Ahh. It is to isolate the alternators from each other. Mostly so the alternator controllers can sense their respective alternator voltages and the outputs can have their own breakers and loadmeters.

Then they are summed to provide main bus voltage to the battery and general systems.

It is one way to do it. My current Columbia 400 has a different approach for dual redundant electrical systems.

That seems odd... alternators are inherently diode-output devices. What's the value add from that secondary puck diode?

Paul

Those switch contacts don't appear in the diagram posted above, is the diagram in error?



A shorted diode wouldn't prevent either alternator from providing power to the bus but as I mentioned, if one overheats for some reason it can fry the other one. Only an open diode would disable an alternator. That's assuming the diode doesn't short one or both terminals to airframe ground but that seems very unlikely without something hitting them.

The big diodes are very unlikely to fail, and I don't know of a case of it happening. They do get quite warm and the heat sink itself is hot (case of the diodes screw into the heat sink). I've never heard of one failing.

If one shorts - that side would still be able to deliver power, but the controller (aka regulator) would likely go out of the control range rapidly. There's about .9v drop across the diode, and the regulation point is higher than the desired bus voltage after the diode. I think that would lead to the system shutting down.

But it has been about ten years ago that I last looked at the electrical system on a PA34.

That dang ganged-multisection master switch bit me twice. The second time was the hardest and the field switches went intermittent and of course, who suspects a brand new part?

The III might be different than the II as well.

I did find a bit of wire out of SCAT tubing laying right on the bus bar when I was trouble shooting it.

I don't know if the posted schematic is from the POH or an actual wiring diagram from the shop manual. If it's from the POH it could indeed be "simplified" but you'd think even then it would include something functionally important like master switch contacts feeding the regulators. That said, turning off the alternators with the master switch is a common arrangement since without that the alternators would continue to power the bus without a battery in the circuit (big opportunity for overvoltage spikes) if you turned the master off while the engines were running and left the alternators on. In some singles that is prevented by mechanically forcing the alternator off when you turn off the master.


Since the regulators sense the bus side of those diodes, a shorted diode would just lower the field current on one alternator to keep the bus voltage in range. The only issue I can see is the control range of the paralleling connection might leave the two alternator currents unbalanced.


No worries, if the SCAT wire managed to bridge bus to airframe that wire would disappear in a brief flash of light and puff of smoke.

Yeah, I don't remember all the details - been a number of years since selling the plane. There's also the secret bridging wire between the regulators and I recall those old fashioned solid state linear alternator regulators seemed to have a very narrow lock-in range. Kind of like an open-loop opamp.

I remember that dang switch though.

That "secret" wire is the one going between the 'P' terminals on the two regulators. And yes it seems there is a limit on how much differential field current the regulators can manage but there's no reason the solid state versions couldn't go all the way to max field on one and zero on the other. The differential limit might be a deliberate attempt to mitigate a failre in one regulator from killing the output from it's mate.

In this particular setup, the paralleling connection simply raises the field current in one regulator when the other is calling for more. IOW, the regulator's field current is a sum of what the internal sensing calls for plus a smaller portion of what the opposite regulator is delivering. That will balance a small difference pretty well but not a large one.


It's funny how well we remember our more frustrating moments.

I have an update.

According the the mechanic, both alternator drive couplings failed. I'm trying to get my head around how that could possibly happen.

And because someone was asking, the schematic I posted is from the PoH.

Frank MacKenzie

Wow. Dual failures like this are very rare. Had to be something, I knew it would be interesting.

Thank you for posting.

Thank you so much for sharing the outcome! ..and glad you made it back safe and didn't have this happen in the clouds!

I had assumed that an alternator coupling was a binary device. That it either turned the alternator or didn't.

Apparently it is possible for them to slip? Thereby producing some power, but not full power.

Does anyone have more experience in this area? Visually, the coupler looks more like a vibration isolation device that should completely shear upon an alternator lock up. Do couplers actually have some amount of slip built into them?

Frank MacKenzie

I managed to answer my own question.

https://www.aircraftspruce.com/catalog/ ... 610-sb.pdf

Continental Service Bulletin SB11-3 step 3:



Are mechanics actually checking this?

Frank MacKenzie

So 15 ft/lbs to slip when new. And about 11.5 to slip after 25 hours.

And I'm going to assume (since there is a new vs. old) that the amount it takes to slip goes down over time.

And once again. When does a mechanic actually check this?

Frank MacKenzie

Before installing an alternator?

I'm puzzled by the rather low acceptable torque limit of 140 lbIn (11.67 lbFt). A 70A 14V alternator at 95% efficiency consumes about 1.375 HP or a little more than 45,000 lbFt/minute. At anything less than about 3900 RPM the torque needed to drive the rotor would exceed 140 lbIn. What's the gear ratio between the crankshaft and the alternator?

I had my coupler slowly die on my Turbo Arrow's TSIO360. Alternator died a year prior with < 1000 hours. New alternator but coupler not replaced. A year later symptoms that showed up were occasional bus voltage drop back down to battery voltage followed by recovery to alternator normal voltage. Over time frequency of occurrence increases. Eventually, only battery voltage on startup, recovering within a minute to normal; then two minutes. Power cycling the field is placebo; by the time it's done, enough time has passed for the heat of the engine to lessen coupler slippage. Damn hard to diagnose. Lesson learned: just replace the coupler with the alternator.

DJ