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Alt field circuit breaker popping - help!

Rick RV-4

Well Known Member
After about 9 hours in my new RV-4, my battery started getting weak, and I noticed my 5A ALT FIELD circuit breaker started popping a few times during the flight. I have a 60A Plane Power alternator wired per their and Bob Knuckoll's plans. After a reset it would stay set for 10-15 minutes. Not sure what the problem was, I looked down at my Odyssey battery after my flight and noticed the plastic around the red positive terminal was slightly melted - not good!.

Thinking I might have a bad battery, I replaced the old battery with a new Odyssey and went flying. All was well on the first flight (no issues), then on the second flight the same thing happened - the ALT FIELD CB popped. I reset it a couple of times, then left it popped and returned to the airport.

I pulled the cowl, and the only thing I saw wrong was some oil in the alternator - the oil came from when I pressure oiled the engine (before my first engine start) and made a mess when I disconnected the plastic oil line I was using on the top front of the case. I'm guessing that up to 1 or 2 tablespoons of oil might have gone into the alternator. I cleaned up what I could but I'm sure I only got about half of it.

Is there any chance this left over oil could be causing my problem? My engine monitor is showing 14.5 volts (when the CB is in) and 12 volts when it is popped, so it doesn't seem as if my voltage regulator has gone bad. I don't see any chafed wires or other obvious problems.

Any suggestions?

Thanks,

Rick
 
Hi Rick...

The OV part of the Regulator will function by short circuiting the field and blowing the fuse / popping the CB. We had this with the Plane Power 60A Alternator in test flying, PP replaced the regulator (but found no fault) and never had problem again. Dynon data, recorded a few times every second showed no voltage spike prior the fuse blowing.

Somebody else I know (who will probably read this) had a similar thing, and redoing some wiring I think solved it.

HTH?
Andy
RV-8 G-HILZ
 
Read with interest, yes I had exactly this problem. I replaced the breaker - no improvement, and replaced all the wiring for the field circuit. Problem solved I thought - and there was a very small nick in the insulation of one of the wires, tho not in a location where I could imagine it having any effect....

Since then I have had the field breaker pop a couple of times, but it always rests easily (alternator field OFF, reset, field back on). It sometimes has happened when testing mags preflight. I don't really understand how that can be and I have wondered about voltages induced in the field circuit wiring by adjacent wires in the same bundle. I have no idea if that notion has any foundation - tho no doubt there are plenty of others on here with the knowledge to comment.

At this stage I have not chosen to change the alternator since the problem is so intermittent and I am always within my batteries range of several airfields if a problem becomes persistent - I'm planning to fly some more hours (currently at around 45 hours) before making that decision.

Chris
RV8 - flying!
 
I might be that someone else

I had a problem with my fuses blowing, too. I use the ATC type fuses. I looked at everything and ultimately sent my alternator back to Plane Power, they inspected it and said nothing was wrong but replaced the voltage regulator anyway. When I reinstalled it, I made sure I connected everything first but only connected to the battery last. Haven't had a problem since. When the fuse blew, I was showing about 11.4 volts and 14.4 when all was fine. Don't know if that helps but you might consider sending it back to Plane Power for a look.

Bill Near
 
Wait a minute guys - it's the FIELD circuit breaker that's popping right? I'm wondering if you got some oil on the alternator brushes and they are no longer making good contact, thus limiting the OUTPUT capacity of the alternator. The regulator will sense a low voltage condition and drive the field current high to try to boost the output of the alternator - but the current can't get out of the winding through oil-soaked brushes. This will result in a continued low-voltage condition and will drive the field circuit to full current and pop the breaker.
 
When this happened to me on my RV-4, it turned out to be a broken lead to one of the rectifier diodes within the alternator. I figure that caused it to need more field current and also may have caused enough voltage ripple to start triggering the over-voltage limiter.
 
Wait a minute guys - it's the FIELD circuit breaker that's popping right? I'm wondering if you got some oil on the alternator brushes and they are no longer making good contact, thus limiting the OUTPUT capacity of the alternator. The regulator will sense a low voltage condition and drive the field current high to try to boost the output of the alternator - but the current can't get out of the winding through oil-soaked brushes. This will result in a continued low-voltage condition and will drive the field circuit to full current and pop the breaker.

The rotor, which is driven through the slip rings/brushes, is the field winding. The stator provides the charging current. If oil caused poor contact between the rings and brushes the regulator would definitely ramp up the voltage to the rotor to drive more charging current, but the oil would prevent field current from flowing - so there wouldn't be a trip in this state. If contact were then made - due to the oil film breaking down - with the voltage ramped up to full, you would get a spike of full charge for a brief moment until the regulator brought the voltage back down. This is similar to a "load dump" - turning off a heavy load suddenly can cause the alternator to provide a brief surge, resulting in a voltage spike. This could perhaps cause a trip due to the surge.

I don't think oil would cause this problem for long, though, unless the alternator was continuously bathed in the stuff - there is good wiping action between the brushes and rings.

