Automotive alternator windings & rectifier

http://www.autoshop101.com/forms/alt_bwoh.pdf
has photos of an alternator with 4 connections for windings (each of 3 phases
+ common), and the 8 diode rectifier. But wiring diagrams show only 3 connections and 6 diodes.
Where does the common winding terminal connect (there is a connection on the rectifier plate)? Where do the other 2 diodes connect?
I handled one of these disassembled units but didn't have time to ohm out the connections.
Thanks.
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http://www.autoshop101.com/forms/alt_bwoh.pdf
has photos of an alternator with 4 connections for windings (each of 3 phases + common), and the 8 diode rectifier. But wiring diagrams show only 3 connections and 6 diodes.
Where does the common winding terminal connect (there is a connection on the rectifier plate)? Where do the other 2 diodes connect?
I handled one of these disassembled units but didn't have time to ohm out the connections.
Thanks.
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On 8/19/2013 11:49 AM, Mike Cook wrote:

Not one of the pages shows four connections brought out from the windings.

I don't see any 8 diode rectifier.

Yup.

Common is created by the way the diodes are connected. >THAT< common is connected to the auto frame.

Hmm.
> Thanks. >
Wo uld you ask your question again and maybe clarify the situation?
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wrote:

The common point is sometimes used for fault detection.
When I was at Motorola SPD in the '60's I developed a circuit that could detect just about any fault, shorted or open diodes and shorted or open windings, by viewing that node. That node, BTW, is often labeled "S". Old GM alternators brought that out as a post on the back of the alternator (before regulators were internal... that's what some of my patents are all about... integrating the regulator).
That was before CAD ;-) If I can find my hand drawing, I'll post it.          ...Jim Thompson
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dumb question.
what EXACTLY does the charge lamp indicate?
what triggers it to go on and off?
can it simply be removed and ignored?
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One cause that can light the lamp is a broken belt. You would like to know as soon as that happens.
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On Mon, 19 Aug 2013 23:20:47 +0000 (UTC), Cydrome Leader

It's called an "IDIOT" light for a reason... as presently implemented it mostly tells you that the alternator is turning and has current in the field winding.
As I designed, the circuit compared the average value of "S" with 1/2 of the "A" terminal. Deviations were correlated with fault type.
Some GM versions enable the alternator regulator to function, so you can't remove it. (Saves GM the cost of a wire ;-)          ...Jim Thompson
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current into the regulator not supplied by the altenator.

basically a working altenator.

you could replace it with a short circuit
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wrote:

The common in the Wye connection (called "Neutral Junction" in the photo on page 18) goes to the 4th pair of diodes (and typically the regulator). The other three pairs of winding ends ("Stator Lead Ends") go to the other three pairs of diodes.

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On 19/08/2013 18:42, Spehro Pefhany wrote:

Not the alternators I've taken apart. The star point is typically a crimp, and serves no other useful purpose.
The field, and regulator power, are normally taken from an additional set of diodes, usually 3, ie one per phase, such that the regulator is effectively isolated from the battery when the engine is not turning.
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That pictured alternator is one used on Hondas. ISTM it's a Nippon-Densi or such.
I have too much experience with same. I went through 3-4 boneyard ones before I bought a rebuilt one from Retarded Auto Parts. That brand/model alternator is nice because the diode array unscrews easily. No unsoldering needed.
That particular alternator has 4 pairs of power diodes; the center point of the wye has a pair from there as well.... and I have no idea why... Jim??
That said, all recent alternators also have a diode trio of tiny diodes. Their function is to rectify enough power to excite the rotor. It takes a few amps to drive it. Until it's up to speed, that excitation comes from the battery, through the red idiot light. When the alternator is up to speed, there is 12V on the battery side of the lamp, and 12v on the load side....and it goes out.
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One problem with this arrangement... if the red idiot light burns out or a wiring fault opens the circuit, the alternator won't "bootstrap" the next time you start the car, and you won't (of course) get any red-light warning. Dead battery, here we come.
Ask me how I know :-(
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That's why you LOOK at the Idiot Light when you turn the ignition switch on, just before you start the engine. A lit ALT bulb means it is present and functioning.
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On Tue, 20 Aug 2013 04:24:20 +0000 (UTC), David Lesher

They are a multitude of ways to implement the "idiot light" function, some used relays, some were a part of the regulator.
I'm in the midst of moving right now, old house sold faster than new one's construction, so I'm stuck in a snow-bird friend's loaner with no access to my (paper) file cabinets.
When I get back on the air I'll retrieve those drawings and post.
Keep in mind I did no automotive after about 1973, so I'm not up on any modern approaches.

Or, like many GM alternators, the rotor was mildly magnetized to provide "kick-start", minimum load requirement hidden by the "daylight-safety" headlamp BS ;-)          ...Jim Thompson
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On Mon, 19 Aug 2013 22:32:24 +0100, the renowned Mike Perkins

Those were alternators with only six power rectifiers, right?

The power rectifiers connected to the Wye common are used to extract 3rd harmonic power- yielding up to 10% additional current capacity- when operating at high speed.
Best regards, Spehro Pefhany
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On 20/08/2013 06:33, Spehro Pefhany wrote:

Yes.

Thanks, I've learnt something today. I was aware that harmonic neutral currents are an issue with mains supplies but never thought they might be an advantage here.
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On 19/08/2013 10:33 PM, Spehro Pefhany wrote:

That doesn't make sense to me. One advantage of 3 phase is the elimination of triplen harmonics. Now, with a 3 phase bridge-will there be a significant 3rd harmonic voltage? Certainly,if the wye neutral is not connected to the DC ground, there is no 3rd harmonic current and power. Generally this is a good thing. Somehow, I appear to be missing something-could you elucidate? A circuit diagram and analysis would help.
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But THAT'S why I'm asking here: this Denso alternator (used in MANY Hondas, Toyotas, and myriad other makes) does have a common terminal connection to the rectifier. The rectifier has EIGHT diodes.
Yes I agree that common configuration is 6 diodes. That's why I'm confused. The system is quite populous and apparently functions. I just don't understand how.
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On 20/08/2013 08:04, Mike Cook wrote:

I can find very little on the web regarding neutral point rectification for car (Lundell) alternators, but there is a lot about 3rd harmonic neutral currents. An example how these manifest in electrical systems for non-linear loads.
http://static.schneider-electric.us/docs/Electrical%20Distribution/Low%20Voltage%20Transformers/Harmonic%20Mitigating/0104ED9501R896.pdf
A conventional automotive alternator uses six diodes to rectify three-phase AC (Alternating Current) into DC (Direct Current). The average voltage of the neutral point is 1/2 of the output DC voltage. While a low output current flows, the voltage at the neutral point is mostly DC, but it also includes an AC portion. As current output increases, the AC portion increases.
When the alternator speed exceeds 2,000 to 3,000 rpm, the peak value of this AC portion exceeds the DC output voltage.
This means that, compared with the output characteristics of the alternator without neutral-point diodes, the output gradually increases from midway by 10 to 15% at a normal rated alternator speed of approx. 5,000 rpm.
http://youronlinemechanic.com/alternator-with-neutral-point-voltage/
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Here's a diagram of what the above link failed to clearly explain
http://autonopedia.org/renewable-energy/generators/alternator-secrets/
so somehow tapping neutral is good. I guess those two diodes bypass current flowing int coils the wrong way or something? As to how speed matters, I'm not following that.
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