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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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 Posted: Tue Dec 07, 2010 6:03 am Post subject: Intermittent Generator |
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Hi Neal,
A diode failure in the alternator was my first thought, too. But when the alternator healed itself and returned to normal function a few minutes later, I wasn't so sure. Is it likely that a diode would fail intermittently?
I listened to Mike Busch's webinar on alternators last night http://www.savvymx.com/index.php/webinar and he said an intermittent problem was likely something to do with the brushes, which was my guess after my alternator healed itself.
Thanks for the help,
Dennis
Time: 01:05:14 PM PST US
From: "Neal George" <n8zg(at)att.net (n8zg(at)att.net)>
Subject: RE: Intermittent Generator Problem
I suspect you're losing an output diode in the alternator.
neal
From: owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)
[mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Dennis
Johnson
Sent: Monday, December 06, 2010 9:59 AM
Subject: Intermittent Generator Problem
While on a cross country flight last week, I got a low voltage indication in
my Lancair Legacy, wired using Z 13-8. I turned on the SD-8 aux alternator
and the voltage increased to the low end of the normal range. I also
noticed a whine sound in my headset. If I turned off the main alternator,
the whine went away but the voltage sagged. (I have an all electric
airplane and the SD-8 won't carry the full load unless I shed loads, turn
the master switch off, and run on the E-bus.)
I turned the main alternator back on and the whine sound in my headset
returned and the voltage returned to the low end of the normal range. I
flew that way for about 15 minutes and the whine suddenly stopped and
voltage jumped up to normal. I turned off the aux alternator and completed
the flight normally.
Everything was perfectly normal for the two hour flight home.
Any ideas what caused this? Since the first symptom was reduced output and
a whine in the headset, I initially suspected failed diodes inside the
alternator, but when it healed itself, I thought it might have been a brush
problem inside the alternator. After 15 minutes, whatever was causing poor
brush contact went away? I'm grasping at straws here.
I am so thankful for Bob's Z 13-8 architecture, which allowed me to continue
the flight without breaking a sweat! Thanks, Bob!
Supporting information:
Lancair Legacy, Continental IO-550 engine, 12 volt gear-driven main
alternator, SD-8 permanent magnet aux alternator, Z 13-8 architecture, all
electric panel.
Dennis Johnson
[quote][b]
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n8zg(at)att.net
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| Posted: Tue Dec 07, 2010 9:14 am Post subject: Intermittent Generator |
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Several possibilities, Dennis,
Assuming you’ve checked the belt for proper tension, and given your description of the circumstances, all the ones I can imagine indicate a heat component. David makes a good point regarding case cracks & conductor breaks that open up as the unit reaches operating temps, then close as it cools. Every component in the case is subject to that failure mode.
You didn’t say how long between take-off and low-voltage indication – still in the climb with lights, pitot heat, gear actuators running, drawing max loads down low where it’s relatively warmer OAT? Or high cruise, cooler OAT and “normal” steady-state loads? When the voltage sagged, where did it stabilize before you enabled the SD-8 – 10.8v? 11.5v? 12.3v?
You reported a whine and sagging voltage – the low end of normal with both the SD-8 and main alt running. Folks who know a lot more about alternators than I have told me that a failing diode pack will often generate enough electrical noise to manifest as a whine in the audio system. I expect arcing/chattering brushes could generate similar noise.
Bob teaches that our GA alternators are predominately 3-phase devices. If you lose one of the three output legs, your alternator can still deliver energy, but it won’t be full rated power. You reported that with the SD-8 running, bringing the main alt back on line raised the bus voltage to the low end of the normal range. This indicates that the main alt was contributing some power, but not enough to carry the load.
A weak field could also manifest as a low voltage. A loose field connector, faulty wire, failing switch/breaker, dirty/worn/contaminated brushes & rings are all possible culprits.
I believe I’d start by checking the ship’s alternator circuits for loose/corroded connections, then pull the alternator for a trip to the local automotive starter & alternator repair house for some bench testing.
neal
==============
Hi Neal,
A diode failure in the alternator was my first thought, too. But when the alternator healed itself and returned to normal function a few minutes later, I wasn't so sure. Is it likely that a diode would fail intermittently?
I listened to Mike Busch's webinar on alternators last night http://www.savvymx.com/index.php/webinar and he said an intermittent problem was likely something to do with the brushes, which was my guess after my alternator healed itself.
Thanks for the help,
Dennis
[quote][b]
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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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| Posted: Wed Dec 08, 2010 8:09 am Post subject: Intermittent Generator |
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Time: 02:50:13 PM PST US
From: "Noel Loveys" <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)>
Subject: RE: Intermittent Generator Problem
It could also be a sticking brush on the field coil.
