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Looking for help with Alternator issue

lr172

Well Known Member
On 8 of the last 10 flights, my Alt Fld breaker has tripped. It is a 7A breaker and it trips about 5 minutes after take off. Voltage is steady at 14.3 - 14.4 when it trips and current output is normal (14-16 amps). I reset it, sometimes have to do it twice, and the rest of the flight is without incident; A couple of these flights were 2 hours long, so clearly only happening early in the flight. A couple flights had no tripping.

I have an ND alternator (45 amp and 2 years old - it was a reman direct from ND, so confident in the quality) with an external transpo VR. I thought it might be resistance on one of the wires, but put an ohmmeter on all of them today and they check out fine. Ground at Alt and VR are 0 ohms. Only thing that I cannot check is resistance from the field wire faston to the alt F spade. It's buried and can't get the probe in there. Plan to clean the spade and put on a new connector.

I have an OV circuit that crowbars at 16 volts, but I don't suspect that as I am not getting voltage warnings (set to warn at 15 volts).

It seems that it is drawing too much current on the field, but unsure as to why - resistance on fld circuit or resistance on B lead output.

I have had issues with resistance on the VR supply line in the past, but they always resulted in an overvoltage and tripping was via the crowbar. Issue turned out to be the screw on the breaker post loosening.

Appreciate any thoughts or ideas. I am questioning that the breaker might be tripping under the rated voltage, but no good way to test it. I initially questioned that the OV circuit was going bad, but this should present more random and not always 5-10 minutes after T/O. Biggest amp draw is right after strart up, but never have an issue there. It is where I would expect a weak link to present itself, but oddly doesn't appear.

Larry
 
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My first thought is to verify the actual amperage on the field circuit, you could just have a weak breaker. Can you insert a digital ammeter in that circuit and go take a quick flight?
 
I have an OV circuit that crowbars at 16 volts, but I don't suspect that as I am not getting voltage warnings (set to warn at 15 volts).
Larry

Disable/bypass the crowbar OV protection and see if the issue persists. Small transients can cause them to trip the breaker, but are too fast in duration to show in your voltmeter.
 
My first thought is to verify the actual amperage on the field circuit, you could just have a weak breaker. Can you insert a digital ammeter in that circuit and go take a quick flight?

Good idea. My ideal w/RMS only has a 1 amp fuse. Maybe I'll go buy a cheap one at HF and wire it in just after the breaker. Those usually have a 10 amp limit.

Larry
 
Disable/bypass the crowbar OV protection and see if the issue persists. Small transients can cause them to trip the breaker, but are too fast in duration to show in your voltmeter.

Will try the that. What could cause the transients to show up? Has worked well for 700 hours. Could issues in the whelen strobe box cause that? A little testing makes me believe it is better with strobes off.
 
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The alt F spade

One end of the Alt field circuit is connected to GND, the other to the F spade.
You should be able to messure a resistanse of 5-15 ohm to GND on the wire going to the F-spade on the alternator.
Use a rubber hammer on the alternator when checking resistanse.

Good luck
 
Will try the that. What could cause the transients to show up? Has worked well for 700 hours. Could issues in the whelen strobe box cause that? A little testing makes me believe it is better with strobes off.

That could be an artifact of total load, and not transients. If your battery is simply absorbing that many amps (depleted, or low sense line voltage causing the alternator to put out too many amps) then the field current may actually be where it should be. The question here is this - is the alternator (and field) doing what it should, or is the field current going wonky high without proper alternator output (meaning the diode pack is at fault), or is the field current fine and the breaker is weak?
 
Thanks for all the ideas. I crimped on a new faston for the field wire at the alternator and did a test flight with no issues. Will do a few more flights to confirm, but it is plausible. High resistance on fld circuit would cause excess current flow on the fld circuit. The fact that it gets better when things get warm, kind of makes sense.

Time will tell.

