262 Dual alternators

[geoffb]

Does anyone have any documentation for the install of the belt drive alternator.  All I’ve got in my plane documents is one bad copy of a schematic from the Mod Squed.  If anyone had the STC package, I’d appreciate more details.  Second alternator is acting up and most of it is buried up behind the electric panel and I’d rather know what I’m supposed to be looking for/at before I approach that rat’s nest.

I’ve got quite the stack of documents covering everything, except the dual alternator mod.  Am I correct in assuming that was an STC?  It’s not included in any part of the 261 conversion package documents.

[Aerodon]

I have the 252 electrical drawings, the dual alternators were standard.  I'll try take some pictures of the relevant sections. 

 

Aerodon

[milotron]

There is not much sophistication to the dual alternator installation. The belt drive uses a voltage regulator same as the gear drive and both connect to the main bus at that same location. They have separate field switches but that is it. They rely on the fact that the belt drive unit spins quicker so it tends to carry more load and become the primary unit for this reason, although they will both split the load to some degree.

 

Aerodon is correct, the second alternator is part of the firewall forward upgrade, but i have seen a couple of 252 with single alternator, although they are not the norm.

[carusoam]

Hmmmm…

There are many second alternators being used from one particular company… B&C

See if they have what you need…

https://mooneyspace.com/search/?q="Second alternator"&quick=1&updated_after=any&sortby=relevancy

Fuzzy memory…..

-a-

[geoffb]

2 hours ago, milotron said:

There is not much sophistication to the dual alternator installation. The belt drive uses a voltage regulator same as the gear drive and both connect to the main bus at that same location. They have separate field switches but that is it. They rely on the fact that the belt drive unit spins quicker so it tends to carry more load and become the primary unit for this reason, although they will both split the load to some degree.

 

Aerodon is correct, the second alternator is part of the firewall forward upgrade, but i have seen a couple of 252 with single alternator, although they are not the norm.

Thank you for that.  Following Don’s input, I looked at the service manual schematics for the 252.  Looks like they used a NLA regulator that had a lead connecting them, I assume to manage the load between the alternators.  The VR802.

from the one drawing I have, looks like the second alternator on my 14v setup just uses a second VR415.  I read somewhere about the two regulators being set to different voltages with alt #2 being set higher so that combined with the faster rotation would have it picking up load first.  My JPI seems to agree with #2 running 14.5 or more and #1 running 13.6-13.8, when running alone.

Issue I’m having is alt #2 output is very unstable with ammeter fluctuating from charge to discharge.  When both online it’s even worse.  So trying to understand how it was supposed to be setup.  Seems that a new regulator isn’t adjustable for voltage, so that adds (removes?) a variable if I need to replace a regulator.

Both regulators behind the electrical panel on the far right side?

Edited November 28, 2021 by geoffb

[milotron]

Both regulators were immediately under the panel on the copilot side, mounted side by side adjacent the center pedestal. I have the 262 like yours and the second regulator was clearly added where it would fit. Your obseravations are the same as mine; two VR415 running isolated. Each has it's fault light also. There is nothing that I could see coordinating their operation aside from the voltage settings and RPM when running.

I had a broken lead to the belt alternator that caused fluctuations like this. It was hidden inside the boot at the end of the connection to not immediately apparent until it was taken apart. Check the simple stuff first.

I am not sure the voltage setting is that critical any more. With newer avionics and LED lighting the electrical loading is considerably less than when these planes were built. So long as the second alternator is running and can carry load it will back up the first unit.

 

My second regulator was literally tyrapped to the underside. The original one is bolted to the tubing to the left, but not in this image.

[Aerodon]

The two ACU's (Alternator control units) are identical, linked together on an EQ Bus wire. 

EQ Bus

OV warning light- to light to to annunciator panel

Field - to alternator

Rem Sense - to Alt breaker

Power - also to Alt switch, then to Alt breaker

From experience in my Seneca, the 'balance' between the units can be problematic, but if your issue is with the #2 alternator with #1 switch off, that eliminates that problem.

The 252 has 3 shunts, one for each alternator output (can't go negative as far as I know), one for the overall load.  Could go negative.

I would check the field wire terminal on the alternator first, these small wires along side larger wires often get damaged when messing around with magnetos and other stuff.  It should be a shielded wire, with the shield to ground at the alternator. When alternators fail, they tend to fail, not be intermittent or fluctuate.  Or half output if some of the diodes fail etc.

You could swap the ACU's and see if the problem changes to the other alternator?

Aerodon