Alternator issues

[201Steve]

The instructions are right there on your schematic. It is a stupid simple install. F1 to VR. F2 to ground. Output. That’s it. 
 

the other of the 5 wires you refer to are probably shielding grounds. The output and the the field wire both have shielded wire. Each end of the shield has to ground in a nearby location. 
 

and then, at least on mine, the ground “post” (the casing of the alternator) also has a jumper leading to the engine block for a solid ground all around. 
 

there’s your 5 or 6 wires. 
 

 

[Ragsf15e]

Approaching this first annual on my new 252, I was trying to figure out why I had so much blowby on my belly (tip to tail).  I traced the tubing from the air oil separator and realized that the return tube was routed to the belly as well as the breather!  The other end where the return should have been connected on the dipstick tube was left completely open which was added pressure in the case (it’s on top in the higher pressure area above the cylinders).  I showed it to my mechanic the first time he saw the airplane and thought he would have some choice words for the folks who had installed the engine and did the two annuals after that (or the owner who didn’t care about his oil soaked belly), but he said, “sometimes s$!t happens. Everyone has a bad day.”

My belly is so much cleaner now though….

Now back to the mysteries of electricity.

[1980Mooney]

For the Hartzell alternators, here are probably the best links for Operating Manual and Troubleshooting. The Operating Manual is for the 28 volt models but provides all the basic description that applies to 14 volt systems. 

No one will publish the Amp rating on the Aux terminal because they don’t want you to use it for anything other than a gauge or diode connection.  So I was likely wrong about “85% of rated” nameplate but it is still enough to burn things up as the OP described…

https://hartzell.aero/wp-content/uploads/2014/12/ES1031-Alt-Owners-Man-Rev-New.pdf

https://hartzell.aero/top-tips-for-troubleshooting-your-aircrafts-alternator/

 

 

[Vance Harral]

1 hour ago, 1980Mooney said:

The Aux terminal is internally tied to common connection of the stator (output) windings and will normally provide about 85% of the main output voltage.  Your A&P just wired 85% (about 50 Amps) directly to ground on that braided wire sheath. 

Ironically, this statement also indicates a fundamental misunderstanding of operational principles.  Voltage is not current.  How many amps might have flowed from AUX to ground in this mis-wired installation depends on the load through the complete circuit, as well as the ability of the center point of the stator wye to actually supply current.  It is extremely unlikely that 50 amps of current every flowed in the circuit, the "burning smell" Grant describes not withstanding.  @PT20J says above this is a "low current" output.  Not sure how he knows that, but there is no one on the forum better informed about the movement of electrons than him.

The thread you linked to above is mine, and my mechanic made exactly the same mistake as Grant's.  Your own confusion about the operational principles, as well as mine (I am a degreed electrical engineer), as well as an identical mistake by two entirely separate mechanics, and the presence of multiple threads asking how all this works, means that the problem is the design and documentation of the system rather than the humans maintaining it.  It is apparently impossible to find a manual specific to the Hartzell ALY-8520 (I had the same trouble doing so as Grant).  Said alternator relies on a case ground which is not shown in the Mooney schematics because it's "implicit", but this is easily confused with the shield termination for the shielded wires in the system.  The Mooney schematics also just ignore the presence of an AUX terminal on the alternator rather than explicitly marking it N/C, which is counter to good design principles.  So what we have here is actually bad design/documentation, and it's no wonder that even well-trained, competent people are confused by the system.  The mistake by Grant's mechanic is entirely forgivable, and accusations of incompetence in making it are weak.

For what it's worth, our airplane was ground run for several minutes in the mis-wired configuration with the AUX terminal grounded.  Once this mistake was corrected, the alternator began operating normally, and has continued to do so for the entire 5 years and several hundred flight hours since I made my post.  I can't say whether Grant's alternator may or may not have been damaged internally, but my own experience shows that this mistake is not necessarily immediately catastrophic.

 

[MikeOH]

I, too, am a degreed and recently retired EE.  @Vance Harral is exactly right.  This represents crap documentation and is begging for an error.  Holding an owner responsible for not being able to pick an A&P that knew all this is ABSURD!  Sure, ultimately the owner may end up paying for the mistake but it's pathetic to consider that a 'just' outcome based on some twisted logic that the owner is always responsible. GMAB!

Rule of thumb for all electrical devices: Be VERY careful of hooking a ground up to ANY terminal post that is ISOLATED from the case of the device.  A true case ground will NOT have any insulation/isolation material (i.e. non-conductive) involved: the post/terminal will thread DIRECTLY into the metal case if it is ground connection.  You can see black/white colored material (insulators) on the AUX post in the photo of the back of the alternator; that's a flag that that post is NOT a ground.

[PT20J]

I certainly don't blame the owner. Owners should be able to take their airplane to a shop and trust that the mechanics know what they are doing. But, sadly, this isn't always the case. And, in my experience, some A&Ps are pretty weak on electrical system knowledge. I agree that the schematic isn't readily interpretable by someone unfamiliar with alternators. But, as I said above, externally regulated alternators have standard connections. 

I'm sure the mechanic learned something and won't make this mistake again. But, since it was a mistake, if it were my airplane, I would expect the mechanic to replace the alternator. The warranty is most likely voided by the incorrect installation.

The fact that the ALT breaker tripped means that a lot of current went through the alternator. The purpose of that breaker is to protect the wiring between the alternator and the bus in case of a fault because the battery is connected to the bus. The breaker is sized to carry more current than the alternator is capable of supplying so that it will only trip if there is a fault that would cause the battery to feed current toward the alternator.

[Grant_Waite]

Thank you all for the replies and input. I did read Vance’s post and thought his situation was exactly what happened to me. A simple mistake but can cause a big headache. I find it odd that some alternators have a designated ground post and others have no marking of a ground post like mine. I guess some alternators ground through the case like mine and some have a post for it. The hole for the screw is just to the right of where that aux pole was so if you’re not paying attention I guess it could happen. I don’t remember but I want to say my old alternator didn’t have a post where it said aux on the back, it was just blank. My plane doesn’t use it so it wouldn’t make sense to have it there anyway. When they overhauled it they obviously put one on it not knowing what plane it was. as I didn’t give them that information. That’s my theory as to how this happened and just carelessness. The mechanic is mobile so it’s convenient to have him and the other A&P do most of my work. They are both very competent but competent people make mistakes. We see that in high profile aviation accidents all the time. The situation will be corrected one way or another and it’s not going to be on my dime if the shop doesn’t want to warranty something that wasn’t their doing. Just still highly frustrating and yes,  I probably should have just got brushes and called it good. I figured it being 12yrs old, it could use the extra tlc.