Persisting alternator problems

[chris mitchell]

I previously posted on an alternator problem (http://www.vansairforce.com/community/showthread.php?t=46156&highlight=alternator+problems).

I'm using a PlanePower alternator wired as per instruction with a 5 amp breaker on the field circuit.

I thought I had cured by replacing some of the wiring. THe wire from the field switch to the alternator had a small nick in it - tho I could never really see how it could short because of the way it was loomed and protected from chafing... Anyway my problem returned and as ever is intermittent, variable and irritating.

For today, I started with master at "bat" then turn the field on - breaker blows. Master to off, reset breaker, master on to bat then field - all is fine. Add radio, lights etc - ie increasing load - no problems. Come to power checks - breaker pops when one mag goes off. Finish power check, master off reset breaker master on - no problems, flew home (35 minutes) no problems, nor any problems with post flight mag check....

I really don't see how the mags have anything to do with the alternator load.

I am now wondering if the mag circuits can induce voltages in the (adjacent) field circuit and breaker?

How many folk have ended up using a eg 7.5 amp breaker for the field circuit I wonder?

Thanks for any insights.


Chris

 

[David-aviator]

Chris,

There is no electrical connection between the mag switches and the alternator. The mag switch circuits go to ground at the mag and nothing else.

The fact that checking a mag causes the alternator field CB to trip indicates there IS an electrical connection between the two. The problem may be with the mag switch wiring. I can't imagine how that could be, but something clearly is not right. Perhaps the rpm drop is what the alternator does not like.

With regard to alternator field CB size, 5 amps has always worked for me. I have the same alternator and it has worked since day one on 5 amp circuit protection.

A possibility exists that the alternator itself is defective. It has been established, alternators do not like being switched off and on with a field switch. It is much better to leave it on always, or remove the switch from the system. You stated the system worked fine for 14 hours. If you have been using the alternator field switch with every start, it may have damaged the alternator.

 

[hgerhardt]

Let's figure out what we know first...

1. Plane Power alternators use a crowbar overvoltage protection device to blow the field breaker when it senses overvoltage. The crowbar simply shorts out the field terminal to ground which blows the breaker.

2. The mag switches short the P-leads to ground when turning the mags OFF. I'm not certain, but I'm fairly sure that the P-leads carry significant voltage spikes when the engine is running, since they're connected to the Primary side of the ignition coils.

Understanding how the two items above work, I can see one of two possible things happening here:

1. There is somehow a connection between one of the P-leads and the field wire, maybe through chafing, causing the field wire to short when the mag switch gets turned off, or...

2. The builder didn't use shielded wire for the P-leads and there is capacitance or inductive coupling between the P-leads and the field wire, which the crowbar senses as an overvoltage condition and thereby blowing the breaker.

I'd guess #2 is more likely, but either way, make sure the P-leads are shielded and also routed separate from any other wiring.

If you swap out the 5A breaker for something bigger, you'll just overload the crowbar assy and possibly cause it to fail when it triggers. Oh... then as a third, but unlikely possibility: maybe the crowbar is defective and overly sensitive to voltage transients.

Heinrich Gerhardt
RV-6

 

[rv9av8tr]

OV sensing

I'd be most suspicious of the overvoltage sensing, causing the breaker to trip.