Noel
Hi Noel,
That's my current guess, too. I think I'll run it some more and see if the problem recurs. If so, I'll remove the alternator and have it repaired.
Thanks,
Dennis
[quote][b]
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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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| Posted: Wed Dec 08, 2010 8:16 am Post subject: Intermittent Generator |
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Hi Neal,
You suggested heat buildup as a possible cause of my intermittent generator problem. The problem began about 45 minutes into the flight. I was at 17,000' MSL and the temperature was about 14 Deg F. The alternator on my IO-550 is gear driven and mounted at the front of the engine, directly behind one of the two air inlets into the cowling, so I can't imagine heat being a problem. I was running normal loads, the battery was pretty much charged and pitot heat was turned off.
I'll take your advice and check all the alternator wiring connectors for loose and corroded connections.
Thanks,
Dennis
[quote][b]
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Thu Dec 09, 2010 6:01 am Post subject: Intermittent Generator |
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You may also want to check the point where the F lead attaches to the alternator. I’m still betting on sticky brushes. My experience is, that only happens when the brushes are well worn.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Dennis Johnson
Sent: December 8, 2010 12:43 PM
To: aeroelectric-list(at)matronics.com
Subject: RE: Intermittent Generator
Hi Neal,
You suggested heat buildup as a possible cause of my intermittent generator problem. The problem began about 45 minutes into the flight. I was at 17,000' MSL and the temperature was about 14 Deg F. The alternator on my IO-550 is gear driven and mounted at the front of the engine, directly behind one of the two air inlets into the cowling, so I can't imagine heat being a problem. I was running normal loads, the battery was pretty much charged and pitot heat was turned off.
I'll take your advice and check all the alternator wiring connectors for loose and corroded connections.
Thanks,
Dennis
01234567890123456
[quote][b]
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Noel Loveys
Kitfox III-A
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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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| Posted: Fri Dec 10, 2010 8:52 am Post subject: Intermittent Generator |
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Hi Noel,
Thanks, I'll check the F lead connection. I'm also guessing that it's sticking brushes. Unless I find some other cause, I'm planning to continue flying it until it fails again and then have the alternator repaired by a reputable shop.
Thanks,
Dennis
Time: 06:01:04 AM PST US
From: "Noel Loveys" <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)>
Subject: RE: RE: Intermittent Generator
You may also want to check the point where the F lead attaches to the
alternator. I'm still betting on sticky brushes. My experience is, that
only happens when the brushes are well worn.
Noel
[quote][b]
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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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| Posted: Wed Dec 15, 2010 5:26 pm Post subject: Intermittent Generator |
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Hi Bob,
Thanks for your reply about my one occurance of an intermittent alternator. I'll try your suggestions for troubleshooting. (By the way, it's an externally regulated 12 volt alternator with a B&C regulator.)
Thanks,
Dennis
Time: 10:14:46 AM PST US
From: "Robert L. Nuckolls, III" <[url=mhtml:{0EDA2854-2A5D-40E6-8E05-7F9995441A89}mid://00001246/!x-usc:mailto:nuckolls.bob(at)aeroelectric.com]nuckolls.bob(at)aeroelectric.com[/url]>
Subject: Re: Intermittent Generator Problem
Any ideas what caused this? Since the first symptom was reduced
output and a whine in the headset, I initially suspected failed
diodes inside the alternator, but when it healed itself, I thought it
might have been a brush problem inside the alternator.
Could be a grounding issue too . . .
After 15 minutes, whatever was causing poor brush contact went
away? I'm grasping at straws here.
I am so thankful for Bob's Z 13-8 architecture, which allowed me to
continue the flight without breaking a sweat! Thanks, Bob!
Let's replace the 'straws' with data. What kind of
alternator/regulator combination are you flying?
If an externally regulated alternator, consider
adding a temporary instrumentation wire with 1000
ohm resistor as close as possible to to the
alternator's field terminal best. See:
[url=mhtml:{0EDA2854-2A5D-40E6-8E05-7F9995441A89}mid://00001246/!x-usc:http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z22K_23K.pdf]http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z22K_23K.pdf[/url]
Get some cheepy volt-ohmeter to monitor voltage
at the cockpit end of the wire in flight. An analog
meter is best suited for this. This would be a good
choice:
[url=mhtml:{0EDA2854-2A5D-40E6-8E05-7F9995441A89}mid://00001246/!x-usc:http://www.radioshack.com/product/index.jsp?productId=2103957]http://www.radioshack.com/product/index.jsp?productId=2103957[/url]
Tape the meter up where you can watch it. Note the
readings when the system is operating normally/
abnormally. Review the topic of Alternator Fault
Isolation in chapter 3 of the 'Connection.
Bob . . .
[quote][b]
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