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  #1  
Old 06-22-2016, 04:20 PM
nigelspeedy's Avatar
nigelspeedy nigelspeedy is offline
 
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Location: Tehachapi, CA
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Default Alternator weirdness, help required.

I have a Dynon Skyview system with EMS. The ammeter shunt is set up so it measures the alternator output. I also have a Vertical Power VPX Pro. The VPX measures the current flowing through it and then calculates the difference between VPX current used and Alt output and figures any difference must be going to/from the battery. I have a Lithium Ion battery and at rest with no load it has about 13.3 volts. I also use solid state relays that have a quirky property that even when off there is a small voltage (~2.3 volts) on the output side. The alternator is a Plane Power 60A and only has about 50 hours on it. With the engine off, if I turn on the alternator I see about 4 amps on the #1 Alt Vse, although in flight this value alternates in the display between 0 & 1 Amp.

Recently I have started seeing some odd behavior and am having difficulty troubleshooting the culprit (EMS, Alternator, Shunt, VPX, me).

It started by seeing a high current output from the alternator before start with the master switch off (I could see it on the Dynon EMS page as I turn them on with the back up battery before start). Clearly the alternator cant put out power if the engine isn't turning.

In flight once every couple of flight hours I will see the alternator output amps drop from the normal +15 amps to some large negative value ~-45 amps. At this point the alternator is switched on and the battery amps are ~14.3 which is above the normal battery voltage of 13.3 so I figure this means the alternator is actually still producing output current. And how could the alternator draw current through the shunt?

If I then turn the alternator off the current displayed remains negative. If I then turn the alternator back on it goes back to normal for a few hours. The screenshots below show the sequence of the discussion above.

Any tips for troubleshooting would be much appreciated.

Cheers

Nigel

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  #2  
Old 06-22-2016, 06:36 PM
Mich48041 Mich48041 is offline
 
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Default

Using a digital meter, monitor the millivolt drop across the shunt, looking for a change of polarity. If the Dynon display changes from positive amps to negative amps, but the voltmeter does not measure a change, then suspect a software problem.
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  #3  
Old 06-22-2016, 06:40 PM
Mich48041 Mich48041 is offline
 
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A bad ground connection can cause weird symptoms.
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  #4  
Old 06-23-2016, 12:09 PM
lr172 lr172 is offline
 
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I don't know much about the VPX. Do you have two devices connected to the shunt? How do both systems know the shunt derived current? If operating properly, a negative number means a current reversal. I am not an expert, but I struggle to see how that much current could flow to the alternator without an alternator failure of some sort and I would expect that to include a change in voltage.

If you have two systems connected to the shunt, I would expect them to be intermittently interfering with one another and causing the issue. If not, I would be looking at the low voltage wiring between the shunt and the EMS, particularly the connectors and possibly shorting of one of those wires to ground.

Larry
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  #5  
Old 06-23-2016, 12:16 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by nigelspeedy View Post
With the engine off, if I turn on the alternator I see about 4 amps on the #1 Alt Vse, although in flight this value alternates in the display between 0 & 1 Amp.
This is expected. At that voltage level (12.8), the internal regulator as asking for max power. Once running and regulating, it is asking for less power. The more power it is asking for the higher the current on the field, which is what your VPX is measuring. If you look, you'll see a relatively linear correlation between field current and alternator output current (while engine is running). While an internally regulated alternator doesn't have an external field input, one does exist and the current to the sense/IGN terminal is directly linked to the current that regulator is feeding to the internal field circuit.

Larry
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Last edited by lr172 : 06-23-2016 at 12:27 PM.
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  #6  
Old 06-23-2016, 02:10 PM
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nigelspeedy nigelspeedy is offline
 
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Default Only one device hooked to the shunt

Hi Larry,

Thanks for the field voltage explanation. There is only the Dynon EMS hooked to the shunt. The Dynon sends this value to the VPX via serial connection. The VPX measures its own current usage (going to aircraft systems) and then subtracts VPX usage from measured alternator output and figures that amount must be going to the battery.

Cheers

Nigel
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  #7  
Old 06-23-2016, 02:21 PM
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Hi Nigel,

Suggest using a meter as Joe mentioned to get an outside report. Let us know, feel free to call me or email if you want to.
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  #8  
Old 06-23-2016, 11:13 PM
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Bill.Peyton Bill.Peyton is offline
 
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Bad connection on one of the sense lines from the shunt to the ems would cause this. That's where I would start
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  #9  
Old 06-24-2016, 03:25 AM
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KRviator KRviator is offline
 
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I have a very similar issue that caused a precautionary landing a couple of days ago during testing.

My configuration is very similar to Nigel's, a PP FS-14 alternator on the vacuum pad, the shunt between the alternator and master contactor, a VP-X for power management and a single-screen SkyView.

In flight, the amps typically hover around 3-4, occasionally 5 or so when the servos are running. During takeoff, I regularly see 30 Amps as the PP alternator typically does not produce meaningful current on the ground.

20 mins after takeoff and established in a climb the alternator output dropped to zero several times, then showed a significant negative value, indicating current flow towards the alternator's B-terminal.

The alternator field is energized through the VP-X as is standard, and I didn't think to check the battery voltage during these events, and am now away from the plane so can't pull the datalog.

A thorough inspection after landing showed no heat, short circuit or other damage to any wiring firewall forward, though it did reveal one of the jam nuts on the mixture cable was loose! And the nut on the starter contactor coil was slightly loose, but as that is also energized through the VP-X, it would have been disabled in flight, so shouldn't be the cause of this issue.

I haven't been able to test-fly the plane again to see if the issue recurs, but am open to ideas as to anything else I might check before further flight.
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  #10  
Old 06-24-2016, 06:12 AM
BillL BillL is offline
 
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Default For consideration in your diagnostics.

Nigel, FWIW, there is another recent thread that confirms some early hour failures of the PP, specifically EI60A stator wires. Several have occurred in the 50 hr range, so it not unheard of.

If Chad's independent check shows it is the alternator, you might check the AC content of the alternator output. It would show high if a phase was missing, or intermittent. An o-scope trace would be ideal. The other failure mode is like Bill mentioned, but usually at higher hours - fretting corrosion in the alternator input terminals, still worth a check to reseat the connector a few times. This usually shows up as intermittent output, and/or improper voltage, usually high.

It might be easier just to take it off and down to the auto parts/alternator store/shop for a bench test.
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Last edited by BillL : 06-24-2016 at 06:18 AM.
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