Pre-buy inspection

[Andreas Riedel]

[Zach Grant L1011jock]

[MULEFLY]

[Andreas Riedel]

[Andreas Riedel]

[MULEFLY]

[Kristin Winter]

Jim's description of the system is a good one. I would add that when an alternator fails on the newer system, it is much less likely to take the other alternator off line. Not so the older system. Over the years, I have had a number of incidents where an overvoltage took the entire charging system off line, instead of just one side.

After the landing gear rebuild, I am going to modify my '69 Twinkie to have a modern paralleling system.

[N3322G]

Andreas,

I'm fortunate to have a 39 with the separate alternator system - works great. One alternator runs my whole panel just fine - over 39 years we've had occasion to experience this.

Hope your friend joins us in the joy of Comanche ownership.

[Kristin Winter]

Pat,

What do you mean by separate alternator system? Have you modified your plane to have a paralleling system instead of the MAIN and AUX voltage regulators? I am interested in that, so I am curious whether this was an STC or a field approval that changed what came from the factory.

[N3322G]

Kristin,

Remember I'm a pilot but hold no awards for aviation mechanical type stuff so terminology may be the issue - no mods to our electrical system - just trying to assure the potential buyer that the 39's stock electrical system has backup advantages with alternators as well as primary and aux voltage regulators (although I have replaced STC'd regulators with 'like' functioning units.)

Still, we do keep a freshly charged 496 on board ... just in case.

[Kristin Winter]

Pat,

Bummer! I was hoping you had converted to a proper paralleling system. The MAIN and AUX regulator system a poor one, particularly considering the ammeter is so lame and will not likely give adequate warning that your system has kicked off line.

Ironically, there are STC's to replace generators with paralleling alternators, but the same kits are not STC'ed to replace non-paralleling alternator systems with paralleling alternator systems. I need to get on Plane Power's case about that. They could likely expand their STC to cover.

I will be in your neck of the woods toward the end of the month.

[15384]

Kris: No matter if you have the original genrators or the new modern alternators. They are feeding a common bus and if they are both working they are in fact feeding this bus at the same time. Thus they are in parallel. This is an easy thing to accomplish since the system is DC. Keep the polarities the same thats how easy it is. This is what we want. With AC it's a harder thing to accomplish. The only way this could be any different is if the planes DC panel had seperate buses and one alternator or generator fed one bus at a time. This is possible but to be practical with this type system one would need a bus tie which would be either a seperate circuit breaker or switch that would connect the two buses should one or the other alternator or generator fail. You could secure the failed alternator or generator by turning its toggle switch off, then close the bus tie and feed both buses from one alternator or generator. Unless your PA39 is different than the 30's you don't have this.

Ammeters have never impressed me as a warning device to signal a voltage failure and it is a voltage failure or lack of that causes trouble first before a lack of amps or too high an amp draw due to low voltage. Especially the ammeters that came stock with our planes. Best way to tell if you have a power failure is to have a digital voltmeter and include in your meter scan every so often a test of the alternators or generators by turning one off at a time and watch the voltage of the one left on. If it is 13.7 or better at cruise speed or at least above 1500 RPM it's working unless you have so much draw that the generator or alternator doesn't have the capacity to produce that much current to run the electronics or the wire feeding the system is too small. But within reason who ever installs addtional electronic gear from the original avionics should keep an eye on the current draw for all the toys and if needed install a larger alternator or generator and if needed larger wire feeding the bus.

I have noticed on my old Trimble GPS it has a message that pops up when system voltage is too low. I have generators and at lower than 1500 RPM like when waiting at the hold line and full electronics on I will sometimes get a message pop up saying low voltage. Depends on what shape my battery is in.

I may be wrong but if I remember right the generator system has a paralleling relay. I have since changed to Zeftronics voltage regulators. This relay works to bring the two sources together. If for some reason one or the other generator fails and stops producing voltage it will no longer energize it's side of the paralleling relay thus preventing one generator from feeding the bus and from back feeding into the other causing either a dead short through the brushes into the armature which would then blow the good one up and or motoring the other generator which would eat up a bunch of current limiting what was available to the bus and the planes electronics which again is no good.

This could be way off and thee are some diodes somewhere in the system preventing a generator from being back fed. But a DC generator is also a DC motor they are both the same creature meaning they do not have diodes inside them to prevent being back fed. Alternators do have internal diodes to prevent being back fed into so there is no need for a paralleling relay. This comes into play if you have one engine running and the other engine stopped or if one fails.

I may have a detail or two off a bit here with my explination. I troubleshoot and repair much better.

If there is some new fancy system out there that you come across let us know. It would be interesting to see how it works.

John

[Kristin Winter]

Paralleling refers to a circuit between the two voltage regulators, each voltage regulator running its own alternator. The problem with left and right voltage regulators is that both alternators need to be putting out the same exact voltage, or one alternator will be doing all the work. The paralleling circuit, keeps the two alternators at the same voltage. This system is referred to as a parallel system, as you have two voltage regulators, with their over voltage relays, running in parallel.

Piper decided to try using just one regulator to run two alternators. Then they had an AUX regulator in case the main failed. The problem is that an anomoly will then kick off both alternators, instead of just one, in a parallel system. If the system kicks off, and the pilot doesn't notice it, the battery will go down to the point that the master relay will open, any you are then out of luck, as you can't get your alternators back on line, no matter what you do.

[Andreas Riedel]

Thanks for the great advice to all of you. In the meantime the turbo was sold and he decided to go for a turbo. So if there is a good plane out there please let me know.
Andreas

[Andreas Riedel]

So far I had two total electrical failures on my generator equipped PA 30 whereby the battery slowly drained till there was nothing left.
The first item leaving was the trimble GPS.
Was VFR clean weather and close to home on a long Int. flight so no problem. For this reason I have kept my old Garmin 100 with battery back up and a handheld on standby.
Should not happen but it did.
Andreas