C-150 Alternator tripping

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bobkiksass

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bobkiksass
Hi, I have a cessna 150 with an o-200 that was overhauled about a year ago, complete with a new alternator. The alternator fuse has tripped twice in the last 2 days, after running the plane for a 30 min approx... and the mechanic tells me that 90% of the time this is a sign of a bad brushes in a failing alternator. Since it has just been replaced during overhaul, what other issues could be causing this. It seems to be happening when I actuate the flaps, but maybe this is just a coincidence ?

Thanks
 
I'm also kind of curious, when your alternator fails, if you have a charged battery .... how long do you think you can continue to run a garmin 430 ?
 
bobkiksass said:
Hi, I have a cessna 150 with an o-200 that was overhauled about a year ago, complete with a new alternator. The alternator fuse has tripped twice in the last 2 days, after running the plane for a 30 min approx... and the mechanic tells me that 90% of the time this is a sign of a bad brushes in a failing alternator. Since it has just been replaced during overhaul, what other issues could be causing this. It seems to be happening when I actuate the flaps, but maybe this is just a coincidence ?

Thanks
Click to expand...

Bad brushes don't blow fuses. They just reduce the alternator's capacity.

Your flap motor system might have a short somewhere along the wiring, but that should blow the flap fuse.

Now, which alternator fuse is blowing? The 5-amp alt field (which really just runs a small relay in the regulator) or the 60-amp output fuse?

Or is the overvolt sensor just tripping the alternator offline without blowing fuses?
 
What exactly do you mean by "the alternator fuse"? I suspect you're talking about a circuit breaker but there's probably two of those, one for the output (30A-60A) and one for the field (2A-5A). If it's a CB, which one is it?

BTW, I don't think bad brushes will cause a CB trip and I suspect your A&P is confusing this symptom with an overvoltage trip (alternator goes offline and works again if you cycle the alternator switch).

If you're tripping the field CB (or blowing a field fuse), there are 4 possibilities: A shorted field winding, a wire in the field circuit shorting to the airframe, a bad CB (note that a loose connection to a CB can make it appear bad due to the heat generated by the associated high resistance), or a bad regulator.

If you're tripping the output CB the cause could be an intermittent short in the alternator diodes (unlikely), a short from the alternator output to the airframe, or a bad CB (see the note above about loose connections).
 
bobkiksass said:
I'm also kind of curious, when your alternator fails, if you have a charged battery .... how long do you think you can continue to run a garmin 430 ?
Click to expand...
That depends on all sorts of things but mostly on how quickly you discover the failure and shed all unnecessary loads.

If you shut everything else down immediately after the alternator fails, you might be good for an hour, especially if you refrain from using the comm transmitter (the current draw of the 430 is 5-7 times greater when transmitting). A more common scenario is letting the alternator failure go unnoticed until something else (like the 430) quits and at that point you'll probably lose everything within seconds.

In the daytime you should turn off the strobes, beacon, landing lights, position lights, pitot heat, and panel lights along with any other avionics (MFD, audio panel, transponder) you don't absolutely need and don't forget the alternator itself (the field current will go to maximum if the regulator is still working). Strobes, beacon, and pitot heat are usually pretty high current, the transponder, panel lights, audio panel are much less but still might add up to enough that the battery life suffers significantly.
 
Yes it was popping the circuit breaker, and it is the 55 amp labeled "alt" I guess that is the output. Thanks for the info, the mechanic suggested he thought it could be a weak circuit breaker, and to test what the voltage is when it trips. Is this what you would do ?
 
bobkiksass said:
Yes it was popping the circuit breaker, and it is the 55 amp labeled "alt" I guess that is the output. Thanks for the info, the mechanic suggested he thought it could be a weak circuit breaker, and to test what the voltage is when it trips. Is this what you would do ?
Click to expand...

Bob. the breaker trips from excess amperage, not excess voltage. Testing the voltage (particularly with a proper functioning voltage regulator) won't tell you much....

-Skip
 
bobkiksass said:
Yes it was popping the circuit breaker, and it is the 55 amp labeled "alt" I guess that is the output. Thanks for the info, the mechanic suggested he thought it could be a weak circuit breaker, and to test what the voltage is when it trips. Is this what you would do ?
Click to expand...

That C/B is to protect the aircraft wiring when the buss is trying to draw too much. That normally occurs when the battery is trying to draw too much amperage to re-charge.

test the battery IAW it's ICAs, When it passes, look for what is getting hot.

Or you jumper around the C/B and watch what smokes. (that's a joke)
 
Tom-D said:
That C/B is to protect the aircraft wiring when the buss is trying to draw too much. That normally occurs when the battery is trying to draw too much amperage to re-charge.

test the battery IAW it's ICAs, When it passes, look for what is getting hot.

Or you jumper around the C/B and watch what smokes. (that's a joke)
Click to expand...

All CB's are to protect wiring.
 
gismo said:
What exactly do you mean by "the alternator fuse"? I suspect you're talking about a circuit breaker but there's probably two of those, one for the output (30A-60A) and one for the field (2A-5A). If it's a CB, which one is it?

BTW, I don't think bad brushes will cause a CB trip and I suspect your A&P is confusing this symptom with an overvoltage trip (alternator goes offline and works again if you cycle the alternator switch).

