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Dual Alternator issue

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Hey, I am troubleshooting an ongoing issue with the second, belt driven alternator. The gear driven one works fine, but when the belt driven one is added, the bus voltage actually goes down. There is a vert low whining noise through the audio panel with this alt on line also, but the measured AC ripple is still within tolerance.

Any thoughts? I have rebuilt the whole alternator except for the diode bridge. New-ish zeftronics regulator that is working correctly according to the indicator lamps. Running out of ideas.

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Not knowing much about the two alternator / Zef system... but I have a question...

Can you turn off either alternator independently?

Want a contact to the second alternator company?   B&G? I posted it around here a while ago...

I would expect some level of funny behavior with both alternators running...  Or is it normal to have both running at the same time...?

As usual... I’m only a PP, not a mechanic...

Best regards,

-a-

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Yes, there is split alternator field switch and each can be turned on or off separately. They are basically two independent alternator and regulator systems connected to the same bus, there is no load sharing or controller that works to share the load.

Normally, I keep both on all the time. The direct drive alternator spins a little slower than the belt drive so takes a bit less load.

 

The second alternator is a standard Kelly Aerospace type unit, like the Prestolite/Ford.

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When getting alternator sounds... that typically occurs when  losing a diode...

having less output from the alternator is often a sign that a diode is no longer operating as normal...

Hmmmm...

How hard is it to find and test the diodes? Some alternators have the diodes on board...

They act like one way electron valves... easy to test when not connected to anything... not sure how to test if they are connected...

When they fail, they can give off a heat signature leaving a black mark around them...

Ordinary PP thoughts only, not a mechanic...

Best regards,

-a-

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Usually there's either a load balancing or paralleling system so that the two alternators share the electrical load (like on a multi-engine aircraft), or one is set up as a primary and the other as a backup.

With a backup setup the regulator for the backup is just set for a lower output voltage than the primary.   e.g., in a 12V system the primary alternator regulator may be set to 14V and the backup to 13V.    If the primary is keeping the system at 14V, the backup contributes nothing to the load, but if the primary fails the backup regulator then comes on-line to keep the system at 13V, above the 12V battery level.  A warning annunciator can discern if the system voltage has dropped to 13V for primary failure, or 12V for total failure.

If you have neither of those systems set up I'd look seriously at how the system is supposed to be engineered.    You don't want the two systems fighting with each other.   It sounds like something isn't set up right.

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Following Eric’s logic for a moment...

1) Confirming the belt tension on the belt driven alternator may make some extra sense...

2) Belts have a tendency to slip even when they feel tight...

3) If the belt has started to slip... its alternator output may have dropped off line...

4) Would that result in the system voltage to be slightly low...?

5) depending on the set up, it’s possible...

6) This is where the two switches to control the two alternators comes in handy...

7) turning off the good alternator will show there is a challenge with the other alternator... 

8) Similar to an inflight mag test... collect some data... upload in savvy... post link here...

PP surmising at best, not an electrician or mechanic...

Best regards,

-a-

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This is the way that Mooney does it, basically two standalone systems. It is fairly primitive but they seem to work fine when all the parts are functional.

 

Diodes required the alternator to be removed and dissembled. They tested fine when I changed the brushes several months ago, but starting to think there is a shorted diode that is causing the winding to act like a load on the system. I just got a new diode bridge in yesterday and will try that next. Not much else to look at it, everything else is new!

 

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