Need help with alternator theory

[paulsteger]

I have a 6a that I bought used 3 yrs ago and recently found out if I turn off the master switch with the engine running, the main bus still has power and all electrical stuff stays on. Not sure how long it?s been like this because my usual shutdown sequence of electrical, mixture, master, mags doesn?t detect this. Here is what I know. It appears to be wired like Van?s sample wiring diagram, except it has no separate voltage regulator, so regulator must be integral to the alternator as car alternators have been for many years. Looking at the schematic it looks like when you shut off the master, the master relay opens up which causes the + side of the battery to disconnect from the master bus. Meanwhile the alt side of the spit master switch disconnects the alternator field from the bus. Without 12 volts to supply the alternator field, the alternator output should drop out, so it also no longer supplies the bus, so bus should go to 0 volts. Mine doesn?t. My master relay does drop out as it should. I can hear it clunk at a low idle. The alt side of my master switch does open up and drop +12 to the alt field. If I start the engine with the field wire disconnected, then the master does shut off the bus, but the battery is discharging with the engine running. So it appears to me that once the field is excited as in a normal start situation, the alt keeps supplying output as long as the engine is running, even if the field input loses its 12 volt input.the alt does charge the battery and supply the electrical needs as it should.
I removed the alternator to have it tested by a car parts jobber. It has no identifying data plate. What company made it? Car place couldn?t test it because he didn?t have a field connector to fit it. It has 3 male spade lugs in a U shape inside a round cavity. Is it from Vans? Lycoming? A paper barcode sticker on it gave no google hits either. I?m grounded till I get this fixed

[MCA]

Hmmm... looks like you did a thorough checkout. Things don't add up.

The only thing I can think of is that the alternator has some type of internal short that causes it to feed itself power. Normally this type of condition leads to an overvoltage condition, but perhaps there is a different failure mode where it still regulates properly.

I recommend replacing the alternator, and please let us know what you find.

[cfiidon]

I am certainly no alternator expert, but could you perhaps have a "self-exciting" alternator? I understand that above a certain RPM they self excite and are sometimes referred to as a "one-wire" alternator.

[DanH]

Good chance your "field" wire isn't a field wire. I recall prior alternator discussion describing some auto alternators as having what can be described as a "wake-up" wire rather than a true field supply for their internal regulator.

Old school alternators supplied wake-up current through the charge light. The alternator was self-exciting after output voltage went above battery voltage as supplied through the light. Same idea, separate wire. Good description with schematic in the old Bosch electrical manual.

[rv8ch]

Originally Posted by paulsteger

...
I removed the alternator to have it tested by a car parts jobber. It has no identifying data plate. What company made it? Car place couldn’t test it because he didn’t have a field connector to fit it. It has 3 male spade lugs in a U shape inside a round cavity. ...

Welcome to VAF. I defer to Dan's superior knowledge, but I think you may want to find a different shop.

Can you post some photos? If you can find an alternator to replace yours, might be quicker, and then you can play with this one on rainy days or during your upcoming weeks of "social distancing" in your house.

If you have some time and want to have some fun, take a tool bag, good shoes, and gloves to a junkyard - lots of cars there with every kind of connector you can imagine. They may also have a good alternator you can buy cheap. It's one of my favorite places to go I have to confess.

[Larco]

DH is correct regarding some auto alternators and the IR 60A that Vans sold before the PP was offered did just that. It was hardly ever noticed because most shut down the engine using mixture so the alternator had stopped turning when turning off master. Try this: Start the engine with field CB or SW out or off, Alt will not charge, activate the field and it will charge, deactivate the field and it will keep charging until the alt stops turning. It was an auto alt that was designed that way who knows why? Because it was noticed at the local airport on a couple of RVs a few years ago we experimented and this was found to be the case. BTW these alternators worked fine and the pilots started to shut down using mixture first, problem became a non issue.

[climberrn]

Originally Posted by rv8ch

Welcome to VAF. I defer to Dan's superior knowledge, but I think you may want to find a different shop.

Can you post some photos? If you can find an alternator to replace yours, might be quicker, and then you can play with this one on rainy days or during your upcoming weeks of "social distancing" in your house.

If you have some time and want to have some fun, take a tool bag, good shoes, and gloves to a junkyard - lots of cars there with every kind of connector you can imagine. They may also have a good alternator you can buy cheap. It's one of my favorite places to go I have to confess.

