TWELVE DIODES INSTEAD OF SIX

[MRSSPOCK]

Does anybody have any idea why my refurbished Denso alternator has twelve diodes, whereas the original had six? It works fine, but I'm mystified how a three phase alternator even uses twelve diodes. Does anyone have a schematic or an explanation. Thanks

 

[GregLocock]

It could be that two 1A diodes are cheaper than one 2A diode.

Cheers

Greg Locock


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[IRstuff]

A 3 phase alternator would require 4 diodes per phase for full wave rectification, so perhaps the original design was only half wave rectifying

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[LionelHutz]

No IRstuff, a 3-phase full-wave rectifier requires 6 diodes in total, 2 per phase.

Does the alternator have 6 leads coming out of the stator? I expect it has two 3-phase windings shifted 30 degrees, at least I think 30 degrees would be used here. That makes DC with twice as many pulses per cycle which reduces the output ripple, reduces the harmonics and most likely increases the current capability.

It might use 60 degrees shift, I'm not sure off hand. The 30 degrees shift is simple to get with a transformer because that is the wye to delta shift angle so it gets used with 12-pulse rectifiers.

Look up 12-pulse rectifiers for more info, it's the same basic principle.

 

[MRSSPOCK]

6 leads only.

Something like this I guess, with three of the winding ends going to ground, i.e. as per the centre of the STAR

 

[LionelHutz]

That shows 3 leads, and no, the center of the star is not grounded.

 

[MRSSPOCK]

My old one looked something like this, (three diode cathodes facing up, and three facing down), so at points A, B and C, one end of each of the three windings is being grounded, so from what I can see, they appear to combine to become the centre of the STAR, so to speak.

 

[LionelHutz]

You can't have both the star point grounded and the negative output side of the 6-diode rectifier grounded at the same time.

I don't know why they would bring out both ends of the windings. I see some stators with the star point brought out already connected together. I don't know of a use for that connection though, never gave it much thought. Any alternator I've seen uses a full-wave rectifier on the output of the stator and that doesn't require a connection to the star point. Possibly, the star point is used for the regulator circuit.

Here is a schematic of dual stators with 12 diodes just as an example. See the angle shift in the 2 sets of stator coils? It appears they missed a connection dot too. It's not very accurate overall but it shows the idea.

https://www.hkona.com/images/alternator_schematic_diagrams-846/554/sume35100l.gif

 

[MRSSPOCK]

Great. That's what I was hoping to see. Maybe now I can make some sense of it. Is this termed six phase, or three phase with three more with a phase shift?

 

[LionelHutz]

You could call it 6 phase if you wanted to.

 

[kontiki99]

I've seen 3 phase, 115V, 400hz, aircraft transformer rectifiers with both a delta and a wye secondary that have a six diode bridge on each. I can't share the schematic though.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[LionelHutz]

Yes, that is a typical 12-pulse rectifier configuration.

 

[bradrs]

Did you verify that the components are diodes? I had seen some technical papers where they use transistors and a controller IC in place of diodes in some newer alternator designs. The transistor has a lot less losses. And there is a version of the circuit that uses 2 transistors in series, instead of 1.