Larry
 
"Disable/bypass the crowbar OV protection and see if the issue persists. Small transients can cause them to trip the breaker, but are too fast in duration to show in your voltmeter."

This is a good thing to check, although short transients are unlikely. However, I had exactly the same issue, in fact first time it happened right after lifting off Teterboro NJ., really neat! Kept happening several times all the way back to Chicago - luckily I have the B&C backup alternator too, motored home with that. Never was able to pinpoint the exact location, but the problem went away when I summarily replaced the field wire from regulator to the alternator, so likely with vibration it was shorting somewhere. As far as the alternator is concerned, no matter what the load, the field current should never exceed about 3 amps or so, except for very unlikely field internal shorting.
 
I'd disconnect the faston at the alternator and connect a load on it, like an automotive headlight bulb, or anything else that pulls around 5A. This will make sure your breaker is ok.
With all that trip history :D it may be a worn out breaker.
 
5 amp load test

Measure the voltage at that 5 amp load to be sure there are no bad connections upstream that will cause a voltage drop.
 
A loose screw on a circuit breaker can generate heat. Heat can cause a breaker to trip.

That is the first thing I checked. Had that happen once before and the symptom was over voltage, as the wire from the CB to the VR is the voltage sense.

Larry
 
I'd disconnect the faston at the alternator and connect a load on it, like an automotive headlight bulb, or anything else that pulls around 5A. This will make sure your breaker is ok.
With all that trip history :D it may be a worn out breaker.

Thanks. If it trips again, I will wire an ammeter in line at the CB to confirm the actual current flow in the circuit. I also think it is the mostly likely culprit if the new connector doesn't resolve this.

Larry
 
I had a Transponder that would pop a breaker about every 3 or 4 hours of flight operation. The breaker had Xponder and ADSB in and out powered with sufficient capacity. This nonsense went on for about a year and always reset after the bkr operation. Enough of this **** so I bought a new to me used GTX327 and installed. Oddly enough it continued doing the same thing! Several more months passed and I began to look for problems . Behind the panel and about 6 inches behind the Xponder backplate, I found a black spot on a white power wire ( to the Xponder). That black spot was directly above a sharp edge in the sub panel cut out. Could not see the conductor wire inside but obviously in rough air and steep decent was enough to pop that breaker. Spliced the power wire ,added extra insulation as well as protection on the cut out , a few tie wraps and have not had any more problems Thats been 4 years ago.
Good luck on the find , and " look closely" That breaker may be actually doing its job!
 
Q. the question is. why, It is a 7A breaker and it trips about 5 minutes after take off. Voltage or other problems.
A. this is not 'full' answer, but a ""hint"", a fuse and circuit breaker, works on heat over time, so a 1amp fuse will hold 1amp for some were around 20 or 30 seconds, then blow.
so if you have a good circuit breaker (7amp)and on ""not" a overvoltage problem. you are drawing a little but under 6.99 amps
.
good luck good day /rick
 
Loose F connector on Alt

The F connector on the alternator is subject to vibration.
If the thin wire is not supported it will eventually brake close to the F spade.
Before it breaks there will be a bad connection.
This will cause normal field current (3A) go to 0A and back on again.
As a result Voltage will sky rocket and the crowbar will trip the field CB.
It is built to do that.
 
Larry,
I am thinking if the spade terminal on F was causing a resistance it would cause the alternator to increase voltage.
I have had this happen.
So I don't know that you replacement of the spade will permanently fix you problem.
 
I have ohm'ed out the whole circuit all the way back to the bus bar and all is <.1 ohms.

Larry

Larry,

Ohmmeters usually pass very little current while making their measurement. Consider the case of taking an ohmmeter reading of a single strand that was stripped out of a 20 gauge wire vs the entire wire (measuring the resistance of the wire with all its strands in the circuit). If you try to put 5 amps through that single strand, the results will be quite different than trying to put 5 amps through a 20 gauge wire.