One other scenario to look for is a loose connection somewhere in the high current charging circuit, all the way from the B lead of the alternator to the battery itself. If this opens even for a fraction of a second the voltage output of the alternator will surge past the high limit, causing the field trip. It's a definite no-no to run an alternator without a battery connected to its output, because the battery acts somewhat as a load leveler, absorbing surges from the alternator without letting the voltage rise to a dangerous level. I've had automotive alternator regulators fail in the full-on (max charge) state, and didn't see an overvoltage condition for quite some time because of the battery absorbing the excess current. However, this condition quickly ruins the battery, usually by boiling off the electrolyte, and eventually there IS an overvoltage condition.

So, barring an outright regulator fault, checking the alternator wiring/charging circuit carefully for loose connections or other irregularities might find the culprit.
 
Alt Field cb pops with engine OFF

I have had the same type of problem as Rick. The Alternator field cb began popping in my RV-10. At first this happened only occasionally and it was easy to reset but on the last two flights the problem has grown worse. Two days ago I had to try multiple times to reset the alternator field. Also, twice now the alternator field cb has popped with the engine OFF. I live in Brazil (Amazon Region) and must call the tower to obtain permission to start the engine. When I turn on the avionics the cb pops out even with the engine off. I would appreciate help on this. I often fly long distances with very few airports in between takeoff and landing. I always fly with a handheld GPS and com radio as a backup and never fly IFR or night, but, even so, I need to fix this problem asap.
 
Plane power has instructions for inspecting the brushes. The document is under "Continued air worthiness" at the top of the home page.
 
I have had the same type of problem as Rick. The Alternator field cb began popping in my RV-10. At first this happened only occasionally and it was easy to reset but on the last two flights the problem has grown worse. Two days ago I had to try multiple times to reset the alternator field. Also, twice now the alternator field cb has popped with the engine OFF. I live in Brazil (Amazon Region) and must call the tower to obtain permission to start the engine. When I turn on the avionics the cb pops out even with the engine off. I would appreciate help on this. I often fly long distances with very few airports in between takeoff and landing. I always fly with a handheld GPS and com radio as a backup and never fly IFR or night, but, even so, I need to fix this problem asap.

I can't help you with the circuit breaker problem, but I will warn you of resetting circuit breakers repeatedly! That's not a good idea! The breaker trips due to a faulty circuit. The amp load that trips it is according to the anticipated load that circuit should carry. Resetting breakers can, and has lead to a fire.

I flew for AA for three decades and their policy changed from resetting the circuit breaker at the Captain's discretion, to NEVER reset a breaker in flight!
A MD11 crashed because the circuit breaker for the cabin entertainment system tripped and was repeatedly reset.
 
I can't help you with the circuit breaker problem, but I will warn you of resetting circuit breakers repeatedly! That's not a good idea! The breaker trips due to a faulty circuit. The amp load that trips it is according to the anticipated load that circuit should carry. Resetting breakers can, and has lead to a fire.

i second this, was in an aircraft with a circuit breaker that kept popping, didn't know the crew had resorted to simply resetting it before each flight and then holding it shut, they did note that it would get hot and "stick" so they assumed it was a failing CB

yeah, so smoke and fire later, that mystery was resolved, faulty wiring not a faulty CB
 
I would add to the above posts. As a general rule you can reset a breaker once. If it pops again its telling you something. Fuel pump breakers should never be reset in flight.

George
 
The PP alternators will draw a full 4-5 amps on the field with a heavy load or low battery (I've measured it). Using a 5 amp breaker in this circuit is marginal at best, most breakers are +/- 20% tolerance so the problem is obvious. What would I do if it were mine... assuming the wiring is of the appropriate gauge, install a 7.5 amp breaker, it will still provide the required protection and eliminate the nuisance pops.
 
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Alternator circuit breaker popping

My RV7 started popping the circuit breaker every now and then, but it got progressively worse. My panel has self resetting circuit breakers and when the alternator field popped the CB it took between 10 and 60 sec to reset. Resetting the popped circuit breakers is not the answer as the CB pops for a reason.
I run a Plane Power alternator which I took out and had tested at a local electrical shop. We could find no fault, but having read up on the Plane Power alternators I chose to exchange the rectifier. We tested the alternator with the new rectifier and it was the same as before showing no different. I reinstalled the alternator into my RV and have since done 10 hours without the circuit breaker popping. I like to assume that one or more diodes on the rectifier were faulty which caused the CB to pop.
It seems to be a common problem on the Plane Power alternators that the diodes on the rectifier start to go and then the CB pops.
 
This makes sense. A bad diode will cause a missing phase in the alternator output, which reduces the average voltage of the alternator.

The regulator corrects for this decline in average voltage by increasing the field current which increases the voltages of the remaining phases. The average can remain the same, but the peak voltage increases. If the peak voltage is high enough, the alternator over-voltage protection will trip the field breaker.

What kills a diode? Two things: ambient heat and current (self heating). Despite PP's instructions, aim a blast tube at the diode array (vents in the rear) and put a heat shield in between any adjacent exhaust pipe and the alternator. That helps reduce the ambient temperature.

As for self heating caused by high currents: This happens if you have a dead or discharged battery, particularly at start-up. Make sure your battery is fully charged before starting and AND NEVER JUMPSTART! Jumpstarting will still leave the battery flat and the alternator will be running at maximum output and minimal cooling for several minutes after startup. Good way to melt diodes.

VV
 
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