and still popped the field breaker. Any further hints here? All the connections seem to be OK and the breaker is only about 7 years old.

[Walt]

I would test the breaker or temp install a slow blow fuse and see what happens.

[Jim P]

Thanks Walt, I'd already been through that guide with all voltages and resistance within limits with exception of the delta between pin 4 and 6. It calls for 1.2v difference between the pins and I'm at 1.41v.

[Kahuna]

I would pull the load side of the field breaker wire and the other end at the regulator and check for any resistance end to end. You could have a short along the wire and easy enough to check. Should be an open circuit.