If you're tripping the field CB (or blowing a field fuse), there are 4 possibilities: A shorted field winding, a wire in the field circuit shorting to the airframe, a bad CB (note that a loose connection to a CB can make it appear bad due to the heat generated by the associated high resistance), or a bad regulator.

If you're tripping the output CB the cause could be an intermittent short in the alternator diodes (unlikely), a short from the alternator output to the airframe, or a bad CB (see the note above about loose connections).
Click to expand...

The alternator field fuse or breaker only feeds the small relay in the regulator that turns the regulator on. The field current is actually taken from the alternator sense terminal (the "A" terminal on the regulator) and that isn't fused at all. Silly, but there it is.

Old airplanes usually have old breakers, and old breakers have oxidized contacts that create resistance and generate more internal heat than the breaker's bimetal strip was calibrated to trip at. Such breakers will pop at lower amperages than new ones. And if a guy crawls under the panel (difficult for an old, stiff guy like me, in a 150) and looks at the bus bar and breaker connections, he'll often see discolored crimp terminals on various circuits, especially the cable from the alternator and ammeter and landing lights. They carry a lot of current, they oxidize inside the crimps, they get hot, and that heat conducts into the breakers and fools them. I've seen melted landing light switches from that, too.

A loose ground connection at the overvolt sensor will cause all sorts of intermittent trips.
 
Those C-150 flap motors are known to get gummed up. If one were dragging, would it cause the CB to pop?
 
iflyvfr said:
Those C-150 flap motors are known to get gummed up. If one were dragging, would it cause the CB to pop?
Click to expand...

It should cause the flap breaker to pop, not the alternator breaker.
 
If the voltage is dropping prior or when the circuit breaker trips, does that mean that the alternator is not maintaining the voltage and be a sign it is failing ?
 
Dan Thomas said:
The alternator field fuse or breaker only feeds the small relay in the regulator that turns the regulator on. The field current is actually taken from the alternator sense terminal (the "A" terminal on the regulator) and that isn't fused at all. Silly, but there it is.
Click to expand...
That depends on the airplane/alternator. I don't know offhand if this it true for a C-150 but in many airplanes the sense line does not power the alternator field and the sensed voltage comes from the alternator output (either directly or via the main bus) not the aux terminal (common connection to the three stator windings). And I've never seen any system where the field was powered by the aux terminal, mostly because the voltage on that terminal is less than 60% of the aircraft bus voltage and that would significantly limit the available current from the alternator.

Old airplanes usually have old breakers, and old breakers have oxidized contacts that create resistance and generate more internal heat than the breaker's bimetal strip was calibrated to trip at. Such breakers will pop at lower amperages than new ones. And if a guy crawls under the panel (difficult for an old, stiff guy like me, in a 150) and looks at the bus bar and breaker connections, he'll often see discolored crimp terminals on various circuits, especially the cable from the alternator and ammeter and landing lights. They carry a lot of current, they oxidize inside the crimps, they get hot, and that heat conducts into the breakers and fools them. I've seen melted landing light switches from that, too.
Click to expand...
Agreed that when breakers age they can trip at lower current than when new and this is especially true of the kind you can't pull. For the push-pull type it's good to pull them at least a couple times each year because that causes the contacts wipe each other removing most of the oxidation when you do that. OTOH, properly crimped terminals do not suffer oxidation in the crimp because it's a "gas tight" connection. The discoloration you've see is more likely due to insufficiently tightened screws on the CBs, heat produced internally in the CBs under heavy load, and/or poor crimps.

A loose ground connection at the overvolt sensor will cause all sorts of intermittent trips.
Click to expand...

True but a loose connection anywhere in the alternator wiring can cause the OVP to trip.
 
Tom-D said:
Or you jumper around the C/B and watch what smokes. (that's a joke)
Click to expand...

I fixed TVs my first two years of high school. We had a tech in the shop that did just that. Bad technique? Yep. But his time to fix was half that of anybody else in the shop.

Jim
 
gismo said:
OTOH, properly crimped terminals do not suffer oxidation in the crimp because it's a "gas tight" connection. The discoloration you've see is more likely due to insufficiently tightened screws on the CBs, heat produced internally in the CBs under heavy load, and/or poor crimps.
Click to expand...

Theoretically, the crimp contact area should be gas-tight, but air still gets into the tiny spaces between the wires and causes oxidation that increases the size of each tiny wire, which tends to expand the crimp a tiny bit. Even the bit of heat generated each time the crcuit is activated can expand the copper enough to gradually loosen things up after many years and many cycles. The overheated crimps I've encountered were mostly factory installed, made by calibrated crimping machinery, but a long time ago.
 
gismo said:
For the push-pull type it's good to pull them at least a couple times each year because that causes the contacts wipe each other removing most of the oxidation when you do that.
Click to expand...

Do that with power off, maybe even battery disconnected, circuit breakers aren't meant to be switches.
 
bobkiksass said:
Yes it was popping the circuit breaker, and it is the 55 amp labeled "alt" I guess that is the output. Thanks for the info, the mechanic suggested he thought it could be a weak circuit breaker, and to test what the voltage is when it trips. Is this what you would do ?
Click to expand...

A fifty year old 55 amp breaker can just wear out. Just yesterday I was chatting about this very subject with a flight school owner who tells me that he's replaced several of these high amp breakers over the years. For one thing, the failure mode is that the ancient breaker doesn't move very much when it pops, unless you look closely you won't even notice it.
 
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