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Weird Electrical Snag...Help!

Rob Erdos

Well Known Member
Hi Guys!

My wife and I were on the last leg home from a wonderful long weekend away in our RV-6. I got some strange electrical indications, and have no idea what I was seeing. If anyone has ideas I would be very appreciative for their insights.

Aircraft Configuration. Our IFR RV-6's electrical system consists of a Plane-Power AL12-E160/B 60A internally regulated alternator and a standby B&C SD-8 alternator. They are wired through a three-position toggle switch as follows: ALT-OFF-STBY. I have an Odyssey PC-680 battery on the firewall. The panel is dual Dynon Skyview, which displays main bus volts and amps.

The electrical schematic is as follows:

[http://i1106.photobucket.com/albums/h373/RVRob/Electrical Schematic_CF-SPB_zpsifkg0aje.gif]

Indication. Operating on the primary alternator, the voltage is normally steady at 14.1 V. The ship normally draws about 15A. Over the last few flights I momentarily thought I saw voltages around 12V, but the voltage recovered just as soon as I noticed it.

On the last flight, the voltage dropped to 12.2 V, then periodically began to fluctuate from 14.1V down to 12+/-0.5V. The current draw seemed steady. I thought, "Rats! My voltage regulator is failing!" I shed some load to about 7A and switched to the standby alternator, but the voltage fluctuations persisted.

I tried turning off everything that I safely could live without. No change.

Upon landing, I selected both alternators off. The battery voltage was 15.1V! When I selected either the Primary or Standby alternator, the voltage dropped to around 14.1V, then began to fluctuate again.

I'm thinking that it might be an internal failure of a cell within the battery.

Has anyone seen anything like this? Ideas greatly appreciated.
 
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OK, as I read this, you are getting weird voltage readings in three different conditions.

1. Normal flight condition, main alternator on, reserve alternator off.

2. Backup flight condition, main alternator off, reserve alternator on.

3. Non flight, ground check, both alternators off.

What are the common denominators in all three of the above conditions?

An internal battery failure would most likely give a low voltage, not a high one.

Have you done anything to verify the accuracy of the voltage sensor?
 
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Check Connections...

I had a situation with a lot of volt/amp fluctuations with a PP alternator and Skyview setup as well.

Got it back on the ground, pulled the cowls and started some basic detective work...

Found the hold-down nut on the main output terminal on the PP alternator had backed off until it was 1 thread away from completely coming off the terminal! Big feed wire from the alternator was just bouncing around making/breaking contact as acro/turbulence acted upon it.

I have made this item something I inspect each time the cowls are pulled now.

Good luck with it!

Rob
 
I agree with Mike; if you're seeing >14V in any mode, it's not the battery. Batteries can't output more than their core design voltage.

If you were seeing low voltage with either alternator on line, then it's highly likely that it's something common to both systems. That would mean either your alternator select switch, or grounds. My vote would be grounds.

Unless, your selector switch isn't rated to handle what the alternator can dish out. Is it actually a switch, and not a contactor? If so, can it handle the full output of the alternator (which is likely to be >60amps)?

Charlie
 
If you saw 15.1 volts on a PC680 with both alternators offline, I'm starting to suspect your voltage indication is at least part of the problem - that's just almost not possible - I would put in a separate volt-meter somewhere in the system to verify that reading.
 
The schematic appears as though you are switching the high current side of the alternators (b-leads) rather than switching the fields on and off. The field of the primary alternator appears to be always powered by your main bus.

What kind of switch are you using for this? Most inexpensive switches are rated somewhere near 15 amps, which means yours will be operating near its limit in steady state operation, and just after starting the engine as the battery is getting recharged, current through the switch would be way over its design capacity.

If this is all true, I would take a look at the switch as it's quite possible the contacts are severely damaged inside, perhaps even welded.

(This switch is also a single point failure that could take both your primary and standby alternators offline, leaving you with only battery power and negating the utility of a standby alternator/generator.)

edit: also, the switch between "norm" and "ext" at the battery is likely seeing higher than designed current after engine start.
 
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I would start with removing the "primary/alternate alternator" switch. This is not how you should wire a standby alternator.

Carl
 
Helpful Stuff

Thanks for your thoughts, guys.

So, it seems that troubleshooting should start with verifying the voltage indications and checking the integrity of the ground. Also, my battery is about three years old, so I may just swap it out on spec.

Apologies if the "schematic" was a bit simplistic. The cockpit switch controls the field current in the conventional sense, and not the alternator output.
 
Thanks for your thoughts, guys.

So, it seems that troubleshooting should start with verifying the voltage indications and checking the integrity of the ground. Also, my battery is about three years old, so I may just swap it out on spec.

Apologies if the "schematic" was a bit simplistic. The cockpit switch controls the field current in the conventional sense, and not the alternator output.

My 680 is on it's 11 year and still going strong, I would trouble shoot first.
 
Upon landing, I selected both alternators off. The battery voltage was 15.1V! When I selected either the Primary or Standby alternator, the voltage dropped to around 14.1V, then began to fluctuate again.

Some alt are self exciting after the field is hot. So killing field wire power may not do what you think. Another good test for this is to turn it off, put a lot of load on the system, and see what the voltage does.

Your problem reads like a loose wire.
 
Face it Rob, it's a cr@p airplane. Just drop it off at my hangar and I'll part it out for you. It's what friends do.
 
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