I?m sorry but I have to disagree. My engine came with one of the automotive alternators and it had a 3 pin plug also. Had an over voltage incident a few years ago. Like yours, mine did not shut off after it was energized. Theory was some diodes failed internally. The reason doesn?t matter as much as what happened. Voltage spiked over 100 volts per EFIS manufacturer. I was unable to shut it off. It quickly burned it self out and produced a puff of electrical smoke. ALL electronics went out. EFIS, radios, audio panel, LEMO powered headsets.... the only thing still on was the engine with dual PMags and my iPhone running ForeFlight.

At 10,000 feet starting to cross the sierras with my wife, this really gets your attention.

The engine ran fine to the nearest airport. Landed, no flaps, with the fan still turning but it quit on rollout. The PMags quit producing their own power below a certain RPM. 800?? Sending most of the panel, PMags and ancillary boxes out for overhaul and leaving the plane 4 hours from home for over a month....

Yes you could go to a junk yard and get a connector. Get a ?lifetime warranty? automotive alternator from your local auto parts store. Many people have. I bought a B&C with the automatic, external over voltage protection. Yes, I may have a failed alternator in the future, but I NEVER want to be in the position where I cant de energize a malfunctioning alternator again. Good luck.

[lr172]

Many IR alternators feed the voltage sense circuit from the B lead. With this type of alternator, it will self-excite once it is producing output. Even if the alt has a 12 volt input pin, doesn't mean that it isn't bridged to the B circuit internally once running. Most auto installations have no need or desire to shut down the alternator with the engine running. I believe wiring in a discrete voltage sense circuit is one of the modifications done to the std alt by B&C and planepower to allow shut down.

Larry

[PerfTech]

....I thought I should chime in here, and make you guys aware of my experience
with this issue, and alternator operation modes! I experienced a very frightening
as well as costly scenario, involving just what this discussion is addressing. So I
thought it only prudent of me to pass on this information. I was cruising along at
about 14,5 (long time from ground) and suddenly smelled burning insolation. Then
cockpit instantly filled with smoke. My first reaction was to get rid of all electrical
power in hopes the problem would go away. I shut off the master, and no luck, it
just instantly got worse. Instead of the panel going dark, it lit up like a flash camera,
blowing out several lights, and worsening the smoke situation. Suddenly I was
painfully aware that I had no control over this runaway electrical situation. My digital
volt meter was flashing over 119 volts, and smoke was almost unbearable. How can
this be, with master and alternator off? The alternator was still putting out voltage
(un-controlled) because the engine was still turning. I shut off the engine, pulled
the nose up to almost a stall and stopped the prop. The volt meter went to zero,
I opened the vents and the smoke situation began to go away. I established a glide,
collected my thoughts on how best to proceed with my dilemma. I had lots of altitude,
so considerable time for planning Etc. I had plenty of airport options, including home
base so home it was. The prop started wind milling so in a test, I switched on the
mags, starting the engine. The volt meter stayed on zero, and no smoke to the
uneventful landing. After a trip to the rest-room, and a change, I spent the next
few days / weeks, and considerable funds figuring out just what had transpired here.
The problem began with a wire coming out of a connector with a bad crimp, grounding
against a Bowden cable and heating up. This in itself wasn't that big of a deal. The real
serious issues started when I turned off the master. With this type of alternator that
once excited, will continue to produce current with field wire voltage removed.
Removing the battery from the equation, removes the regulator's ability to sense the
voltage it is supposed to control. It thinks the battery is dead, so it runs wild and
producing all it can to fix the problem. The subsequent massive voltage surge took
its toll on the panel, killing the radios, electric gyros, virtually all the Vans china gauges,
and all the lighting that was turned on as well. My permanent fix that will assure this
can never happen again ? I used another heavy duty continues hold relay to divorce
all alternator wiring from the system when turned off. The alternator battery wire,
is switched via this relay. The diode protected alternator switch now controls this relay,
thus totally disconnecting all electrical to the unit when switched off. This experience
was very frightening, humbling, and enlightening. It all happened so fast, and snow-
balled. I was not prepared to say the least, and the outcome could have been disastrous,
had luck not played some part. I hope this post can prevent another from the same,
or worse. Thanks, Allan.......

[climberrn]

I love your solution Allan about the additional relay. With the external OV protection I have I don?t think it is necessary but that would allow the ability to safely run an internally regulated alternator.

On a side note, any wires in the plane (large gauge, or main wires to CB panel) in addition to the crimp I placed a short section of marine heat shrink with the adhesive. In the event a wire was not crimped properly, the heat shrink ?should? prevent it from coming apart and shorting out. Just some added insurance.

Sorry for the drift.