Put another way; consider this example. An automobile heater blower motor ceased to run. The feed wire showed 12 volts when disconnected from the motor. The motor ran fine when when jumpered to the battery. The feed wire had broken all but one or two strands. It would probably have ohm'ed out at <.1 ohms, but could not pass enough amperage to turn the motor.
 
Larry,

I had this happen on an well experienced B and C alternator on my RV6. Ended up being the braided wire from one of the brushes to the field terminal. The brush lead only had a couple strands remaining causing it to to "make and break" intermittently and the result was the field breaker popping. Basically the same as described would result from a field wiring breaking down in a similar fashion. Essentially that maybe happing upstream (in the alternator brush block) inside the alternator. Worth checking. I replaced the brush block on mine and have had zero issues. The brushed themselves had lots of life left, the braided leads one brush did not. I suspect this was a manufacturing error but the alternator did run about 1400 hrs before this happened.

Bill
 
I just wrote a column on this. Two things are usually the cause of the field CB tripping----overvoltage and a short. Usually, there could be some sign of an overvoltage event in the logs (glass panels).

Most likely, it is an intermittent short. Start at the field wire connection at the alternator and carefully run your flashlight and fingers along it's entire length all the way back to the regulator and switch.

I luckily found one this past week in the shortest time ever. The EGT probe metal sheath on cylinder one had rubbed a very tiny hole in the insulation of the field wire 4" from the alternator. It only popped the breaker intermittently on flights. After we removed the cowling and ran the engine it didn't fail. That told me something had moved in the slipstream, so I set to work finding it. :)

Vic
 
Larry,
I am thinking if the spade terminal on F was causing a resistance it would cause the alternator to increase voltage.
I have had this happen.
So I don't know that you replacement of the spade will permanently fix you problem.

You'll have to explain why that is for me. I now that resistance on the VR sense line will create an over voltage situation, by being tricked into believing that the buss voltage is lower than it really is and upping the alt output voltage accordingly by increasing fld current. This actually happened on this plane two years ago - screw on breaker backed off. However, the VR modulates the Alt output by adjusting the current on the field. If there is resistance in the fld circuit, between the VR and ALt, The VR will have to increase the amount of current sent to the Alt to make up for that lost due to the excess resistance, in order to maintain the desired buss voltage. If resistance is bad enough, eventually the current capacity of the breaker is exceeded and it trips. What am I missing?
 
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I just wrote a column on this. Two things are usually the cause of the field CB tripping----overvoltage and a short. Usually, there could be some sign of an overvoltage event in the logs (glass panels).

Most likely, it is an intermittent short. Start at the field wire connection at the alternator and carefully run your flashlight and fingers along it's entire length all the way back to the regulator and switch.

I luckily found one this past week in the shortest time ever. The EGT probe metal sheath on cylinder one had rubbed a very tiny hole in the insulation of the field wire 4" from the alternator. It only popped the breaker intermittently on flights. After we removed the cowling and ran the engine it didn't fail. That told me something had moved in the slipstream, so I set to work finding it. :)

Vic


Thanks Vic. I hadn't thought of a possible short on the Fld wire. I checked it near the terminations, but not the whole run. Seems unlikely though, as the fld wire and B lead wire are both inside of a wire sock to protect from chaffing. Anything is possible though.

Larry
 
Larry,

I had this happen on an well experienced B and C alternator on my RV6. Ended up being the braided wire from one of the brushes to the field terminal. The brush lead only had a couple strands remaining causing it to to "make and break" intermittently and the result was the field breaker popping. Basically the same as described would result from a field wiring breaking down in a similar fashion. Essentially that maybe happing upstream (in the alternator brush block) inside the alternator. Worth checking. I replaced the brush block on mine and have had zero issues. The brushed themselves had lots of life left, the braided leads one brush did not. I suspect this was a manufacturing error but the alternator did run about 1400 hrs before this happened.

Bill

Thanks for the tip. if this continues, I'll pull the Alt off and have a look at the brushes.
 
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