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#128590 - 06/17/03 11:14 AM alternator excitement question
DaveBinVa Offline
Junior Member

Registered: 06/16/03
Posts: 5
Loc: Va
I have a friend who was first a B-17 (http://www.wpafb.af.mil/museum/air_power/ap16.htm) gunner/flight engineer with the USAAF in WWII, then an aircraft electrician maintaining SACs B-36 Peacemaker (http://www.wpafb.af.mil/museum/air_power/ap39.htm) bombers. He related to me a story that had always puzzled him and it puzzles me as well.

On the B-36 there was an alternator that used a brushless exciter. From the aircrafts batteries was the positive going to the positive terminal, then from the negative terminal to a voltage regulator, then to the aircraft frame completeing the circuit.

There was also a radio filter across the positive and negative terminals of the alternator. If this filter was removed or the wire disconnected, the alternator would not excite. But if it was connected the alternator would excite.

Why is this, it has stumped me.

He also related a story about these alternators not working on one particular aircraft yet working on others. I dont know any other details about this so it may be the installers actually hooking it up right the second time.

Any help with this would be greatly appreciated as he said this was one of the few things he never really understood during his career.

DaveB.inVa

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#128591 - 07/01/03 07:11 AM Re: alternator excitement question
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
It could have something to do with residual magnetism and generator-field flashing.

Induction generators rely on connection to an AC-power system for excitation, but in odd cases can self excite with power-factor-correction capacitors, but not reliably.

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#128592 - 07/07/03 12:19 AM Re: alternator excitement question
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
The only thing I can think of is that the RFI Filter was also used to create a pulse train, which was pushed back into the windings and induced a current into the rotor.
A crude, yet effective creation of a somewhat steady state AC, via segments of RLC network arrays (or chopped via SCRs or FETs).

Alternately, the filter would contain the AC within the Genny and not let it out to plain-old DC devices, so maybe removing the filter placed an extremely low XL or XC (or both) in front of the AC, which resulted in it's major portion flowing externally from the Genny (or reducing the resonant Hz to a very low or high value - out of sync with the genny and/or unable to produce an effective excitation current across the rotor).

Maybe the regulator required some pulse train from the filter for operation? (like a fail safe device? no filter operation - no genny output, attempting to minimize RFI errors on flight equipment???).

What did the filter look like?
A simple Capacitor will pass AC and block DC, so that could be a key item to drive the regulator!

Anyone else?!?!?!

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128593 - 10/27/04 01:38 PM Re: alternator excitement question
SimonUK Offline
Member

Registered: 10/15/04
Posts: 50
Loc: Ayrshire, Scotland, UK.
Hi,

I'm new to this forum and am following it with great interest.

The following link should maybe help explain this anomoly though you will have to scroll through all the posts to get the gist of it.
http://www.enginads.com/sprkarcs.cgi/read/6073&expand/1

The 'missing' capacitance of the RFI filter could be enough to stop exciting the alternator.

Now I'm returning to reading as many posts as I can on this amazing site.

Regards Simon.

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#128594 - 10/27/04 01:52 PM Re: alternator excitement question
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Simon,

Welcome to ECN! Thanks for the link, too.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128595 - 10/27/04 04:27 PM Re: alternator excitement question
SimonUK Offline
Member

Registered: 10/15/04
Posts: 50
Loc: Ayrshire, Scotland, UK.
Thank you very much for the welcome.

there's a lot of good reading here which just goes to show how rusty I am after 16 years working on torpedos of all things.

Just greatfull my hobbies have kept part of my brains working.

I emailed some pics of one of my projects for the nostalgia board before realising how big they were so I hope I haven't caused any problems.

Simon.

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