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  #1  
Old 01-24-2023, 01:07 PM
tracy tracy is offline
 
Join Date: Dec 2012
Location: chattanooga,tn
Posts: 309
Default Alternator failure

I read Ed’s post on alternator failure and wanted to throw this in. I also had an alternator failure coming from Beaufort SC to Chattanooga. Well, not really a failure but a drop in volts to 13.5 causing flags. It was fluctuating between 13.5 and 14 at first, then 13.5 for remainder of flight. My question is, wouldn’t the brushes, as in my case, start producing less and less voltage over time as to giving us a heads up about alternator replacement? Mine had 930 hrs and was a denso. I could have replaced brushes, but felt a new one was in order.
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  #2  
Old 01-24-2023, 01:52 PM
Avanza Avanza is offline
 
Join Date: Jul 2020
Location: Vastervik Sweden
Posts: 216
Smile Voltage drop

Yes, voltage drop may indicate worn brushes.
This is specially true when the alternator is more loaded.
The brushes normally last a long time, on a car >100.000 miles.
This is because they are powering the rotor, current is low.
I have a normal charging voltage of 14.5V.
Flag is set to 14.0V, then I should have time for service.
To check, you just turn on a load that takes several amps.
I the voltage drops to your flag level, check the brushes.

https://www.youtube.com/watch?v=xvrCtq6__5A

Good luck
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  #3  
Old 01-25-2023, 10:21 AM
664781 664781 is offline
 
Join Date: Jan 2005
Location: Powder Springs, Ga
Posts: 366
Default Low voltage

My failure was a slow decrease in voltage. Started at 14.4 then 14.0 then finally got down to 13.5 over about a 2 month period. Took it to a old school alternator repair shop and asked what failed. Brushes were worn some but the internal voltage regulator was my failure. We changed brushes, diodes, bearings and voltage regulator. All good and back to 14.5 volts.

Your mileage may vary!
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  #4  
Old 01-25-2023, 10:26 AM
wawrzynskivp wawrzynskivp is offline
 
Join Date: Feb 2018
Location: Incline Village Nv
Posts: 373
Default A Weird Alternator Problem

I had a fuse inline with the field current that had continuity problems. This showed itself as wandering alternator output voltage between 11.5V-14.5V

Took a while to conclude it was the fuse.
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  #5  
Old 01-25-2023, 01:00 PM
planenutz planenutz is offline
 
Join Date: Jan 2005
Location: Arrowtown, New Zealand
Posts: 111
Default

I had issues with the electrical system in the Bolkow 209 last year that had me stumped. The battery slowly went flat (it was new) and in the Old Skool panel there was no voltmeter to warn me. The engine cranked perfectly but after 40 minutes of flight the first sign I had a problem was when the radio wouldn't transmit and the Fuel Flow screen went blank. Mmmm.... no radio, no Xponder, no flaps and no undercarrige.

After checking out the electrical system I found northing wrong. I replaced the external regulator but couldn't get the system to charge more than 12.3 volts output. I took the alternator to a specialist who tested it in front of me and found it to be working perfectly. It was about 200 hours TSN. Long story short, the issue was in the Master Switch. Over 1600 hours of operation the contact points within the switch had gotten dirty and it seems this was interferring with the ability for the regulator to control the alternator. I cleaned the contacts within the Master Switch, reassembled it, and now everything is perfect.

Even the simplest of electrical designs can catch you out.
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  #6  
Old 01-25-2023, 02:08 PM
Untainted123's Avatar
Untainted123 Untainted123 is offline
 
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Location: Azle, TX
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Default

Quote:
Originally Posted by planenutz View Post
Long story short, the issue was in the Master Switch. Over 1600 hours of operation the contact points within the switch had gotten dirty and it seems this was interferring with the ability for the regulator to control the alternator.
This sounds a lot like what happens when you don't have a diode on the solenoid: the field collapses and induces a current that causes a spark to jump across the now-open master switch, which causes damage each time it does so.

This video describes it well: https://www.youtube.com/watch?v=HJWoA8taECU

You can skip to about 45sec to see what happens. In your case maybe it was actually just dirty, but if you don't have a diode on the master solenoid, you can add one easy.
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  #7  
Old 01-25-2023, 03:24 PM
planenutz planenutz is offline
 
Join Date: Jan 2005
Location: Arrowtown, New Zealand
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Default

Yes, good thinking there Untainted. And thanks for the video link - that's really good watching.

In fact it does have a diode as this was the subject of an AD many years ago for us here in NZ but you're exactly right... it would indeed be the result of not having one installed. The contact points within the switch weren't damaged (fortunately) however they were certainly dirty. A good clean and some lube should see us right for another 1000 hours.

It took me days of head-scratching to track this problem down. Like the continuity issue with the inline fuse mentioned above, these things can give you unexpected outcomes in what would otherwise appear to be a simple system with few obvious failure modes.
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Last edited by planenutz : 01-25-2023 at 03:26 PM.
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  #8  
Old 01-25-2023, 04:57 PM
gasman gasman is offline
 
Join Date: Mar 2007
Location: Sonoma County
Posts: 4,772
Default

Quote:
Originally Posted by Untainted123 View Post
This sounds a lot like what happens when you don't have a diode on the solenoid: the field collapses and induces a current that causes a spark to jump across the now-open master switch, which causes damage each time it does so.

This video describes it well: https://www.youtube.com/watch?v=HJWoA8taECU

You can skip to about 45sec to see what happens. In your case maybe it was actually just dirty, but if you don't have a diode on the master solenoid, you can add one easy.
This video is a very good example of with and without a diode on the solenoid. But as you can see in the video, he was switching the master side only, and the coil in the master solenoid is what caused the arc. The other side of the switch that does not get switched in the video, controls the field and does not have the arcing issue.

All RV models follow a standard master switch wiring schematic that has the master switch only activating the master solenoid with a grounding leg. This single switch cannot also control the field. A DPST switch or a split switch like in the video or two separate switches are used to operate the master and the field.

If the master switch was cleaned to solve the alternator problem, it was no doubt a split switch with dirty contacts on the field (ALT.) side.
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