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Volts constant but amps will fall off and run negative

aeroaddict

MEMBER
Boise ID area
Plane; CC EX-2, 60 hours on it, Dynon instruments.

This occurs without any pattern (intermittently) and I cannot duplicate the fault, but always happens when airborne. If I where to average the fault it would be roughly every 6 hours???

On the EMS display the amps will fail off and go negative; ~10 to 12 amps, voltage will always remain constant. I have a 50 amp breaker between the alternator (40 amp) and shunt. the breaker does not 'pop'. When the fault occurs, I will turn off the 50 amp breaker, then turn it on and things return to normal; ~6 to 8 amps. I have checked connections, looked over everything and can find nothing.

Any ideas and/or help would be appreciated.
 
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loose screw on 50 amp breaker??(or at other ends of those wires) had a noise issue we chased for years that was this... it would "weld" tight for a while, then become loose and 'noisy'
 
The only time I experienced negative amps with normal volts was when my shunt was wired backwards. Mine wasn't intermittent.
 
Just guessing - Bad breaker, that opens when it shouldn't but doesn't pop - dunno if that's possible. Voltage taken from "upstream" of the breaker, and amps showing flow from battery to loads. Maybe???
 
What charging system? If it's an alternator system, you should never have a negative current flow due to the rectifier diodes. Sounds like a problem with one of those diodes. Loose connection or possible diode break down when it gets hot.

Web
 
I’d look at the shunt anyhow. Just in case - it might not be backwards, but may be mis-wired.


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Great replies, thanks.

It is an alternator and the voltage remains constant; 14.2V. Yes there is an OV relay. I have also verified the voltage readout on the screen to the battery, they are the same.
 
Your OVP is activating erroneously. It gets reset when you cycle the breaker. I had this same issue in a Comanche after an alternator conversion. SOMETIMES when the flaps were moved, it would trip. SOMETIMES when the gear was moved it would trip. The manufacturer solution was to tell me to have my gear and flap motor rebuilt because they were “noisy “. That didn’t work. The problem is these circuits are generally not designed to ignore short duration voltage spikes which you could see on an oscilloscope but never on a volt meter. My solution: I pulled the breaker that powered the OVP and flew for another ten years with no problem. Yes, I know what could have happened, but it didn’t.

If you have a separate breaker for the OVP try pulling that rather than the big breaker next time that should also reset it. If that’s the problem.
 
Thanks, I'll try the OV Relay breaker next time. I have pulled the OV Relay breaker on the ground and the results mimic as if pulling the 50 amp breaker; amps drop then return on reset.
 
What model alternator and regulator?

It's intermittent, so that rules out miswired components. Over voltage relays cut off regulator/field power, which would not cause a reverse current condition. The rectifier diodes in all alternators physically prevent reverse current conditions. Therefore, the only reason to have reverse current flow would be a diode failure or a short to ground on the output circuit. And since the voltage shows correct and the shunt acts in concert with the ammeter to show current value and flow, any short circuit has to be between the output terminal of the alternator and the shunt.

Web
 
That makes a lot of sense. Alternator is a Denso and the voltage regulator I assumed was the shunt. From the B pole of the alternator, there is a alternator noise filter and a transient suppressor, then a OV relay between that and the 50 amp breaker, then off to the shunt. Could a faulty circuit breaker or OV Relay cause the intermittant amp drop? In other words, when a relay or breaker fail, would they just fail and not be able to reset?
 
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An open circuit breaker or OV relay would just show as 0 amps current flow. Not a negative flow. And if the OV relay opened, you would also have no voltage or, depending on how it's wired, possibly battery voltage.

When circuit breakers pop, it takes a few seconds for them to cool down enough to reset. If you try to reset when they are still hot, the button just won't reset.

Web
 
Don't discount the diode breakdown. They fail when hot, so when you recycle the breaker, they have a few seconds to cool down.

Do you know what model alternator and regulator are installed? This info is important for troubleshooting.

Web
 
Sometimes when diodes fail they create AC ripple current...causes noise/whine on the radio more than normal. My experience.

Gary
 
Sorry. Our posts crossed paths about the alternator model.

It's an automotive alternator (I'm not a fan of them on aircraft, but we can argue that later) so pull it off and take it in to an automotive shop that has a test bench for them. Tell them you suspect an intermittent diode failure so that they leave a load on it for a while. If you don't want to screw with this, these alternators are relatively inexpensive so you could just install a NEW alternator. While the alternator is out, physically trace the large, output wire back to the shunt location, checking for evidence of a short to the airframe.

If you replace the alternator, get a new one. Not an ovehauled/rebuilt one.

Web
 
Yup, that is what I was thinking. I will remove the alternator and get it tested. As this has happened previously I will take the opportunity to trace out (again) the whole system.

And also check the small wires and pinout of those wires.

Thanks
 
bad/loose connection on shunt small wire(s) going to gauge? maybe its just lying to you.....

The shunt will have the two fat wires from the alternator output circuit and two small (maybe 22 gauge?) wires that go from the shunt to the amp/volt gauge. Not sure what would have to happen for the reverse current indication to come from poor connections alone. Both volts and amps should go to zero or battery voltage. Also, the two small wires are used by the gauge to sense both voltage and amps. If they shorted or opened it should again indicate zero.

Under normal conditions, current flow would be from the alternator, through the shunt, to the bus. For the current to flow from the bus and back through the shunt would need something unusual to happen.

Web
 
Is your circuit breaker one of this type? https://www.sager.com/w31-x2m1g-5-2...MIhMDZ2rGd6AIVBGKGCh1pNgymEAQYAiABEgKeTvD_BwE
20_pr1h40027b.jpg
It's possible that it is tripping internally without moving the toggle. So you do not realize it is actually tripped. Without knowing anything about your wiring schematic it is not possible to know where the meter readings are coming from. If the breaker is tripped, it will feel differently when you cycle it off then on than when you normally turn it off and on. The off direction will be softer if it had tripped. Maybe the breaker needs to be replaced? Perhaps it is rated at too low an amperage for your system? This sounds similar to an issue I had with mine. Changing the main breaker from a 15 to 20 amp solved the issue.
 
It's possible that it is tripping internally without moving the toggle. So you do not realize it is actually tripped. Without knowing anything about your wiring schematic it is not possible to know where the meter readings are coming from. If the breaker is tripped, it will feel differently when you cycle it off then on than when you normally turn it off and on. The off direction will be softer if it had tripped. Maybe the breaker needs to be replaced? Perhaps it is rated at too low an amperage for your system? This sounds similar to an issue I had with mine. Changing the main breaker from a 15 to 20 amp solved the issue.
What I was guessing, but you said it better.
 
If the either the output or field breaker tripped, it would result in a 0 amps reading, NOT a negative reading.

Web
 
If the either the output or field breaker tripped, it would result in a 0 amps reading, NOT a negative reading.

Web
If the ammeter was wired to the battery side it could show the discharge from the battery. That would be negative. Mine does this.
 
Aeroadditct

Did you wire this alternator or do you have the diagrams that show the wiring? Tell us if the shunt has been installed on the alternator output or on the battery feed to the bus.

Web
 
If your voltage is steady, I would think it's a fault in the Dynon. Have you verified voltage with a portable meter?
 
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